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C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | The effective average over the distribution by degrees of sexual activity, c, is given explicitly as
c = Σi2Ni/ΣiNi = m + σ2/m
(2)
where m is the mean and σ2 the variance of the distribution of the number of new sexual partners per unit of time8. Thus, c is not simply the mean but the mean plus the ratio of variance to... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | systematic reporting. By contrast, the opportunistic infections cancers4,5,12,19, and subsequent mortality characteristic of the destruction of the immune system in AIDS, leads to fairly reliable reporting20. There is, however, one study of hepatitis B virus (HBV) in a cohort of 6,875 homosexual and bisexual males in S... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | UK, United Kingdom). d, The relationship between sexual activity amongst a sample of homosexual/bisexual males (from San Francisco, USA) as measured by the number of male partners over a two-year period, and the percentage of each group (based on sex partners) who were seropositive for HIV antibodies (data from ref. 26... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | Fig. 3. Data on the distribution of the incubation period of AIDS derived from longitudinal studies of transfusion recipients (data from ref. 31). Observed cases of AIDS are recorded as a function of the year of transfusion (the assumed point of acquisition of infection) and the year of diagnosis. A Weibull distributio... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | number of partners per unit of time may significantly overestimate the number of new partners per unit of time, which, or that equating D to the incubation period, may overestimate the average duration of infectiousness. It may also be that the high values of c arise from sampling biased towards the high activity group... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | Although much more information is available about the incidence of AIDS than of HIV infection (Table 1), it is harder to tease estimates of epidemiological parameters out of these. Incidence of AIDS depends not only on the transmission factors β and c, but also on the incubation period and on the fraction, f, of those ... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | If indeed the probability per unit time, to develop AIDS (for that fraction f who do indeed develop it) increases linearly with time from infection as αt, the result is a Weibull distribution with γ = 2 and ν = α for the hazard function8. This assumption differs from conventional epidemiological models, where infected ... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | acquire infection, and eventually are removed, relatively early in the epidemic; transmission among the remaining, less active, individuals may be relatively weak. 中文
Figure 5 may be used, in combination with two factual observations, to make a rough assessment of R0 for HIV among male homosexuals. First, the studies i... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | An estimate of the value of R0 in the early stages of the epidemic is also valuable in indicating the magnitude of the social changes needed to bring R0 below unity. If R0 is around 5–10 or more, then reductions by a factor of 5–10 or more in βc are needed. Because c depends disproportionately on those in the highly se... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | At present, it is not possible to tell whether the severity of the epidemic will be increased or decreased if a larger fraction of those infected develop AIDS, for the relative infectiousness of the two categories is unknown. For public health planning it is clearly important to attempt to acquire such data. 中文
Dynamic... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | the sexually active community was set at 100,000 per 32 yr. 中文
Many of the features presented in Fig. 6 show qualitative agreement with observation. The rise in incidence of infection (seropositivity) is initially exponential, but soon shows a more linear rise. And, the rise in incidence of AIDS lags that in the propo... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | c. The data (Table 1) suggest the doubling time for heterosexually-transmitted HIV infection is roughly equal to that among homosexual males, which is in accord with our simple expectation. 中文
As the epidemic progresses, a high proportion of homosexual males become infected (Fig. 2e). On the other hand, the pool of fem... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | significantly smaller than R0 for purely male homosexual transmission. Whether R0 is greater than unity, such that HIV infection can maintain itself and spread by purely heterosexual transmission, is at present unclear. There is an urgent need for studies to measure c1 and c2 in different communities (stratified by age... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | associated with the age-related degree of sexual activity amongst heterosexuals27,47,48,56,57. We note, however, that in the early and approximately exponential phase of the epidemic, the ratio of the number of seropositive males to seropositive females is not unity, but is roughly (β1c1/β2c2)1/2. 中文
It is generally th... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | For public health planning, the dominant unknown is f, the fraction infected who will eventually develop AIDS. Estimates of this parameter have been increasing in recent years, but on present evidence the possibility cannot be ruled out that it is as low as 20% or as high as virtually 100%. Thus any current predictions... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | In developed countries, at present and into the near future, it is probable that sexually-transmitted HIV infections among females are likely to come mainly from bisexual males. Whether subsequent spread of infection from such females to heterosexual male partners is likely to reach significant levels, and more importa... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | Robert M. May is in the Biology Department, Princeton University, Princeton, New Jersey 08544, USA. Roy M. Anderson is in the Parasite Epidemiology Research Group, Department of Pure and Applied Biology, Imperial College, University of London, London SW72BB, UK.
References:
Hahn B. H. et al. Science 232, 1548-1552 (1... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | Centers for Disease Control MMWR 34, 613-615 (1985).
Lui, K. J. et al. Proc. Natl. Acad. Sci. U.S.A. 83, 3051-3055 (1986).
Peterman, T. A. et al. J. Am. med. Ass. 254, 2913-2917 (1985).
Moss, A. R. et al. J. infect. Dis. 152, 152-161 (1985).
Weber, J. N. et al. Lancet i 1179-1182 (1986).
Stevens, C. E. et al. J. Am. me... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | 英文
尽管在人类免疫缺陷病毒(HIV——艾滋病,即获得性免疫缺陷综合征的发病原因)[1-5]的基本生物学特性上的认识已有显著进展,但是公共卫生计划仍然受到流行病学不确定因素的阻碍[4-6]。要想获得关于感染性的典型持续时间和感染强度,或者被感染者发展为艾滋病的比例(以及多久之后发展为艾滋病)这些方面的准确信息,只有在这些事件发生的同时进行仔细设计的研究才可以,也就是说这需要很多年。缺少这些数据,目前HIV传染动力学的数学模型就不能用来准确预测艾滋病将来的发展趋势,但是它们有助于间接评价某些流行病学参数,弄清楚预测未来发展趋势需要哪些数据,在多种特定推测的基础上,对个人的传染过程和限定人群中性活动的模式(或者改变)做出预测,通俗地讲... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | 英文
图1.1978~1985年期间,被统计的病人中HIV抗原血清阳性反应呈增长趋势。在旧金山[12]、伦敦[23]和纽约[35]研究的是男性同性恋和双性恋。在意大利[24]研究的是吸毒者。 英文
我们的HIV模型表明,如图1中所示的大致呈直线增长的模式,并不是标准流行病(在同类的混合群体中)的特点。在英国和其他的欧洲国家,这种病毒首次出现的时间要比美国晚几年(图2a, b和c),传染病的蔓延仍旧处于早期阶段。结果大概从它最初在欧洲出现以后[23,24](图1),才有集中于HIV的血清学研究。HIV感染的最初指数增长可以用倍增时间来描述,td,与生长速率Λ有关。公式(1)变为td=(ln2)/Λ。 英文
图2.a,在美国,19... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | c,在1984年,伦敦调查到的同性恋和双性恋男性居民(来自麦克马纳斯未发表的数据)。数据表示每年的男性伴侣的数量。d,1986年11月,在英格兰调查到的18~44岁之间的异性恋居民(哈里斯研究组织;安德森和梅德利未发表数据)。数据表示每年异性性伴侣的数量(样本大小=823, m=1.41, σ2=4.36)。进一步的调查发现(参考文献27),从1982年11月到1984年11月,旧金山的男同性恋每月的伴侣平均数下降,从5.9降为2.5。 英文
由于公众对艾滋病的认识有所提高,美国男性同性恋之间的性活动模式已发生改变(比如,表现为直肠淋病的发病率明显下降[29,30]),可能导致β和c的改变[27]。因此,我们的讨论主要适合于流行病... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | 英文
对在染病早期阶段R0的估值是有用的,它可以表明使R0低于整体值的社会习性变化幅度。如果R0的值在5~10左右或者更高,那么βc需要减小5~10倍或更多。因为c不成比例地取决于那些使高度性活跃群体中的人改变自己习惯——较少的性伴侣和安全的性行为——的项目是最有效的。但是如果这些人不太可能响应公共健康教育,那么使R0低于整体值是很困难的。 英文
死亡率
如果感染者最终都发展为艾滋病并随后死亡,那么流行病的严重程度,也就是累计死亡率就会达到最大,这种推测是不准确的。死亡率主要取决于已经发展为AIDS的感染者和没有发展为AIDS的感染者二者的传染性的持续时间。如果后者与未被感染的人有相似的预期寿命,但是他们生活中仍保持传染性,那么他们... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | 英文
双性恋男性传染HIV给女性只是一个过程,它最初的动力学主要是由同性恋男性之间的传播R0决定的,其后出现了纯粹异性恋接触的传播和持续时间的问题。这种异性恋之间HIV传播的感染增值指数R0′的计算公式是
R0′= (β1β2c1c2)1/2D
(7) 英文
β1和β2分别是被感染的女性和易受感染的男性之间接触以及被感染的男性和易受感染的女性之间接触的传播参数;c1和c2在前面(2)中已经给出,分别是女性和男性获得新的异性性伴侣的比率分布。 英文
虽然数据受传播和性活动的参数的限制,但是图4的数据显示c1和c2明显地比男性同性恋之间的c小。此外,似乎β1<β2,它们两个都小于男性同性恋之间的MM。因此,总的来说,(β1β2c1c2)... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | 我们强调,流行病学上最重要的是获得新的性伴侣的平均几率不一定要和单位时间里性伴侣的平均数相同。一些作者认识到,高度性活跃的个体在传染动力学中起到了不成比例的重要作用;公式(2)量化了这个观察结果,明确了流行病相关的数量不是新伴侣的平均值,而是均方除以平均值之后的结果。 英文
在现在和不久的将来,在发达国家女性中通过性传播HIV可能主要来自男性双性恋。从这样的女性到异性恋的男性伴侣随后的传播是否有可能达到显著的水平和最重要的是纯粹的异性恋之间的HIV传播是否可以自我维持(R0′>1),都取决于传播参数β1,β2,c1 和 c2的估计值。 英文
我们已经阐明如何间接地从原始的倍增时间估算出男性同性恋之间的传染c值,但是β1c1和β2c2... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | shock-produced but primary, formed by stellar condensation as a metastable phase. It appears that interstellar dust contains diamond. 中文
THINKING that Cδ might be the C60 cluster “buckminsterfullerene”5, we isolated a new sample from the Allende C3V chondrite, aiming for maximum purity rather than maximum yield. After... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | Fig. 1. Dark-field transmission electron micrograph of Allende CJ, in which bright areas are diamond grains diffracting electrons into one part of the innermost ring of the diffraction pattern shown in Fig. 2. As most grains do not have the proper orientation, the true abundance of diamond is perhaps five times higher ... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | Curiously the infrared spectrum strongly resembles that of soot, with peaks at 2.93, 3.17, 5.59, 6.13 and 7.14 μm. For soot, these features are attributed to the reaction of surface atoms with H and O, and presumably such surface alteration also occurs for fine-grained diamond. Approximately 9% of the atoms in 50-Å dia... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | ‡ White; all other samples are black (except Indarch CF, which is dark grey) owing to the presence of graphitic carbon.
§ Excluding an isotopically normal Xe component released at 800 °C, before the release of Xe-HL (≥1,000 °C). It occurs only in C2 chondrites23. Its carrier, Cζ is carbonaceous but totally uncharacteri... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | while ignoring the more abundant, gas-poor carbon grains? Third, all four meteorites have essentially similar Cδ, with comparable gas contents and isotopic compositions (Table 1). If the diamonds were shock-produced, we would expect greater variations, depending on the proportions of the two types of carbon converted t... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | diamond, being metastable by only ~2 kJ mol–1, could form instead. Indeed, such preferential formation of diamond has been observed in the laboratory15. The processes studied so far involve decomposition of CH4 or organic compounds in the presence of large amounts of hydrogen, either by ordinary pyrolysis or by plasma ... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | Second, if diamond is detected in circumstellar dust, its formation conditions (as inferred from the atmosphere of the central star) should be determined and duplicated, insofar as possible, in the laboratory. Possibly nature makes diamond more efficiently than even the best laboratory synthesis discovered so far. 中文
F... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | Lipschutz, M. E. & Anders, E. Geochim. Cosmochim. Acta 24, 83-105 (1961).
Clarke, R. S. Jr, Appleman, D. E. & Ross, D. R. Nature 291, 396-398 (1981).
Ott, U., Kronenbitter, J., Flores, J. & Chang, S. Geochim. Cosmochim. Acta 48, 267-280 (1984).
Göbel, R., Ott, U. & Begemann, F. J. Geophys. Res. 83, 855-867 (1978).
Land... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | 电子衍射(图2)确认了大部分的颗粒均为金刚石,获得的晶面间距与公认值(显示在括号中)一致:2.06(2.06)Å、1.26(1.26)Å、1.07(1.075)Å、0.893(0.8916)Å、0.817(0.8182)Å、0.73(0.728)Å、0.68(0.686)Å。分析型电子显微镜的元素分析显示样品中有很少量的氯(污染物或者来自盐酸的键合在表面的氯?)、铬和铁(微米尺寸以下的铬铁矿[7])。在70~550°C进行的高分辨质谱检测表明仅有少量的挥发性物质,很可能是污染物。出现的峰的质荷比至少可以达到733,但是没有哪个含量较多的组分的峰对应于C60或者其他的Cn离子。 英文
图1.样品阿连德 CJ的暗场透射电子显微镜图片,... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | 之前的分步燃烧测量显示,少量氮、少量碳和Xe-HL都在同样的温度区间(500~600°C)被释放[1,3,8],这表明它们可能处在同一相中。然而,这种相关性必须在金刚石含量高的样品中进行确认。(在相同条件下陆生金刚石的燃烧温度较之高约250°C[9],但这是较为粗大的颗粒尺寸的一个寻常结果。当活化能为60 kcal·mol–1时,800°C下单位面积的燃烧速率是550°C的4.8×103倍,但是这个差异会被晶粒尺寸70倍的差异完全抵消,也就是说尺寸为3,500 Å而非50 Å)。 英文
以前金刚石在一些陨石中也被发现过(大部分的橄辉无球粒陨石和两种铁陨石),但是它们显然是由于在地球上或太空中的撞击形成的[10,11]。看上去Cδ并非... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | 英文
最后,对于陨石中的金刚石,应该对其中所有可检测的元素进行同位素分析。如果氙可以通过来自超新星的离子注入被捕获,那么它应该伴随有元素周期表中的其他元素。由于具有非比寻常的耐受性,金刚石可能记录了含有恒星的核合成和化学信息以及太阳系形成前的信息。 英文
本研究部分由美国国家航空航天局支持。 英文
(李琦 翻译;顾镇南 审稿)
Oceanic Phytoplankton, Atmospheric Sulphur, Cloud Albedo and Climate
R. J. Charlson et al.
Editor’s Note
In the 1970s James Lovelock developed the Gaia hy... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | CLIMATIC influences of the biota are usually thought of in connection with biological release and uptake of CO2 and CH4 and the effect of these gases on the infrared radiative properties of the atmosphere1. However, the atmospheric aerosol also participates in the radiation balance, and Shaw2 has proposed that the aero... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | Assuming that sulphuric acid and sulphate aerosols are indeed the only significant contributors to CCN over the oceans (as will be concluded below), we have to consider what processes are responsible for the production of the volatile sulphur compounds that are the precursors of sulphuric acid in the atmosphere. In thi... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | thermodynamically stable form of sulphur in the presence of oxygen. Sulphate is reduced by organisms through two mechanisms, “assimilatory” and “dissimilatory” sulphate reduction. The dissimilatory pathway is restricted to sulphate-reducing bacteria in anaerobic environments; due to physical and microbial restrictions ... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | In the marine environment, most volatile sulphur is emitted in the form of DMS which is excreted by living planktonic algae. In contrast to microbial decomposition which produces a complex mixture of volatile sulphur species, algal metabolism yields DMS as the only volatile sulphur species. The biological function of t... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | Two reasons can be given for this unexpected behaviour. First, the concentration of DMS in surface sea water depends not only on the rate of DMS production, but also on its rate of removal. The two dominant removal processes are ventilation to the atmosphere and the photochemical and microbial breakdown of DMS in the w... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | We can now evaluate the role of sulphur gases in general (and DMS in particular) as sources of and of CCN in the unpolluted marine troposphere. Because submicrometre particles are not directly emitted by land or ocean surfaces, they are created in the atmosphere only by chemical reactions. As far as is known, the only ... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | The rates of DMS oxidation obtained from laboratory experiments, and the corresponding lifetimes, are consistent with observed concentrations, the observed flux out of the atmosphere in rain and the calculated flux into the atmosphere from the ocean surface5,17,18. Thus, the lack of alternative sources of sulphur-beari... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | Koehler26 argued that most CCN are composed of water-soluble materials, such that the marine should qualify. It is observed that sea-salt particle concentrations at cloud height are typically not more than 1 cm–3, so that sea-salt itself cannot be the main CCN23,27,28. Water-soluble materials other than sea-salt and do... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | Other data also support the idea that particles are the main contributor to the CCN. Much of the light-scattering aerosol in marine air is volatile at elevated temperatures, evaporating in a fashion identical to sulphates with a range of compositions from H2SO4 to (NH4)2SO4, as deduced from temperature-and-humidity-con... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | where ρ is the density of water in the appropriate units. Various studies have examined the effect of holding one of these three variables constant while the other two change. Paltridge39, Charlock40, and Somerville and Remer41 considered cloud albedo changes due to a climatic warming which might increase the liquid wa... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | different values of N. Figure 1 shows the change in albedo Δa from that of Twomey’s reference cloud, caused by variation of N/N0 in the range 8 to 1/8, where N0 is the number-density of droplets in the reference cloud. Different thicknesses of the reference cloud are represented on the vertical axis by their albedos. T... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | calculations used , but the figure also is approximately valid for other reference clouds: the plotted values of Δa are in error by less than 20% if albedo < 0.9 and (that is, anywhere in the range of reff found in real clouds by Hegg66). The size distributions used for the calculations are almost monodisperse, broaden... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | Real clouds are not homogeneous, and the albedo of a cloud with a horizontally inhomogeneous distribution of droplets is always less than that of a hypothetical homogeneous cloud44,45, because the albedo-versus-optical-depth function is nonlinear, concave downwards. We therefore compare observed clouds to modelled clou... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | 中文
Table 1. Climatic effect caused by increasing CCN concentration over the ocean
* Only the areas covered by low stratus and stratocumulus (St/Sc) clouds and middle altostratus and altocumulus (As/Ac) are considered, because the change in albedo is smaller for convective clouds and for nimbostratus because they are t... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | ** Because of absorption in the 0.6-μm band of ozone above the cloud, the visible-channel albedo at TOA is smaller than at TOC, by a factor of about 0.9 (ref. 47). A further factor of about 0.9 is needed to convert visible-channel TOA albedo to solar TOA albedo47,63, because cloud albedo is lower in the near-infrared t... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | itself which is ~0.1. This empirically derived quantity is similar to the estimates of Shaw2. A 30% increase in the number of particles (over the oceans only) with no change in their mean size would increase δbsp by 9×10–4, increasing the planetary albedo over dark surfaces by the same amount, but having no effect in a... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | Although we do believe that increased DMS emission should influence both the mass concentration and number population of CCN composed of (and hence cloud albedo), we do not understand quantitatively the relationship of source strength of DMS to CCN number concentration. It seems likely that increased DMS fluxes would i... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | k. The magnitude of this effect on CCN would depend on the relative rates of loss of DMS to the atmosphere and in the water column. 中文
Empirically, we find that the largest flux of DMS comes from the tropical and equatorial oceans5. This suggests that the most important climatic role of DMS is to contribute to elevated... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | place by pumping of CO2 then the mechanism is now close to the limit of its capacity to operate. Atmospheric CO2 has been reduced from about 30% of the atmosphere at the start of life to the present 300 p.p.m.v., a factor of 1,000. It was suggested that the decrease of CO2 through its declining greenhouse effect had co... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | for the system. 中文
With algae it seems likely that the local problem that led to the biosynthesis of DMSP was salt stress. As a result of tidal movement algae are left exposed and drying on the beach at least twice daily. Neutral solutes, such as glycerol or dimethyl sulphoxide are well known to protect against the adv... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | A large proportion of the current biosynthesis of DMSP is in the open oceans distant from the land surfaces. Is this DMSP also made for the relief of salt stress, or is it a redundant mechanism kept in action because of glacial epochs when the sea or part of it was saltier? Interglacials have occupied only one tenth of... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | Lastly, DMS is the principal component of the present biogeochemical flux of sulphur to the atmosphere. But it is not the only one; some sulphur is emitted as H2S, COS and CS2. COS, both from direct emission and as an oxidation product of CS2, is stable in the troposphere long enough to be an important source of sulphu... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | counteract the increasing luminosity of the Sun and/or that it has already counteracted the influence of the recent increase in CO2 and other “greenhouse” gases). However, the data required to demonstrate the latter effect have not been and are not now being acquired. 中文
The portion of this work done in the USA was sup... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | Bates, T. S., Cline, J. D., Gammon, R. H. & Kelly-Hansen, S. R. J. Geophys. Res. (in the press).
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Warren, S. G., Hahn, C. & London, J. J. Clim. Appl. Meteor. 24, 6... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | 到目前为止,这一颇具吸引力的假说在物理和生物方面还未得到量化,不过对于偏远的海洋大气圈来说,近来的三个发现可能会促使这种量化成为可能。(1)普遍生长于海洋中的绝大多数浮游植物都可以分泌二甲硫醚(DMS),这些二甲硫醚逸入大气后发生化学反应,生成硫酸盐和甲烷磺酸盐(MSA)气溶胶。(2)这种非海盐硫酸盐气溶胶在海气边界层中到处都是。(3)气溶胶颗粒是海洋大气圈中的云凝结核(CCN),它们可能几乎全部都是这样的非海盐硫酸盐。 英文
本文我们将证明,在未受污染的偏远海洋大气圈中,由浮游植物分泌出二甲硫醚的量足以使其成为云凝结核的气相前驱体。我们重新检测了大气辐射传输过程中,特别是地球反照率主要影响因素的云层之中,硫酸盐气溶胶的物理作用,同... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | 即使在沿海地区以及上升流区这些高生产力区,其二甲硫醚通量也不是很高(基于多个研究小组的大量数据[5],分别为5.7 mmol·m–2·yr–1和2.8 mmol·m–2·yr–1)。海洋中最温暖、盐度最高且日照最强烈的区域向大气释放二甲硫醚的速率似乎也最高,并与初级生产率无关。这是我们在讨论可能的气候反馈机制时必须注意的关键问题。 英文
关于这种意外现象可能有两个原因。首先,表层海水中二甲硫醚的浓度不仅仅取决于二甲硫醚的生成速率,还取决于其移除速率。两种主要的移除作用分别为向大气的通风量以及水体中二甲硫醚的光化学分解和微生物分解作用[12,13]。微生物对二甲硫醚的移除作用会随海水中可代谢生成二甲硫醚细菌的密度而增加,也会随海水中二... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | 相比之下,现今陆地空气的特性很可能与弗莱彻的结论一致。 英文
克勒[26]认为,多数云凝结核是由水溶性物质组成的,这样海洋非海盐硫酸盐就可以作为云凝结核。据观测,在云层的高度上,海盐的粒子浓度一般不高于1 cm–3,所以,海盐本身不会成为云凝结核的主体[23,27,28]。除海盐与非海盐硫酸盐之外,海洋空气中确实还存在其他水溶性物质,但数量不大。硝酸盐可能在较粗的颗粒中找到,故硝酸盐对总体数量的贡献不大[29]。亚微米级颗粒中存在有机化合物,但其质量浓度大概仅为非海盐硫酸盐的十分之一(佐勒,个人交流)。它们的数浓度及来源尚不清楚。 英文
几十年来,人们一直认为海洋空气中天然云凝结核是由硫酸盐组成的,并且霍布斯[27]曾提出这些天然云... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | 图1所示为图米提出的参考云层的反照率变化Δa,它是由N/N0在8~1/8之间变化所引起的,其中N0是参考云层中云滴的数密度。参考云层的不同厚度在竖轴上以其反照率表示。因此,图米的图12.6给出的信息就包含于本文图1的左右两边,同时图1还展示了N发生任何微小变化将引起的效应。重叠部分是我们的研究结果与图米的结果相一致的部分,这说明,如果将结果表示为参考云的反照率的函数而不是云层厚度的函数,则因N变化而引起的反照率的变化对太阳天顶角并不敏感。由图1可知,当a = 0.5时,云的反照率对N最敏感,但当0.3 < a < 0.8且N/N0变化较小时,Δa基本独立于a。取参考云层的有效云滴半径 μm,对图1作了计算,但对于在4~500μm之间... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | † 海洋上空St/Sc的纬向平均值(部分覆盖区)在低纬度地区的18%到高纬度地区的50%之间变化[61],全球海洋的平均值为34%,季节平均值(参考文献61及沃伦等的其他资料,完稿中)。白天(受日光照射)接近于日平均值。当海洋上空观测到St/Sc时,约有52%的时间也会在其上方看到As/Ac[62]。当海洋As/Ac的量占到50%时(沃伦等,完稿中),则有50%×52%=26%的St/Sc与As/Ac重叠。那么,非重叠St/Sc所覆盖的大洋面积比就为34%×(1.0–0.26)=25.2%,即地表的17.6%。
‡ 白天As/Ac对海洋的覆盖率为22.4%(参考文献61及沃伦等,完稿中),但与St/Sc、积云或积雨云不重叠的As/A... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | 英文
在寻找二甲硫醚由海洋向大气输入的质量通量F与全球气候之间的反馈机制(图2)时,我们可以从组成通量方程的变量入手:
F = A· k·Δc
我们可以改变发生气体交换的大洋总表面积(A)、传输速度(k)或气–海界面上的浓度梯度(Δc)中的任何一个。相对于大气来说,海洋是高度超饱和的(至少超出3个数量级),所以浓度梯度这一变量实际上相当于表层海水中二甲硫醚的浓度。 英文
图2.一种可能的气候反馈循环概念图解。矩形内为可测定量,椭圆内为连接矩形内各个量的作用。椭圆内的+、–号表示前一个矩形内的量的正向改变对后一个矩形内的量的影响,采用凯洛格[74]的符号表示法。该循环中最不确定的关系是云的反照率对二甲硫醚排放量的影响;要起到调节气候... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | 因此潮间带生物对渗透压调节物质的需求可能是合成二甲基磺基丙酯的起因。 英文
我们终于开始看到出于盐胁迫保护的局部利己行为与全球硫循环之间的地球生理学关系了。二甲基磺基丙酯生成作用中的副产物就是其分解产物二甲硫醚。该化合物或其气溶胶氧化产物会从沿海地区向内陆输送,并在海洋下风向位置的陆地表面形成硫沉降。陆地上的硫是趋于减少的,而来自海洋的这种营养元素可以提高生产力以及风化速率,进而导致营养盐回流至海洋生态系统。表面上朴素的利他行为实际上是一种无意识的利己行为。来自二甲硫醚的硫比海盐气溶胶的传输距离要远,因为气态二甲硫醚转变成气溶胶硫酸盐要经过多个步骤;而且所形成的气溶胶颗粒较小,因此其生命周期也长得多。 英文
目前,二甲基磺基丙酯的合... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | 英文
(齐红艳 翻译;刘新 审稿)
Structure of the Repressor–Operator Complex of Bacteriophage 434
J. E. Anderson et al.
Editor’s Note
In 1967, molecular biologist Mark Ptashne demonstrated negative gene regulation in the form of a repressor protein binding directly to specific DNA sequences. Two decades later, with the help of colle... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | together with experiments reported in an accompanying paper and elsewhere2,3, reveals the basis of specific binding and its modulation by the exact DNA sequence of the operator. 中文
In an account of this structure at 7 Å resolution4, we described the general features of the complex. The repressor DNA-binding domain (R1-... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | origin4. The small magnitude of the heavy-atom differences and weak intensities beyond 7 Å made conventional refinement of heavy-atom positions impractical. We therefore used a two-step strategy to extend phases, first from 7 Å to 5 Å and then from 5 Å to 3.2 Å. In the first step, heavy-atom positions were refined at 7... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | Nposs = maximum number of reflections possible in corresponding resolution range.
The percentage of observed and possible data with F > 2σ (F) is indicated. Data from native crystals were collected to 3.2 Å using CEA-25 X-ray film and oscillation photography with Elliot GX-6 and GX-13 X-ray generators. The films were s... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | present in the native data set, whether or not present in the initial SIR data set.
† Phases calculated from coordinates fit to the averaged maps from a were refined by averaging Fo exp (iαc) maps. The envelope was constructed from the coordinates on which the initial phases were based. Rav = ∑| Fo–Fc |/∑|Fo|
中文
The ... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | The initial model, constructed as just described, was used to obtain starting phases for a second round of noncrystallographic symmetry phase refinement. The order in the crystals is anisotropic, with intensities measurable to spacings of 3.2 Å in the directions of noncrystallographic symmetry axes formed by stacked pr... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | Phases from the initial model were used to initiate eight further cycles of noncrystallographic symmetry refinement of phases between 15 and 3.2 Å. The computation converged stably to Rav = 0.27, with an overall phase change of 43° (Table 2, lower part). Significant adjustments were indicated by this map, and we extend... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | base pairs (See f). b, Overall view of the 434 repressor-operator complex. The central 14mer (red) is bound to two R1-69 domains (light blue and dark blue). Adjacent 14mers, stacked as in the crystal, are also shown (violet). The view is perpendicular to the operator dyad, which lies in the centre of the diagram. c, DN... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | As in any structure determination at this resolution, interactions such as hydrogen bonds must be inferred from the position and orientation of the participating groups. We believe our present structure to be sufficiently well determined that the protein–DNA interactions can be described correctly and that likely hydro... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | Protein Conformation
R1-69 is a cluster of four α-helices, with a C-terminal extension. Two of the helices, α2 and α3, form a helix–turn–helix motif. The residues found in each of the four helices are shown in Table 3. For comparison, the corresponding residues in the N-terminal domain of λ repressor5, determined by co... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | helix 3, since its carbonyl appears to hydrogen bond to N31. 中文
Interactions occur between the two R1-69 subunits bound to one 14mer. The two monomers are in contact across the dyad near residues 57–58. The aliphatic chain of Arg 41 and a residue near the C-terminus in one subunit lie against the aromatic ring of Phe 4... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | The three tightest contacts (as judged both from the structure and by the strength of ethylation interference) all appear to involve peptide backbone NH groups. At P14-, the NH of Gln 17 approaches the phosphate oxygens closely enough to form a hydrogen bond. Residue 17 is at the N-terminus of α2, and its peptide NH gr... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | The side chain of Arg 43 projects into the minor groove near base pair 7. The groove is somewhat narrower than average at this position, and the guanidinium group could probably have significant coulombic interaction with phosphates 9 and 10′ on either side of the groove (Fig. 4). The possibility of direct interaction ... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | A10′ (base pair 5); a refined model will be required for confident assignment. 中文
Fig. 5. a, Stereo view of helix α3 and base pairs 1–5, viewed from the N-terminus of α3. One base pair of the adjacent 14-mer is also present. Included are residues 28–35 of R1-69. b, Schematic diagram as a key to a. Note the similarity ... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | Base-pair contacts. The interactions of Gln 28 with base pair 1 and of Gln 29 with base-pairs 2 and 3 are consistent with the conservation of base pairs 1–3 in all 12 operator half-sites, because changes in any of these base pairs would destroy one or more of the hydrogen bonds or disturb van der Waals contacts with th... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | clear whether the bend is present in unbound operator DNA, or whether protein binding induces it. 中文
Central base pairs. The configuration of the operator at its central base pairs is consistent with sequence preferences at positions 6–9. A more detailed discussion appears in the accompanying paper, but we summarize th... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | relationship of the two half-sites and requires compensatory variation in the protein dimer contacts, with a corresponding loss in the overall energy of the complex. We expect dimer contacts to be more extensive and less deformable in the intact repressor, and indeed alterations in central base-pair sequence have great... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | minor groove) could extend to provide compensating electrostatic stabilization. Such non-specific complexes may be intermediates in operator binding, because proteins like 434 repressor are believed to locate their sites by initial nonspecific interaction with distant sequences, followed by diffusion along the backbone... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | The distinction between these zones is only heuristic and it is clear that a perturbation at one position will affect the fit elsewhere. For example, the entire operator half-site must be configured correctly against the repressor monomer for any of the interactions with base pairs to be meaningful. Given such a config... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | Rossmann, M. G. & Blow, D. M. Acta crystallogr. 16, 39-45 (1973).
Bricogne, G. Acta crystallogr. A130, 395-405 (1974).
Bricogne, G. Acta crystallogr. A132, 832-847 (1976).
Hogle, J., Chow, M. & Filman, D. Science 229, 1359 (1985).
Jones, T. A. J. appl. Crystallogr. 11, 268-272 (1978).
Hendrickson, W. A. & Konnert, J. H... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | 这一结构揭示了阻遏蛋白如何同时识别DNA的特殊构象并与一排碱基对相接触的,该现象使阻遏蛋白具有了特异性。这样的结构使基因表达的可控开关成为可能,并为了解基因调控机制提供了框架。 英文
噬菌体434阻遏蛋白的DNA结合域与人工合成的434操纵基因DNA所形成的特异复合物的晶体结构显示了决定序列依赖性亲和力的相互作用方式。阻遏蛋白通过和特异DNA构象的互补作用及与DNA大沟中的碱基对直接作用的方式识别其操纵基因。 英文
本文所描述的是噬菌体阻遏蛋白的DNA结合域与人工合成的操纵基因DNA所形成的特异复合物的结构。所有描述都是基于对晶体的X射线晶体学衍射图谱的分析,该晶体的衍射分辨率在某些方向上达到3.2 Å,在其余方向约为4.5 Å... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | 母体晶体收集到了分辨率达3.2 Å的数据,使用的是CEA-25型X射线胶片和振荡摄影术以及埃利奥特GX-6、GX-13型X射线发射机。胶片用Optronics P1000型胶片扫描仪以50μm光栅产生的光进行扫描。积分强度则由程序SCAN12(参考文献34)衍生出的一个胶片扫描程序SCANFILM获得。分辨率达7 Å的数据也是在使用哈佛软件包的Xentronics面探测器原型机上收集的[35]。这些测量提供了因光太强而不能在胶片上进行测量的母体反射强度。分辨率达3.2 Å的衍生物数据由同晶置换的晶体在面探测器上收集,该同晶置换中将14碱基DNA中第七位的胸腺嘧啶改变为5–溴代脱氧核糖[4]。由面探测器和胶片得到的母体数据经过处理和合... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | 碱基对和侧链的密度变得更加清晰,多肽骨架的密度也更加连续。起始模型中某些侧链较差的定位并没有影响其在最终密度图中的正确位置,这说明模型中的错误没有使相位修正后的结果偏移(图2b和c)。在芜菁皱缩病毒结构的测定过程中曾利用中间模型来起始第二轮的相位修正,也生成了对实际结构的类似修正[19]。三重非晶体学冗余度在相位修正中的作用已在先前的工作中得到充分展示——例如,在测定流感病毒血细胞凝集素的结构时,数据的准确度与我们的相近[20]。模型的倒易空间修正正在进行中。本文中模型的R因子为0.44(霍利、安德森和哈里森,未发表数据)。 英文
图1.434阻遏蛋白–操纵基因复合物中各要素的总结性图示,其方向与图b所示的计算机模拟图对应。R1-... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | α碳的位置显示,相比于λ阻遏蛋白,434阻遏蛋白缺失了螺旋α1和α2间的两个残基以及α4起始处的一个残基。两个模型在434的58位残基(对应于λ的75位残基)出现分歧,也正是从这个位置开始两个蛋白的氨基酸序列不再相似。各种疏水侧链组成四螺旋簇的内部结构,该结构还因几个极性键而得以稳定——尤其是Gln 17–Glu 32间和Arg 5–Glu 35间的联接。R1-69的最后六个残基在我们的电子密度图上不是很明确。它们似乎朝着P11附近的DNA骨架延伸。 英文
表3.α螺旋中的氨基酸残基
λ阻遏蛋白N端结构域的Cα骨架[5]叠加在R1-69骨架上,使α2和α3的相合性最佳(参考文献4)。α螺旋残基的分配是根据骨架构象的视图而定的。三维... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | 如果它们参与了接触,这个相互作用可能会帮助两个操纵基因半位点建立最惠的相对取向,因为结合在一个半位点上的R1-69结构域会使其C末端与另一个半位点的DNA骨架相接触。 英文
碱基对的接触。螺旋α3中的三个谷氨酰胺(残基28、29和33)从大沟中伸出(如图5)。Gln 28接近A1(碱基对1),从而使碱基的N7与Gln的Nε、碱基的N6与Gln的Oε之间形成氢键。这种“双配位”氢键类型被西曼等人描述为腺嘌呤特异性的相互作用模式[26]。Gln 29位于螺旋α3。它的Cβ和Cγ与T12′(碱基对3)的5–甲基基团通过范德华作用相接触,其Nε可以与G13′(碱基对2)的O6(也可能是N7)形成氢键。Gln 29的Oε可能与其主链上的NH基... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | 当Arg 43突变为丙氨酸后,伸入小沟中,中间碱基对序列的偏好性依旧存在。在本模型中胍基并没有与碱基形成氢键,Ala 43突变体的结合亲和力实验显示任何与小沟的接触没有明显影响其对6~9序列的辨别力。因此,我们认为是DNA的构象而不是与碱基对自身的接触影响了结合。 英文
位于操纵基因中央的结构是怎样影响亲和性的呢?一种解释是两个半位点之间正确的相对取向需要特定的构象。尽管在多种浓度的溶液中该蛋白均以单体形式存在,然而在与DNA形成的复合物中两个R1-69的结构域有显著的相互作用,包括每个亚基中对DNA骨架的接触非常重要的那些部分互补堆积(特别是残基41~44:见图4)。因此与DNA结合的单体表面的相对取向受到单个R1-69亚基间相互... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | 英文
(侯彦婕 翻译;吴琳 审稿)
Iron Deficiency Limits Phytoplankton Growth in the North-East Pacific Subarctic
John H. Martin and Steve E. Fitzwater
Editor’s Note
Oceanographer John Martin discovered that in several regions of the world’s oceans the main inorganic nutrients of plant plankton are incompletely consumed, because plan... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | WATER, with its resident phytoplankton, was collected from a depth of 20 m at Ocean Station “PAPA” (50.0° N; 145.0° W), using ultra-clean trace-element sampling techniques (30-litre Go-Flo bottles on Kevlar line; samples processed inside a positive pressure portable laboratory)6,7. Aliquots of the raw water were placed... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | Chlorophyll concentrations remained fairly constant for all treatments on days 1–3 (Table 1, Fig. 1). On day 4, however, rapid growth began and substantial chlorophyll increases were observed in all of the bottles with added Fe. Accelerated growth continued into day 5 and appeared to peak in the 10 and 10+ (10 nmol Fe ... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | Similar experiments were also performed 625 km north-west (55.5° N; 147.5° W) and 625 km south-east (45.0° N; 142.89° W) of Ocean Station “PAPA” (50.0° N; 145.0° W). Essentially the same results were found; chlorophyll data are shown in Fig. 2. These data, together with those in Fig. 1, provide consistent evidence that... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | In addition to the necessity for Fe at the cellular level, it is possible that oceanic Fe availability may be important in determining global atmospheric CO2 levels and, hence, affecting global climate. The major role of the Antarctic Ocean in the removal of CO2 from the atmosphere has been noted in several studies13-1... | 400套付费套装书 |
C:\Users\yuan\Documents\400套付费套装书\04-100套\epub\0305. 《自然》百年科学经典(英汉对照版)(第七卷)(1985-1992).epub | .epub | 41,865,664 | 2025-09-26T19:07:39.767346 | are 2.5±0.9 ng g–1during the present interglacial (Holocene) and are not significantly different from those of the previous interglacial (3.0±1.5 ng Al g–1, 127 kyr BP). This would be ~0.03–0.04 nmol of Fe g–1 based upon Taylor’s25 crustal abundance estimates (8.23% Al and 5.63% Fe; Fe:Al ratio=0.68). Levels of CO2, in... | 400套付费套装书 |
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