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sci-modeling-bench
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design-bench

An upstream start codon lies in a 5′ leader before the main coding-sequence start. It becomes an upstream open reading frame only when initiation there defines a translated frame extending to a termination codon. Upstream translation can reduce, redirect, or conditionally regulate main-protein synthesis through leaky scanning, ribosome dissociation, reinitiation, overlap, stalling, and peptide-dependent mechanisms.

Scope

Covered

Distinction among uAUGs, upstream initiation sites, and uORFs.
Reading frames, termination, overlap, leaky scanning, and reinitiation.
Common mechanisms and context dependence of uORF regulation.

Not covered

An annotation convention for one dataset.
A universal claim that every upstream AUG represses translation.
A computational method for identifying useful sequence candidates.

Key concepts and notation

Term	Definition
uAUG	AUG located upstream of the main start codon
uTIS	Upstream translation initiation site
uORF	Translated upstream frame from a start site to a stop codon
Main ORF	Open reading frame encoding the principal annotated protein
Leaky scanning	Continued scanning past a potential initiation site
Reinitiation	New initiation after a ribosome translated and terminated an upstream ORF
Overlapping uORF	uORF whose translated interval overlaps the main ORF

Core knowledge

A start codon is not by itself an open reading frame

An AUG triplet upstream of the main start is a potential initiation site. Whether it produces an uORF depends on recognition by scanning complexes and the downstream frame. Each start establishes one of three reading frames; the first in-frame stop codon delimits the corresponding ORF. Near-cognate codons can also serve as upstream starts in some contexts [1,2].

Competing fates of scanning ribosomes

Some scanning complexes initiate at an upstream site, while others bypass it. The fraction following each path depends on start-codon identity, neighboring sequence, RNA structure, and initiation-factor state. Ribosomes that translate an uORF may dissociate at its stop codon or retain/reacquire factors and reinitiate downstream [2,3].

Reinitiation commonly depends on uORF length, the intercistronic distance between the uORF stop and downstream start, and the time available to regain an initiation-competent state. An uORF overlapping the main ORF can prevent ordinary downstream reinitiation because the main start has already been passed in another frame [1–3].

Regulatory outcomes are diverse

Upstream translation often lowers initiation at a downstream main ORF by diverting scanning complexes. It can also create conditional regulation. Changes in initiation-factor availability, metabolites, stress, or ribosome behavior can alter bypass and reinitiation. Some uORF-encoded peptides cause sequence-dependent ribosome stalling, while other uORFs act without a conserved peptide [1,3].

Multiple upstream starts can interact. Recognition of one site changes the population of ribosomes that reaches later sites, so their effects need not be independent or additive.

Position and frame are mechanistically relevant

The same AUG sequence can have different consequences when moved because start-to-cap distance, surrounding structure, reading frame, stop position, and distance to the main start change. “Upstream AUG present” is therefore a coarse property that does not fully specify the translational mechanism.

Conditions, limitations, and uncertainty

Sequence annotation identifies potential ORFs, not necessarily translated ORFs; experimental evidence can come from ribosome profiling, proteomics, or reporter perturbation.
Near-cognate initiation and reinitiation efficiencies vary by organism and cell state.
A translated uORF can affect RNA stability through pathways such as nonsense-mediated decay as well as affect translation.
The absence of an AUG-initiated uORF does not exclude other 5′-leader regulation.
The magnitude and even direction of an uORF effect cannot be assigned from its presence alone.

Related knowledge resources

kozak_context_and_start_codon_recognition: recognition probability of upstream and main starts.
five_prime_utr_regulatory_elements: other interacting leader elements.

References

Wethmar K. The regulatory potential of upstream open reading frames in eukaryotic gene expression. Wiley Interdisciplinary Reviews: RNA. 2014;5:765–778. https://doi.org/10.1002/wrna.1245
Hinnebusch AG, Ivanov IP, Sonenberg N. Translational control by 5′-untranslated regions of eukaryotic mRNAs. Science. 2016;352:1413–1416. https://doi.org/10.1126/science.aad9868
Young SK, Wek RC. Upstream open reading frames differentially regulate gene-specific translation in the integrated stress response. Journal of Biological Chemistry. 2016;291:16927–16935. https://doi.org/10.1074/jbc.R116.733899