def clean_split_sequence(seq):
    seq = seq.upper()
    seq = seq.replace('U', 'T')

    for base in seq:
        if base not in {'A', 'T', 'G', 'C'}:
            raise ValueError(f"Invalid character '{base}' found in sequence. Only A, T, G, C, and U are allowed.")

    spaced_seq = " ".join([seq[i:i+3] for i in range(0, len(seq), 3)])

    return spaced_seq