Immortal Coding Sequences (Ohno, 1984; Ohno and Epplen, 1983)

A few genes is all that retrovirusess possess in their genome; inherent replication error rate of reverse transcriptase is in the order of 10 high-3/base pair/year (Gojobori and Yokoyama, 1985). For bacteria it is about 10 high-4/base pair/year. In contrast, the number of gene loci in the mammalian genome approaches 10 high 5; inherent replication error rate amounts to about 10 high-9 base pair/year (Ohno, 1985). Thus, when it comes to DNA replication, mammalia utilize a rather refined copying and editing machinery, to keep all genes functional, whereas the genome of retroviruses can afford to be enterprising. Non-enzymatic nucleic acid replication can be expected to have a replication error rate not less than the error rate of reverse transcriptase. This implies that an assigned polypeptide chain experiences a lOO percent amino acid sequence change every 1,000 years, a rather cumbersome situation for life to emerge.

Fig. 11.32. Transcription of the Beatles' "Yellow Submarine" (John Lennon and Paul McCartney, 1966) into its base sequence. Repeats of base oligomers are evident. Striking is the abundance of thymine which occurs almost six times more frequently than adenine. It is extraordinary that no initiator nor terminator codon is employed, at least in the first critical six lines. The expression of this sequence, that is its polypeptide chain, bears interesting features. The amino acid polymer has a hydroxyl (serine) beginning and serine reappears at the start of the next four lines too. High aromaticity (phenylalanine) throughout the score is striking. Sulfur-crosslinkage via cys teine (cys) nicely connects the general theme. The rest are fillers except for a few basic amino acids which appear to give the beat. In summary, serine may stand for silk, phenylalanine for skin, and cysteine for hair, all three of which are a Beatles' trade-mark. However, the most refined phenomenon is the presence of a single tryptophane -almost at the end. This amino acid releases upon digestion indols which are known to put you nicely to sleep. This may suggest, that the air in the "Yellow Submarine" was eventually either low in oxygen, or high in carbon monoxide or both

Premature chain shifts are the most coding sequences may sustain. They may come about by base substitution that changes an amino acid specifying codon to a chain terminator. Furthermore, deletions and insertions of bases that are not multiples of three in numbers displace the reading frame. Resulting unusual reading frames tend to be full of chain terminators and a shortening of chain lengths results, which may deprive the polypeptide of its assigned function. However, should the number of bases in a hypothetical abiotic oligomer not be a multiple of three, coding sequences that arise by means of oligomeric repeats are quasi "immune" to the above described base change dilemma as shown for a heptameric repeat: The 21-base long sequence (3 x 7) becomes the unit periodicity and encodes the heptapeptidic periodical polypeptide chain. Abase change, for instance from a cystein codon TGC to a terminator TGA, can silence only one of the open reading frames, and readingframe shifts are of no consequence because the periodicity of polypeptide chains is maintained. Hence, a good measure of immortality is ingrained in coding sequences that are repeats of N = 3 n+- 1 or 2 base oligomers. Such types of oligomeric repeats have the further advantage of giving longer periodicities to the polypeptide chains they encode. In evolution, new genes come into existence from redundant copies of pre-existing genes. For instance, the adaptive immune system of vertebrates has apparently evolved by plagiarizing one ancestral gene (Mostow et al., 1984). De novo recruitments of truly new coding sequences from the non-coding base sequences have shown themselves to be near-exact repeats, thus encoding polypeptide chains of the exact periodicity.