Hypothetical clues suggesting that 'AU' and 'UA' were the first 'beginning' and 'end' codons: is their ancient polymerase extant present as a protein domain fossil in reverse transcriptase and telomerase?
Tryptophan and cysteine are excluded from the histones, except histone H3, presumably to decrease premature terminations, since the first two letters of their codons are the same as that of the termination codon UGA. Though tyrosine codon also begins with 'UG', any mutation in tyrosine codon causing premature termination is corrected more professionally by the suppressor tRNA system. Given this background, a hypothetical model for the first replication is proposed. The first termination codon is 'UA' and the first initiation codon is 'AU', in 'UAU' centred RNA rings, which are replicated both clockwise and counter-clockwise. Replication of these would result in the formation of palindromic strands, which may have led to the formation of palindromic sequences in the genome, such as those forming hairpin loops. Their polymerase may be extant in reverse transcriptase (RT), because of tendencies to 'turn the corner' and to form 'hairpin loops', when RT is used for DNA synthesis in vitro. The catalytic subunit of telomerase hTERT in close contact to its RNA template may be another site to find this ancient 'polymerase'.
Altinoz
Meric A
MA
Department of Neuroimmunology, McGill Neurological Institute and Hospital, Que., Canada. maltinoz@hotmail.com
D003062:Codon; D012321:DNA-Directed RNA Polymerases / Q000378:metabolism*; D005580:Fossils; D006657:Histones / Q000737:chemistry; D012343:RNA, Transfer / Q000235:genetics; D012194:RNA-Directed DNA Polymerase / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D019098:Telomerase / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D014443:Tyrosine / Q000737:chemistry
D003062:Codon; D006657:Histones; D014443:Tyrosine; D012343:RNA, Transfer; D012194:RNA-Directed DNA Polymerase; D019098:Telomerase; D012321:DNA-Directed RNA Polymerases