SEQUENCE FOSSILS, TRIPLET EXPANSION, AND RECONSTRUCTION OF EARLIEST CODONS

mRNA sequences are known to carry a hidden periodical pattern (GCU)(n) , which may be considered a remnant of sequence organization of mRNA e arly in its evolution, dominated by codons for alanine and their point mutation derivatives. A similar pattern is characteristic of the mast er (consensus) tRNA sequence derived in 1981 by Eigen and Winkler-Oswa titsch. The master IRNA sequence is thought to represent one of the ea rliest mRNA. From analysis of literature and from our own calculations presented in this work, the (GCU)(n) pattern appears to be the most e xpandable in the norm and in disease. The speculation is put forward t hat (GCU)(n) and polyalanine have been key players at the beginning of the triplet code, and the first codons, apart from the GCU triplet, w ere point change derivatives of the generic triplet GCU, coding for am ino acids present in the early prebiotic-biotic environment. The set o f the earliest amino acids is derived on the basis of structural simpl icity, presence in imitated prebiotic conditions and involvement with class II aminoacyl-tRNA synthetases. The set consists of six amino aci ds: Ala, Asp, Gly, Pro, Ser and Thr. All these amino acids are, indeed , encoded by the GCU triplet and its derivatives, as predicted. Thus, the pairs GCN (Ala), GAU (Asp), GGU (Gly), CCU (Pro), UCU (Ser) and AC U (Thr) can be viewed as an early triplet code. (C) 1997 Elsevier Scie nce B.V.