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 early in its evolution, dominated by codons for alanine and their point mutation derivatives. A similar pattern is characteristic of the master (consensus) tRNA sequence derived in 1981 by Eigen and Winkler-Oswatitsch. The master tRNA sequence is thought to represent one of the earliest mRNA. From analysis of literature and from our own calculations presented in this work, the (GCU)n pattern appears to be the most expandable in the norm and in disease. The speculation is put forward that (GCU)n and polyalanine have been key players at the beginning of the triplet code, and the first codons, apart from the GCU triplet, were point change derivatives of the generic triplet GCU, coding for amino acids present in the early prebiotic-biotic environment. The set of the earliest amino acids is derived on the basis of structural simplicity, presence in imitated prebiotic conditions and involvement with class II aminoacyl-tRNA synthetases. The set consists of six amino acids: 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 ACU (Thr) can be viewed as an early triplet code.