Nucleotide tetramers TCGA and CTAG: viral DNA and the genetic code (hypothesis)

Abstract

Introduction. The published and our own data show that CTAG and, to a lesser extent, TCGA tetra-nucleotides have significantly lower concentrations in frequency profiles (FPs) of herpesvirus DNAs compared to other complete, bilaterally symmetrical tetra-nucleotides.The aim of the study is to present a comparative analysis of CTAG and TCGA tetra-nucleotide FPs in viral DNAs.Materials and methods. We have analyzed FPs and other characteristics of the two above tetramers in DNAs of at least one species of viruses of each genus (or each subfamily, if the classification into genera was not available), complying with the size limit requirements (minimum 100,000 base pairs) — a total of more than 200 species of viruses. The analysis was performed using the GenBank database.Results. Two groups of characteristics of TCGA and CTAG tetramers have been described. One of them covers the results of the FP analysis for these tetranucleotides in viral DNAs and shows that DNAs with GC:AT 2 are characterized by nCGn FP symmetries while these symmetries are frequently distorted in nTAn FP due to CTAG underrepresentation. The other group of tetramer characteristics demonstrates differences in their FPs in complete viral DNAs and in their genomes (a coding part, which can reach 80% in some studied viruses, thus making the analysis of their DNAs more significant than the analysis of DNAs of cellular live forms) and suggests that these tetramers may have participated in the origin of the universal genetic code.Discussion. Assumedly, the genetic code started evolving amid C+G prevailing in "pre-code" DNA polymers; then the initial code forms evolved further to their final structure where TCGA and CTAG tetramers hold a central position, encapsulating the previous stages of this evolution. The nCGn FP symmetries typical of the "complete" DNA of Herpes simplex viruses disappear in the sequence of the second codon letters of the genome of these viruses, implying that their functions differ from functions of other letters and emphasizing the reasonableness of presenting the genetic code as a calligram where the second line is not symmetrical.