Stanisław Cebrat (Department of Genomics, University of Wrocław) Optimisation of asymmetric mutational pressure and selection pressure around the universal genetic code. Lecture 1 (see Lecture 2, Lecture 3)

Abstract: One of the hypotheses explaining the origin of the genetic code assumes that its evolution has minimised the deleterious effects of nucleotide substitutions on the amino acid composition of coded proteins. To estimate the level of such an optimization of the genetic code it is necessary to consider the real mutational and selection pressures operating at the level of genomes and proteomes. Even for the simplest cases of small bacterial genomes the mutational and selection pressures are different for genes located on the differently replicating DNA strands: the leading and lagging ones. Assuming the probability of amino acid substitutions in proteins as a measure of a code.s susceptibility to errors, it is possible to calculated the optimal codes for genes located on the leading and lagging strands separately. The optimal code for genes located on one DNA strand was simultaneously worse than the universal code for the genes located on the other strand. Furthermore, only 23 of 20 million random codes were better than universal one for genes located on both strands simultaneously while about two orders of magnitude more codes were better for each of the two strands separately. The result indicates that the existing universal code, the mutational pressure, the codon and amino acid compositions are highly optimised for the both differently replicating DNA strands. B. Lewin, Genes VIII, Oxford University Press