[Inverse correlation between GC-content of bacterial genomes and the level of preterminal codons usage in them]

The purpose of this work was to find the character of the dependence between the GC-content of the gene and the level of preterminal codons usage inside it. 84 codon usage tables were used as the material (each of them contains average frequencies of codon usage for all encoding districts belonging to one bacterial specie). We also used nucleotide sequences encoding for active centers of bacterial adenylate cyclases. Inverse correlation between the GC-content (G+C) and the total level of preterminal codons usage (PCU) was observed (R = - 0.97). For nucleotide sequences encoding for active centers from adenylate cyclases class I the coefficient of correlation between G+C and PCU is -0.75. For sequences encoding for active centers from adenylate cyclases class III the coefficient of correlation between G+C and PCU is -0.91. Preterminal codons are mostly GC-deficient relative to their synonymous nonpreterminal codons and in absolute terms. The cause of the inverse correlation between G+C and the level of preterminal codons usage is an increase in their frequencies of usage due to mutational AT-pressure and the decrease of their frequencies of usage due to mutational GC-pressure. The evidence of the frequent fixation of nonsense mutations in GC-deficient bacteria was found. The practical conclusion is the following: the higher is the GC-content of gene or genome, the lower is the probability of nonsense mutation inside it.