Ongoing evolution of strand composition in bacterial genomes.

We tried to identify the substitutions involved in the establishment of replication strand bias, which has been recognized as an important evolutionary factor in the evolution of bacterial genomes. First, we analyzed the composition asymmetry of 28 complete bacterial genomes and used it to test the possibility that asymmetric deamination of cytosine might be at the origin of the bias. The model showed significant correlation to the data but left unexplained a significant portion of the variance and indicated a systematic underestimation of GC skews in comparison with TA skews. Second, we analyzed the substitutions acting on the genes from five fully sequenced Chlamydia genomes that had not suffered strand switch since speciation. This analysis showed that substitutions were not at equilibrium in Chlamydia trachomatis or in C. muridarum and that strand bias is still an on-going process in these genes. Third, we identified substitutions involved in the adaptation of genes that had switched strands after speciation. These genes adapted quickly to the skewed composition of the new strand, mostly due to C-->T, A-->G, and C-->G asymmetric substitutions. This observation was reinforced by the analysis of genes that switched strands after divergence between Bacillus subtilis and B. halodurans. Finally, we propose a more extended model based on the analysis of the substitution asymmetries of CHLAMYDIA: This model fits well with the data provided by bacterial genomes presenting strong strand bias.