Evolutionary forces in shaping the codon and amino acid usages in Blochmannia floridanus

Endosymbiotic relationship has great effect on ecological system. Codon and amino acid usages bias of endosymbiotic bacteria Blochmannia floridanus (whose host is an ant Camponotus floridanus) was investigated using experimentally known genes of this organism. Correspondence Analysis on RSCU values show that there exists only one single explanatory major axis that is linked to the strand specific mutational biases. Majority of the genes have a tendency to concentrate on the leading strand, which may be related to the adaptive property related to the replication mechanisms. Amino acid usages were markedly different between the highly and lowly expressed genes in this organism and in particular, GC rich amino acids were found to occur significantly higher in highly expressed genes than the lowly expressed genes. Comparative analyses of the orthologous genes of Escherichia coli and Blochmannia floridanus show that highly expressed genes are significantly more conserved than lowly expressed genes. Based on our results we concluded that strand specific mutational bias is strongly operational in selecting the codon usage in this organism. Replicational-transcriptional selection can be invoked from the presence of majority of highly expressed genes in the leading strand. Conservation of GC rich amino acids in the highly expressed genes to its ancestor is the major source of variation in amino acid usages in the organism. Hydrophobicity of the genes is the second major source in differentiating the genes according to their amino acid usages in this organism.