Evolution of optimal codon choices in the family Enterobacteriaceae.

The Enterobacteriaceae are a large family of Proteobacteria that include many well-known prokaryotic genera, such as Escherichia, Yersinia and Salmonella. The main ideas of synonymous codon usage (CU) evolution and translational selection have been deeply influenced by studies with these bacterial groups. In this work we report the analysis of the CU pattern of completely sequenced bacterial genomes that belong to the Enterobacteriaceae. The effect of selection in translation acting at the levels of speed and accuracy, and phylogenetic trends within this group are described. Preferred (optimal) codons were identified. The evolutionary dynamics of these codons were studied and following a Bayesian approach these preferences were traced back to the common ancestor of the family. We found that there is some level of variation in selection among the analysed micro-organisms that is probably associated with lineage-specific trends. The codon bias was largely conserved across the evolutionary time of the family in highly expressed genes and protein conserved regions, suggesting a major role of negative selection. In this sense, the results support the idea that the extant CU bias is finely tuned over the ancestral well-conserved pool of tRNAs.

Trends in amino acid usage across the class Mollicutes.

Many studies have explored the mechanisms involved in relative amino acid usage (RAAU) in a variety of prokaryotes and eukaryotes. A strong bias was observed in highly expressed genes (HEGs) of endosymbiotic bacteria. By means of correspondence analysis, we studied the major trends affecting internal variability of RAAU in Mollicutes. The principal trend is related to the usage of smaller, less aromatic, and GC-rich coded amino acids in HEGs. Given the nature of the genetic code, these properties are linked among them. Expectedly, we found a slow evolutionary rate of HEGs, which is likely driven by purifying selection. On the other hand, the rest of the genes accumulate rapid changes as a result of the extreme mutational bias toward A + T of the genomes and genetic drift, increasing internal variability. Amino acid changes across the phylogeny of the group were traced in order to estimate the mean molecular weight and aromaticity trends in each branch. Finally, we compared amino acid usage bias within and between Mollicutes and the free-living Firmicutes.

Selected codon usage bias in members of the class Mollicutes.

Mollicutes are parasitic microorganisms mainly characterized by small cell sizes, reduced genomes and great A and T mutational bias. We analyzed the codon usage patterns of the completely sequenced genomes of bacteria that belong to this class. We found that for many organisms not only mutational bias but also selection has a major effect on codon usage. Through a comparative perspective and based on three widely used criteria we were able to classify Mollicutes according to the effect of selection on codon usage. We found conserved optimal codons in many species and study the tRNA gene pool in each genome. Previous results are reinforced by the fact that, when selection is operative, the putative optimal codons found match the respective cognate tRNA. Finally, we trace selection effect backwards to the common ancestor of the class and estimate the phylogenetic inertia associated with this character. We discuss the possible scenarios that explain the observed evolutionary patterns.