Selection, mutations and codon usage in a bacterial model

We present a statistical model of bacterial evolution based on the coupling between codon usage and tRNA abundance. Such a model interprets this aspect of the evolutionary process as a balance between the codon homogenization effect due to mutation process and the improvement of the translation phase due to natural selection. We develop a thermodynamical description of the asymptotic state of the model. The analysis of naturally occurring sequences shows that the effect of natural selection on codon bias affects genes whose products are largely required at maximal growth rate conditions or undergo rapid transient increases.