Effects of Genic Base Composition on Growth Rate in G+C-rich Genomes

摘要

The source and significance of the wide variation in the genomic base composition of bacteria have been a matter of continued debate. Although the variation was originally attributed to a strictly neutral process, i.e. , species-specific differences in mutational patterns, recent genomic comparisons have shown that bacteria with G+C-rich genomes experience a mutational bias toward A+T. This difference between the mutational input to a genome and its overall base composition suggests the action of natural selection. Here, we examine if selection acts on G+C contents in Caulobacter crescentus and Pseudomonas aeruginosa , which both have very G+C-rich genomes, by testing whether the expression of gene variants that differ only in their base compositions at synonymous sites affects cellular growth rates. In C. crescentus , expression of the more A+T-rich gene variants decelerated growth, indicating that selection on genic base composition is, in part, responsible for the high G+C content of this genome. In contrast, no comparable effect was observed in P. aeruginosa , which has similarly high genome G+C contents. Selection for increased genic G+C-contents in C. crescentus acts independently of the species-specific codon usage pattern and represents an additional selective force operating in bacterial genomes.