Department of Ecology and Evolutionary Biology, Yale University, New Haven, CT 06520, USA.
Proc Natl Acad Sci U S A. 2012 Sep 4;109(36):14504-7. doi: 10.1073/pnas.1205683109. Epub 2012 Aug 20.
Bacteria display considerable variation in their overall base compositions, which range from 13% to over 75% G+C. This variation in genomic base compositions has long been considered to be a strictly neutral character, due solely to differences in the mutational process; however, recent sequence comparisons indicate that mutational input alone cannot produce the observed base compositions, implying a role for natural selection. Because bacterial genomes have high gene content, forces that operate on the base composition of individual genes could help shape the overall genomic base composition. To explore this possibility, we tested whether genes that encode the same protein but vary only in their base compositions at synonymous sites have effects on bacterial fitness. Escherichia coli strains harboring G+C-rich versions of genes display higher growth rates, indicating that despite a pervasive mutational bias toward A+T, a selective force, independent of adaptive codon use, is driving genes toward higher G+C contents.
细菌的总碱基组成差异很大，范围从 13%到超过 75%的 G+C。这种基因组碱基组成的变化长期以来被认为是一种严格的中性特征，仅仅是由于突变过程的差异；然而，最近的序列比较表明，仅突变输入不能产生观察到的碱基组成，这意味着自然选择的作用。由于细菌基因组具有高基因含量，作用于个别基因碱基组成的力量可能有助于塑造整体基因组碱基组成。为了探索这种可能性，我们测试了编码相同蛋白质但仅在同义位点的碱基组成上有所不同的基因是否会影响细菌的适应性。携带富含 G+C 的基因的大肠杆菌菌株显示出更高的生长速度，这表明尽管存在普遍的偏向 A+T 的突变，但一种独立于适应性密码子使用的选择压力正在促使基因朝着更高的 G+C 含量发展。
