In every kingdom of life, GC->AT transitions occur more frequently than any other type of mutation due to the spontaneous deamination of cytidine. In eukaryotic genomes, this slow loss of GC base pairs is counteracted by biased gene conversion which increases genomic GC content as part of the recombination process. However, this type of biased gene conversion has not been observed in bacterial genomes so we hypothesized that GC->AT transitions cause a reduction of genomic GC content in prokaryotic genomes on an evolutionary time scale. To test this hypothesis, we used a phylogenetic approach to analyze triplets of closely related genomes representing a wide range of the bacterial kingdom. The resulting data indicate that genomic GC content is slowly declining in bacterial genomes where GC base pairs comprise 40% or more of the total genome. In contrast, genomes containing less than 40% GC base pairs have fewer opportunities for GC->AT transitions to occur so genomic GC content is relatively stable or actually increasing at a slow rate. It should be noted that this observed change in genomic GC content is the net change in shared parts of the genome and does not apply to parts of the genome that have been lost or acquired since the genomes being compared shared common ancestor. However, a more detailed analysis of two Caulobacter genomes revealed that the acquisition of mobile elements by the two genomes actually reduced the total genome content as well.

中文翻译：

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由于突变率偏高，细菌基因组GC含量缓慢向下漂移

在每个生命王国中，GC-> AT转换是在进化时期内累积的主要突变类型。在真核生物基因组中，这种缓慢的GC碱基对丢失可通过有偏向的基因转化来抵消，这种转化会增加基因组GC的含量，这是重组过程的一部分。然而，在细菌基因组中尚未观察到这种类型的有偏向的基因转换，因此我们假设GC-> AT转换导致原核基因组中基因组GC含量的降低。为了检验该假设，我们使用了系统发育方法来分析代表广泛细菌王国的紧密相关基因组的三胞胎。所得数据表明，细菌基因组中的基因组GC含量正在缓慢下降，其中GC碱基对占总基因组的40％或更多。相反，