Clark M A, Baumann L, Thao M L, Moran N A, Baumann P
Microbiology Section, University of California, Davis, California 95616-8665, USA.
J Bacteriol. 2001 Mar;183(6):1853-61. doi: 10.1128/JB.183.6.1853-1861.2001.
Psyllids, like aphids, feed on plant phloem sap and are obligately associated with prokaryotic endosymbionts acquired through vertical transmission from an ancestral infection. We have sequenced 37 kb of DNA of the genome of Carsonella ruddii, the endosymbiont of psyllids, and found that it has a number of unusual properties revealing a more extreme case of degeneration than was previously reported from studies of eubacterial genomes, including that of the aphid endosymbiont Buchnera aphidicola. Among the unusual properties are an exceptionally low guanine-plus-cytosine content (19.9%), almost complete absence of intergenic spaces, operon fusion, and lack of the usual promoter sequences upstream of 16S rDNA. These features suggest the synthesis of long mRNAs and translational coupling. The most extreme instances of base compositional bias occur in the genes encoding proteins that have less highly conserved amino acid sequences; the guanine-plus-cytosine content of some protein-coding sequences is as low as 10%. The shift in base composition has a large effect on proteins: in polypeptides of C. ruddii, half of the residues consist of five amino acids with codons low in guanine plus cytosine. Furthermore, the proteins of C. ruddii are reduced in size, with an average of about 9% fewer amino acids than in homologous proteins of related bacteria. These observations suggest that the C. ruddii genome is not subject to constraints that limit the evolution of other known eubacteria.
木虱与蚜虫一样，以植物韧皮部汁液为食，并且必然与通过垂直传播从祖先感染中获得的原核内共生体相关联。我们对木虱内共生体鲁氏卡索氏菌（Carsonella ruddii）的基因组37 kb的DNA进行了测序，发现它具有许多不同寻常的特性，揭示了一种比之前从真细菌基因组研究（包括蚜虫内共生体蚜虫布赫纳氏菌Buchnera aphidicola的研究）中报道的更为极端的退化情况。这些不同寻常的特性包括鸟嘌呤加胞嘧啶含量异常低（19.9%）、几乎完全没有基因间隔区、操纵子融合以及16S rDNA上游缺乏通常的启动子序列。这些特征表明存在长mRNA的合成以及翻译偶联。碱基组成偏向的最极端情况出现在编码氨基酸序列保守性较低的蛋白质的基因中；一些蛋白质编码序列的鸟嘌呤加胞嘧啶含量低至10%。碱基组成的变化对蛋白质有很大影响：在鲁氏卡索氏菌的多肽中，一半的残基由鸟嘌呤加胞嘧啶含量低的密码子所对应的五种氨基酸组成。此外，鲁氏卡索氏菌的蛋白质在大小上有所减少，与相关细菌的同源蛋白质相比，平均氨基酸数量少约9%。这些观察结果表明，鲁氏卡索氏菌的基因组不受限制其他已知真细菌进化的约束。
