Toft Christina, Fares Mario A
Department of Genetics, Smurfit Institute of Genetics, University of Dublin, Trinity College, Dublin, Ireland.
Mol Biol Evol. 2008 Sep;25(9):2069-76. doi: 10.1093/molbev/msn153. Epub 2008 Jul 17.
Genome shrinkage is a common feature of most intracellular pathogens and symbionts. Reduction of genome sizes is among the best-characterized evolutionary ways of intracellular organisms to save and avoid maintaining expensive redundant biological processes. Endosymbiotic bacteria of insects are examples of biological economy taken to completion because their genomes are dramatically reduced. These bacteria are nonmotile, and their biochemical processes are intimately related to those of their host. Because of this relationship, many of the processes in these bacteria have been either lost or have suffered massive remodeling to adapt to the intracellular symbiotic lifestyle. An example of such changes is the flagellum structure that is essential for bacterial motility and infectivity. Our analysis indicates that genes responsible for flagellar assembly have been partially or totally lost in most intracellular symbionts of gamma-Proteobacteria. Comparative genomic analyses show that flagellar genes have been differentially lost in endosymbiotic bacteria of insects. Only proteins involved in protein export within the flagella assembly pathway (type III secretion system and the basal body) have been kept in most of the endosymbionts, whereas those involved in building the filament and hook of flagella have only in few instances been kept, indicating a change in the functional purpose of this pathway. In some endosymbionts, genes controlling protein-export switch and hook length have undergone functional divergence as shown through an analysis of their evolutionary dynamics. Based on our results, we suggest that genes of flagellum have diverged functionally as to specialize in the export of proteins from the bacterium to the host.
基因组缩减是大多数细胞内病原体和共生体的一个共同特征。基因组大小的减小是细胞内生物为节省并避免维持昂贵的冗余生物过程而采用的最具特征的进化方式之一。昆虫的内共生细菌就是生物经济性达到极致的例子，因为它们的基因组大幅缩减。这些细菌不具有运动能力，其生化过程与宿主密切相关。由于这种关系，这些细菌中的许多过程要么已经丧失，要么经历了大规模重塑以适应细胞内共生生活方式。这种变化的一个例子是对细菌运动性和感染性至关重要的鞭毛结构。我们的分析表明，在大多数γ-变形菌纲的细胞内共生体中，负责鞭毛组装的基因已部分或完全丧失。比较基因组分析表明，鞭毛基因在昆虫内共生细菌中存在差异丧失。在大多数内共生体中，仅保留了鞭毛组装途径（III型分泌系统和基体）中参与蛋白质输出的蛋白质，而参与构建鞭毛丝和钩的蛋白质仅在少数情况下被保留，这表明该途径的功能目的发生了变化。通过对其进化动态的分析表明，在一些内共生体中，控制蛋白质输出开关和钩长度的基因已经发生了功能分化。基于我们的结果，我们认为鞭毛基因在功能上已经发生分化，专门用于从细菌向宿主输出蛋白质。
