Gophna Uri, Ron Eliora Z, Graur Dan
Department of Molecular Microbiology and Biotechnology, George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv, Israel 69978.
Gene. 2003 Jul 17;312:151-63. doi: 10.1016/s0378-1119(03)00612-7.
Type III secretion systems (TTSS) are unique bacterial mechanisms that mediate elaborate interactions with their hosts. The fact that several of the TTSS proteins are closely related to flagellar export proteins has led to the suggestion that TTSS had evolved from flagella. Here we reconstruct the evolutionary history of four conserved type III secretion proteins and their phylogenetic relationships with flagellar paralogs. Our analysis indicates that the TTSS and the flagellar export mechanism share a common ancestor, but have evolved independently from one another. The suggestion that TTSS genes have evolved from genes encoding flagellar proteins is effectively refuted. A comparison of the species tree, as deduced from 16S rDNA sequences, to the protein phylogenetic trees has led to the identification of several major lateral transfer events involving clusters of TTSS genes. It is hypothesized that horizontal gene transfer has occurred much earlier and more frequently than previously inferred for TTSS genes and is, consequently, a major force shaping the evolution of species that harbor type III secretion systems.
III型分泌系统（TTSS）是细菌介导与宿主进行复杂相互作用的独特机制。TTSS的几种蛋白质与鞭毛输出蛋白密切相关，这一事实使人认为TTSS是从鞭毛进化而来的。在此，我们重建了四种保守的III型分泌蛋白的进化史及其与鞭毛旁系同源物的系统发育关系。我们的分析表明，TTSS和鞭毛输出机制有一个共同的祖先，但彼此独立进化。TTSS基因是从编码鞭毛蛋白的基因进化而来的这一观点被有效反驳。将根据16S rDNA序列推导的物种树与蛋白质系统发育树进行比较，已识别出涉及TTSS基因簇的几个主要横向转移事件。据推测，水平基因转移发生的时间比之前推断的TTSS基因要早得多，频率也更高，因此是塑造拥有III型分泌系统物种进化的主要力量。
