Host-Pathogen O-Methyltransferase Similarity and Its Specific Presence in Highly Virulent Strains of Francisella tularensis Suggests Molecular Mimicry

Whole genome comparative studies of many bacterial pathogens have shown an overall high similarity of gene content (>95%) between phylogenetically distinct subspecies. In highly clonal species that share the bulk of their genomes subtle changes in gene content and small-scale polymorphisms, especially those that may alter gene expression and protein-protein interactions, are more likely to have a significant effect on the pathogen's biology. In order to better understand molecular attributes that may mediate the adaptation of virulence in infectious bacteria, a comparative study was done to further analyze the evolution of a gene encoding an o-methyltransferase that was previously identified as a candidate virulence factor due to its conservation specifically in highly pathogenic Francisella tularensis subsp. tularensis strains. The o-methyltransferase gene is located in the genomic neighborhood of a known pathogenicity island and predicted site of rearrangement. Distinct o-methyltransferase subtypes are present in different Francisella tularensis subspecies. Related protein families were identified in several host species as well as species of pathogenic bacteria that are otherwise very distant phylogenetically from Francisella, including species of Mycobacterium. A conserved sequence motif profile is present in the mammalian host and pathogen protein sequences, and sites of non-synonymous variation conserved in Francisella subspecies specific o-methyltransferases map proximally to the predicted active site of the orthologous human protein structure. Altogether, evidence suggests a role of the F. t. subsp. tularensis protein in a mechanism of molecular mimicry, similar perhaps to Legionella and Coxiella. These findings therefore provide insights into the evolution of niche-restriction and virulence in Francisella, and have broader implications regarding the molecular mechanisms that mediate host-pathogen relationships.

针对多种细菌性致病菌的全基因组比较研究显示，系统发育分化的不同亚种之间，其基因内容总体相似度超过95%。在共享大部分基因组的高度克隆化物种中，基因内容的细微变化与小规模多态性——尤其是那些可能改变基因表达及蛋白质-蛋白质相互作用的变异——往往会对致病菌的生物学特性产生显著影响。为了更好地解析可能介导传染性致病菌毒力适应的分子特征，本研究开展了比较基因组学分析，以进一步探究某一编码O-甲基转移酶（o-methyltransferase）的基因的演化历程。该基因此前因仅在高致病性土拉弗朗西斯菌土拉亚种（Francisella tularensis subsp. tularensis）菌株中保守存在，而被认定为候选毒力因子。该O-甲基转移酶基因位于已知致病岛（pathogenicity island）的基因组邻区内，且处于预测的基因组重排位点附近。不同土拉弗朗西斯菌亚种携带各异的O-甲基转移酶亚型。在多种宿主物种以及系统发育上与弗朗西斯菌亲缘关系极远的其他致病菌属（包括分枝杆菌属Mycobacterium）中，均鉴定到了与之同源的蛋白质家族。在哺乳动物宿主与致病菌的蛋白质序列中，均存在保守的序列基序特征；且土拉弗朗西斯菌亚种特异性O-甲基转移酶中保守的非同义变异位点，在空间上紧邻其同源人类蛋白质结构的预测活性位点。综合来看，现有证据表明土拉弗朗西斯菌土拉亚种的该蛋白参与分子模拟（molecular mimicry）机制，其功能或与军团菌属（Legionella）、考克斯体属（Coxiella）的相关蛋白类似。综上，本研究结果为解析弗朗西斯菌的生态位限制演化与毒力演化提供了新视角，同时对介导宿主-致病菌互作的分子机制研究具有更广泛的借鉴意义。