High allelic diversity in the methyltransferase gene of a phase variable type III restriction-modification system has implications for the fitness of Haemophilus influenzae
High allelic diversity in the methyltransferase gene of a phase variable type III restriction-modification system has implications for the fitness of Haemophilus influenzae
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DOI：
10.1093/nar/gkl568
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发表时间：
2006-01-01
影响因子：
14.9
通讯作者：
Moxon, E. Richard
中科院分区：
文献类型：
--
作者：
Bayliss, Christopher D.;Callaghan, Martin J.;Moxon, E. Richard
相变限制修饰（R-M）系统在真细菌中广泛存在。流感嗜血杆菌编码III型R-M系统的相变量同源物。对22株不可分型流感嗜血杆菌分离株的序列分析显示，mod基因的中心部分有一个高变区，而res基因则是保守的。最大似然（ML）分析表明，在这个高变区之外的大多数位点经历了强烈的负选择，但在邻近区域的少数位点有正选择的证据。对61个III型模型基因的系统发育分析显示，这些流感嗜血杆菌模型等位基因与来自致病性奈瑟球菌的模型基因聚类，并且基于序列分析，模型复合体在这些物种之间存在水平转移。Neisserial mode等位基因也包含一个高变区，所有mode等位基因在重复通道中都表现出变异性。我们认为这个高变区编码了Mod蛋白的靶识别域（TRD），这种变异导致了R-M系统识别序列的改变。我们认为，这种R-M系统的高等位基因多样性和相位可变特性是由于噬菌体种群多样性施加的选择压力而产生的，但也对这些细菌物种的其他适应性属性有影响。
Phase variable restriction-modification (R-M) systems are widespread in Eubacteria. Haemophilus influenzae encodes a phase variable homolog of Type III R-M systems. Sequence analysis of this system in 22 non-typeable H.influenzae isolates revealed a hypervariable region in the central portion of the mod gene whereas the res gene was conserved. Maximum likelihood (ML) analysis indicated that most sites outside this hypervariable region experienced strong negative selection but evidence of positive selection for a few sites in adjacent regions. A phylogenetic analysis of 61 Type III mod genes revealed clustering of these H.influenzae mod alleles with mod genes from pathogenic Neisseriae and, based on sequence analysis, horizontal transfer of the mod-res complex between these species. Neisserial mod alleles also contained a hypervariable region and all mod alleles exhibited variability in the repeat tract. We propose that this hypervariable region encodes the target recognition domain (TRD) of the Mod protein and that variability results in alterations to the recognition sequence of this R-M system. We argue that the high allelic diversity and phase variable nature of this R-M system have arisen due to selective pressures exerted by diversity in bacteriophage populations but also have implications for other fitness attributes of these bacterial species.