Orsi R H, Ripoll D R, Yeung M, Nightingale K K, Wiedmann M
Department of Food Science, Cornell University, Ithaca, NY, USA.
Computational Biology Service Unit, Cornell Theory Center, Cornell University, Ithaca, NY, USA.
Microbiology (Reading). 2007 Aug;153(Pt 8):2666-2678. doi: 10.1099/mic.0.2007/007310-0.
The surface molecule InlA interacts with E-cadherin to promote invasion of Listeria monocytogenes into selected host cells. DNA sequencing of inlA for 40 L. monocytogenes isolates revealed 107 synonymous and 45 nonsynonymous substitutions. A frameshift mutation in a homopolymeric tract encoding part of the InlA signal peptide was identified in three lineage II isolates, which also showed reduced ability to invade human intestinal epithelial cells. Phylogenies showed clear separation of inlA sequences into lineages I and II. Thirteen inlA recombination events, predominantly involving lineage II strains as recipients (12 events), were detected and a number of amino acid residues were shown to be under positive selection. Four of the 45 non-synonymous changes were found to be under positive selection with posterior probabilities >95 %. Mapping of polymorphic and positively selected amino acid sites on the partial crystal structure for InlA showed that the internalin surface of the leucine-rich repeat (LRR) region that faces the InlA receptor E-cadherin does not include any polymorphic sites; all polymorphic and positively selected amino acids mapped to the outer face of the LRR region or to other InlA regions. The data show that (i) inlA is highly polymorphic and evolution of inlA involved a considerable number of recombination events in lineage II isolates; (ii) positive selection at specific amino acid sites appears to contribute to evolution of inlA, including fixation of recombinant events; and (iii) single-nucleotide deletions in a lineage II-specific 3' homopolymeric tract in inlA lead to complete loss of InlA or to production of truncated InlA, which conveys reduced invasiveness. In conclusion, inlA has a complex evolutionary history, which is consistent with L. monocytogenes' natural history as an environmental pathogen with broad host-range, including its adaptation to environments and hosts where different inlA alleles may provide a selective advantage or where inlA may not be required.
表面分子InlA与E-钙黏蛋白相互作用，促进单核细胞增生李斯特菌侵入特定宿主细胞。对40株单核细胞增生李斯特菌分离株的inlA进行DNA测序，发现107个同义替换和45个非同义替换。在3株II型谱系分离株中，鉴定出编码InlA信号肽部分的同聚物区域存在移码突变，这些分离株侵入人肠道上皮细胞的能力也有所降低。系统发育分析表明，inlA序列可清晰分为I型和II型谱系。检测到13次inlA重组事件，主要涉及II型谱系菌株作为受体（12次事件），并且一些氨基酸残基显示受到正选择。45个非同义变化中有4个被发现受到后验概率>95%的正选择。将多态性和正选择的氨基酸位点映射到InlA的部分晶体结构上，结果显示富含亮氨酸重复序列（LRR）区域面向InlA受体E-钙黏蛋白的内化素表面不包含任何多态性位点；所有多态性和正选择的氨基酸都映射到LRR区域的外表面或其他InlA区域。数据表明：（i）inlA具有高度多态性，inlA的进化在II型谱系分离株中涉及大量重组事件；（ii）特定氨基酸位点的正选择似乎有助于inlA的进化，包括重组事件的固定；（iii）inlA中II型谱系特异性3'同聚物区域的单核苷酸缺失导致InlA完全丧失或产生截短的InlA，其侵袭性降低。总之，inlA具有复杂的进化史，这与单核细胞增生李斯特菌作为一种具有广泛宿主范围的环境病原体的自然史一致，包括其对不同inlA等位基因可能提供选择优势或可能不需要inlA的环境和宿主的适应。
