Molecular Food Microbiology Laboratory, Department of Food Science, Purdue University, West Lafayette, IN 47907, USA.
Gene. 2011 Dec 10;489(2):76-85. doi: 10.1016/j.gene.2011.08.015. Epub 2011 Sep 17.
The translocation of proteins across the bacterial cell wall is carried out by the general secretory (Sec) system. Most bacteria have a single copy of the secA gene, with the exception of a few Gram-positive bacteria, which have an additional copy of secA, designated secA2. secA2 is present in Listeria monocytogenes and is responsible for secretion and translocation of several proteins including virulence factors; however, little is known about the secA2 gene and its genetic organization in nonpathogenic members of the genus Listeria. The goal of this study was to determine the presence of secA2 locus and analyze the genetic relatedness among pathogenic and nonpathogenic Listeria species. Cloning experiments revealed that secA2 is present in all analyzed pathogenic (L. monocytogenes and L. ivanovii) and nonpathogenic (L. welshimeri, L. innocua, L. seeligeri, L. grayi and L. marthii) Listeria species except L. rocourtiae. Likewise, SecA2 transcripts were also detected in all species. Sequence analysis further revealed that 2331 nucleotides (776 amino acids) are conserved in L. monocytogenes, L. welshimeri, L. innocua and L. marthii. Three nucleotides are deleted in L. ivanovii and L. seeligeri and six in L. grayi, resulting in amino acid counts of 775, 775 and 774, respectively. secA2 is flanked upstream by iap (encoding p60) and downstream by a putative membrane protein (lmo0583, lmo f2365_0613) in all analyzed Listeria species, demonstrating conserved genetic organization of the secA2 locus in pathogenic and nonpathogenic species. Deletion of secA2 in L. innocua impaired accumulation of SecA2 substrate, N-acetyl muramidase (NamA) in the cell wall, providing evidence for the presence of functional SecA2 in nonpathogenic Listeria.
蛋白质在细菌细胞壁中的跨膜转运是由一般分泌（Sec）系统完成的。大多数细菌只有一个 secA 基因的拷贝，除了少数革兰氏阳性菌，它们有一个额外的 secA 拷贝，称为 secA2。secA2 存在于李斯特菌属的李斯特菌和单核细胞增生李斯特菌中，负责包括毒力因子在内的几种蛋白质的分泌和转运；然而，对于非致病性李斯特菌属成员中的 secA2 基因及其遗传组织知之甚少。本研究的目的是确定 secA2 基因座的存在，并分析致病性和非致病性李斯特菌种之间的遗传相关性。克隆实验表明，secA2 存在于所有分析的致病性（单核细胞增生李斯特菌和李斯特菌 ivanovii）和非致病性（李斯特菌 welshimeri、李斯特菌 innocua、李斯特菌 seeligeri、李斯特菌 grayi 和李斯特菌 marthii）李斯特菌种中，除了李斯特菌 rocourtiae。同样，在所有物种中也检测到 SecA2 转录本。序列分析进一步表明，李斯特菌 monocytogenes、李斯特菌 welshimeri、李斯特菌 innocua 和李斯特菌 marthii 中有 2331 个核苷酸（776 个氨基酸）保守。李斯特菌 ivanovii 和李斯特菌 seeligeri 中有 3 个核苷酸缺失，李斯特菌 grayi 中有 6 个核苷酸缺失，导致氨基酸计数分别为 775、775 和 774。在所有分析的李斯特菌种中，secA2 上游由 iap（编码 p60）侧翼，下游由一个假定的膜蛋白（lmo0583、lmo f2365_0613）侧翼，表明致病性和非致病性物种中 secA2 基因座的遗传组织保守。李斯特菌 innocua 中 secA2 的缺失损害了细胞壁中 SecA2 底物 N-乙酰胞壁酸酶（NamA）的积累，为非致病性李斯特菌中存在功能性 SecA2 提供了证据。
