Albers Urs, Tiaden André, Spirig Thomas, Al Alam Denise, Goyert Sanna M, Gangloff Sophie C, Hilbi Hubert
ETH Zürich, Institute of Microbiology, Wolfgang-Pauli Strasse 10, HCI G405, 8093 Zürich, Switzerland.
EA3796, IFR53, UFR of Pharmacy, University of Reims Champagne Ardenne, 1 Avenue du Marechal Juin, 51100 Reims, France.
Microbiology (Reading). 2007 Nov;153(Pt 11):3817-3829. doi: 10.1099/mic.0.2007/009829-0.
Legionella pneumophila is an opportunistic pathogen that in the environment colonizes biofilms and replicates within amoebae. The bacteria employ the intracellular multiplication/defective organelle trafficking (Icm/Dot) type IV secretion system to grow intracellularly in a specific vacuole. Using Acanthamoeba castellanii as a host cell, we have previously identified lcsC (Legionella cytotoxic suppressor), a paralogue of the lipid A disaccharide synthase lpxB, as a cytotoxic factor of L. pneumophila. A bioinformatic analysis of the genome revealed that L. pneumophila is unique in harbouring two paralogues of lpxB and two and three paralogues of the lipid A biosynthesis acyltransferases lpxA and lpxD, respectively. LcsC (lpxB1) forms a transcriptional unit with gnnA, encoding a putative UDP-GlcNAc oxidase in the biosynthetic pathway leading to 3-aminoglucosamine analogues of lipid A. LpxB2 clusters with lpxD2, lpxA2 and lpxL paralogues, encoding secondary acyltransferases. LcsC/lpxB1 and lpxB2 were found to partially complement the growth defect of an Escherichia coli lpxB conditional mutant strain, indicating that both corresponding enzymes possess lipid A disaccharide synthase activity. The two L. pneumophila lpxB paralogues are not functionally equivalent, since expression of lcsC/lpxB1 but not lpxB2 in an L. pneumophila icmG mutant is cytotoxic for A. castellanii, and LPS purified from the two strains triggers CD14-dependent tumour necrosis factor (TNF)alpha production by macrophages with a different potency. The lpxB and lpxA paralogues are expressed under various growth conditions, including broth, biofilms and in A. castellanii. While the flagellar gene flaA is mainly expressed in late stationary phase, the lpxB and lpxA paralogues are preferentially expressed in the exponential and early stationary phases. Upon exposure to hypotonic stress and nutrient deprivation, lpxA1, and to a lesser extent lcsC/lpxB1, is upregulated. The differential regulation of lpxB or lpxA paralogues in response to changing environmental conditions might allow L. pneumophila to adapt its lipid A structure.
嗜肺军团菌是一种机会致病菌，在环境中可定殖于生物膜并在变形虫内复制。该细菌利用细胞内增殖/缺陷细胞器运输（Icm/Dot）IV型分泌系统在特定液泡内进行细胞内生长。我们之前以卡氏棘阿米巴作为宿主细胞，鉴定出脂质A二糖合酶lpxB的旁系同源物lcsC（嗜肺军团菌细胞毒性抑制因子）为嗜肺军团菌的一种细胞毒性因子。对该基因组的生物信息学分析表明，嗜肺军团菌的独特之处在于分别含有两个lpxB旁系同源物以及两个和三个脂质A生物合成酰基转移酶lpxA和lpxD的旁系同源物。LcsC（lpxB1）与gnnA形成一个转录单元，gnnA在导致脂质A的3-氨基葡糖类似物的生物合成途径中编码一种假定的UDP-N-乙酰葡糖胺氧化酶。LpxB2与lpxD2、lpxA2和lpxL旁系同源物成簇，后者编码二级酰基转移酶。研究发现LcsC/lpxB1和lpxB2可部分弥补大肠杆菌lpxB条件突变株的生长缺陷，这表明这两种相应的酶均具有脂质A二糖合酶活性。嗜肺军团菌的两个lpxB旁系同源物在功能上并不等同，因为在嗜肺军团菌icmG突变体中表达lcsC/lpxB1而非lpxB2对卡氏棘阿米巴具有细胞毒性，并且从这两种菌株中纯化的脂多糖以不同的效力触发巨噬细胞产生依赖CD14的肿瘤坏死因子（TNF）α。lpxB和lpxA旁系同源物在包括肉汤、生物膜以及在卡氏棘阿米巴内等各种生长条件下均有表达。虽然鞭毛基因flaA主要在稳定期末期表达，但lpxB和lpxA旁系同源物在指数期和稳定期早期优先表达。在受到低渗应激和营养剥夺时，lpxA1以及程度较轻的lcsC/lpxB1会上调。lpxB或lpxA旁系同源物对不断变化的环境条件的差异调节可能使嗜肺军团菌能够调整其脂质A结构。
