Lessard Christian, Cochu Armelle, Lemay Jean-Dominique, Roy Denis, Vaillancourt Katy, Frenette Michel, Moineau Sylvain, Vadeboncoeur Christian
Groupe de Recherche en Ecologie Buccale, Département de Biochimie et de Microbiologie, Faculté des Sciences et de Génie, and Faculté de Médecine Dentaire, Université Laval, Québec City, Quebec G1K 7P4, Canada.
J Bacteriol. 2003 Dec;185(23):6764-72. doi: 10.1128/JB.185.23.6764-6772.2003.
The oral bacterium Streptococcus salivarius takes up lactose via a transporter called LacS that shares 95% identity with the LacS from Streptococcus thermophilus, a phylogenetically closely related organism. S. thermophilus releases galactose into the medium during growth on lactose. Expulsion of galactose is mediated via LacS and stimulated by phosphorylation of the transporter by HPr(His approximately P), a phosphocarrier of the phosphoenolpyruvate:sugar phosphotransferase transport system (PTS). Unlike S. thermophilus, S. salivarius grew on lactose without expelling galactose and took up galactose and lactose concomitantly when it is grown in a medium containing both sugars. Analysis of the C-terminal end of S. salivarius LacS revealed a IIA-like domain (IIA(LacS)) almost identical to the IIA domain of S. thermophilus LacS. Experiments performed with purified proteins showed that S. salivarius IIA(LacS) was reversibly phosphorylated on a histidine residue at position 552 not only by HPr(His approximately P) but also by HPr(Ser-P)(His approximately P), a doubly phosphorylated form of HPr present in large amounts in rapidly growing S. salivarius cells. Two other major S. salivarius PTS proteins, IIAB(L)(Man) and IIAB(H)(Man), were unable to phosphorylate IIA(LacS). The effect of LacS phosphorylation on growth was studied with strain G71, an S. salivarius enzyme I-negative mutant that cannot synthesize HPr(His approximately P) or HPr(Ser-P)(His approximately P). These results indicated that (i) the wild-type and mutant strains had identical generation times on lactose, (ii) neither strain expelled galactose during growth on lactose, (iii) both strains metabolized lactose and galactose concomitantly when grown in a medium containing both sugars, and (iv) the growth of the mutant was slightly reduced on galactose.
口腔细菌唾液链球菌通过一种名为LacS的转运蛋白摄取乳糖，该转运蛋白与嗜热链球菌的LacS有95%的同源性，嗜热链球菌是一种在系统发育上密切相关的生物体。嗜热链球菌在以乳糖为生长底物时会将半乳糖释放到培养基中。半乳糖的排出是通过LacS介导的，并受到磷酸烯醇丙酮酸：糖磷酸转移酶运输系统（PTS）的磷酸载体HPr（HisP）对转运蛋白的磷酸化作用的刺激。与嗜热链球菌不同，唾液链球菌在乳糖上生长时不会排出半乳糖，并且当它在含有两种糖的培养基中生长时会同时摄取半乳糖和乳糖。对唾液链球菌LacS的C末端分析揭示了一个与嗜热链球菌LacS的IIA结构域几乎相同的IIA样结构域（IIA（LacS））。用纯化蛋白进行的实验表明，唾液链球菌IIA（LacS）不仅能被HPr（HisP），而且能被在快速生长的唾液链球菌细胞中大量存在的HPr（Ser-P）（HisP）（一种HPr的双磷酸化形式）在第552位组氨酸残基上可逆磷酸化。另外两种主要的唾液链球菌PTS蛋白，IIAB（L）（Man）和IIAB（H）（Man），不能使IIA（LacS）磷酸化。用菌株G71研究了LacS磷酸化对生长的影响，G71是一种唾液链球菌酶I阴性突变体，不能合成HPr（HisP）或HPr（Ser-P）（His~P）。这些结果表明：（i）野生型和突变型菌株在乳糖上的代时相同；（ii）两菌株在以乳糖为生长底物时均不排出半乳糖；（iii）当在含有两种糖的培养基中生长时，两菌株均能同时代谢乳糖和半乳糖；（iv）突变体在半乳糖上的生长略有降低。
