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Phosphorylation of Streptococcus salivarius Lactose Permease (LacS) by HPr(HisP) and HPr(Ser-P)(HisP) and Effects on Growth

Groupe de Recherche en Écologie 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

Received 18 June 2003/ Accepted 9 September 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(HisP), 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(HisP) but also by HPr(Ser-P)(HisP), 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(HisP) or HPr(Ser-P)(HisP). 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.

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