Phosphorylation of Streptococcus salivarius lactose permease (LacS) by HPr(His∼P) and HPr(Ser-P)(His∼P) and effects on growth

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(Hissimilar toP), 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(Hissimilar toP) but also by HPr(Sersimilar toP)(Hissimilar toP), a doubly phosphorylated form of HPr present in large amounts in rapidly growing S. salivarius cells. Two other major S. salivarius PTS proteins, IIBLMan and IIAB(H)(Man) were unable to phosphorylate IIA. The effect of LacS phosphorylation on growth was studied with strain G71, an S. salivarius enzyme I-negative mutant that cannot synthesize HPr(Hissimilar toP) or HPr(SerP)(Hissimilar toP). 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.