PROPERTIES OF A STREPTOCOCCUS-SALIVARIUS SPONTANEOUS MUTANT IN WHICH THE METHIONINE AT POSITION 48 IN THE PROTEIN HPR HAS BEEN REPLACED BY A VALINE

HPr is a protein of the phosphoenolpyruvate:sugar phosphotransferase s ystem (PTS) that participates in the concomitant transport and phospho rylation of sugars in bacteria. In gram-positive bacteria, HPr is also reversibly phosphorylated at a seryl residue at position 46 (Ser-46) by a metabolite-activated ATP-dependent kinase and a P(i)-dependent HP r(Ser-P) phosphatase. We report in this article the isolation of a spo ntaneous mutant (mutant A66) from a streptococcus (Streptococcus saliv arius) in which the methionine at position 48 (Met-48) in the protein HPr has been replaced by a valine (Val). The mutation inhibited the ph osphorylation of HPr on Ser-46 by the ATP-dependent kinase but did not prevent phosphorylation of HPr by enzyme I or the phosphorylation of enzyme 11 complexes by HPr(His-P). The results, however, suggested tha t replacement of Met-48 by Val decreased the affinity of enzyme I for HPr or the affinity of enzyme II proteins for HPr(His-P) or both. Char acterization of mutant A66 demonstrated that it has pleiotropic proper ties, including the lack of III(L)Man, a specific protein of the manno se PTS; decreased levels of HPr; derepression of some cytoplasmic prot eins; reduced growth on PTS as well as on non-PTS sugars; and aberrant growth in medium containing a mixture of sugars.