Phenotypic consequences resulting from a methionine-to-valine substitutionat position 48 in the HPr protein of Streptococcus salivarius

In gram-positive bacteria, the HPr protein of the phosphoenolpyruvate:sugar phosphotransferase system (PTS) can be phosphorylated on a histidine resid ue at position 15 (His(15)) by enzyme I (EI) of the PTS and on a serine res idue at position 46 (Ser(46)) by an ATP-dependent protein kinase (His simil ar to P and Ser-P, respectively). We have isolated from Streptococcus saliv arius ATCC 25975, by independent selection from separate cultures, two spon taneous mutants (Ga3.78 and Ga3.14) that possess a missense mutation in pts H (the gene encoding HPr) replacing the methionine at position 48 by a vali ne. The mutation did not prevent the phosphorylation of HPr at His(15) by E I nor the phosphorylation at Ser(46) by the ATP-dependent HPr kinase. The l evels of HPr(Ser-P) in glucose-grown cells of the parental and mutant Ga3.7 8 mere virtually the same. However, mutant cells growing on glucose produce d two- to threefold less HPr(Ser-P)(His-P) than the wild-type strain, while the levels of free HPr and Wr(His similar to P) were increased 18- and 3-f old, respectively. The mutants grew as well as the wild-type strain on PTS sugars (glucose, fructose, and mannose) and on the non-PTS sugars lactose a nd melibiose, However, the growth rate of both mutants on galactose, also a non-PTS sugar, decreased rapidly with time. The M48V substitution had only a minor effect an the repression of alpha-galactosidase, beta-galactosidas e, and galactokinase by glucose, but this mutation abolished diauxie by ren dering cells unable to prevent the catabolism of a non-PTS sugar (lactose, galactose, and melibiose) when glucose was available. The results suggested that the capacity of the wild-type cells to preferentially metabolize gluc ose over non-PTS sugars resulted mainly from inhibition of the catabolism o f these secondary energy sources via a HPr-dependent mechanism. This mechan ism was activated following glucose but not lactose metabolism and it did n ot involve Wr(Ser-P) as the only regulatory molecule.