Diversity of Streptococcus salivarius ptsH mutants that can be isolated inthe presence of 2-deoxyglucose and galactose and characterization of two mutants synthesizing reduced levels of HPr, a phosphocarrier of the phosphoenolpyruvate : sugar phosphotransferase system

In streptococci, HPr, a phosphocarrier of the phosphoenolpyruvate:sugar pho sphotransferase transport system (PTS), undergoes multiple posttranslationa l chemical modifications resulting in the formation of HPr(His similar toP) , HPr(Ser-P), and HPr(Ser-P)(His similar toP), whose cellular concentration s vary with growth conditions. Distinct physiological functions are associa ted with specific forms of HPr. We do not know, however, the cellular thres holds below which these forms become unable to fulfill their functions and to what extent modifications in the cellular concentrations of the differen t forms of HPr modify cellular physiology. In this study, we present a glim pse of the diversity of Streptococcus salivarius ptsH mutants that can be i solated by positive selection on a solid medium containing 2-deoxyglucose a nd galactose and identify 13 amino acids that are essential for HPr to prop erly accomplish its physiological functions. We also report the characteriz ation of two S. salivarius mutants that produced approximately two- and thr eefoldless HPr and enzyme I (EI) respectively. The data indicated that (i) a reduction in the synthesis of HPr due to a mutation in the Shine-Dalgarno sequence of ptsH reduced ptsI expression; (ii) a threefold reduction in El and HPr cellular levels did not affect PTS transport capacity; (iii) a two fold reduction in HPr synthesis was sufficient to reduce the rate at which cells metabolized PTS sugars, increase generation times on PTS sugars and t o a lesser extent on non-PTS sugars, and impede the exclusion of non-PTS su gars by PTS sugars; (iv) a threefold reduction in HPr synthesis caused a st rong derepression of the genes coding for alpha -galactosidase, beta -galac tosidase, and galactokinase when the cells were grown at the expense of a P TS sugar but did not affect the synthesis of ct-galactosidase when cells we re grown at the expense of lactose, a noninducing non-PTS sugar; and (v) no correlation was found between the magnitude of enzyme derepression and the cellular levels of HPr(Ser-P).