Journal of Bacteriology, September 2001, p. 5145-5154, Vol. 183, No. 17
0021-9193/01/$04.00+0 DOI: 10.1128/JB.183.17.5145-5154.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Groupe de recherche en écologie buccale (GREB), Département de biochimie et de microbiologie, Faculté des sciences et de génie, and Faculté de médecine dentaire, Université Laval, Québec, Canada G1K 7P4
Received 27 March 2001/Accepted 15 June 2001
In streptococci, HPr, a phosphocarrier of the
phosphoenolpyruvate:sugar phosphotransferase transport system (PTS),
undergoes multiple posttranslational chemical modifications resulting
in the formation of HPr(His~P), HPr(Ser-P), and HPr(Ser-P)(His~P), whose cellular concentrations vary with growth conditions. Distinct physiological functions are associated with specific forms of HPr. We
do not know, however, the cellular thresholds below which these forms
become unable to fulfill their functions and to what extent
modifications in the cellular concentrations of the different forms of
HPr modify cellular physiology. In this study, we present a glimpse of
the diversity of Streptococcus salivarius ptsH mutants that
can be isolated by positive selection on a solid medium containing 2-deoxyglucose and galactose and identify 13 amino acids that are
essential for HPr to properly accomplish its physiological functions.
We also report the characterization of two S. salivarius mutants that produced approximately two- and threefoldless 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 EI and HPr cellular levels
did not affect PTS transport capacity; (iii) a twofold reduction in HPr
synthesis was sufficient to reduce the rate at which cells metabolized
PTS sugars, increase generation times on PTS sugars and to a lesser
extent on non-PTS sugars, and impede the exclusion of non-PTS sugars by
PTS sugars; (iv) a threefold reduction in HPr synthesis caused a strong
derepression of the genes coding for
-galactosidase,
-galactosidase, and galactokinase when the cells were grown at the
expense of a PTS sugar but did not affect the synthesis of
-galactosidase when cells were 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).
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