Phenotypic consequences resulting from a methionine-to-valine substitution at 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 residue at position 15 (His(15)) by enzyme I (EI) of the PTS and on a serine residue at position 46 (Ser(46)) by an ATP-dependent protein kinase (His approximately P and Ser-P, respectively). We have isolated from Streptococcus salivarius ATCC 25975, by independent selection from separate cultures, two spontaneous mutants (Ga3.78 and Ga3.14) that possess a missense mutation in ptsH (the gene encoding HPr) replacing the methionine at position 48 by a valine. The mutation did not prevent the phosphorylation of HPr at His(15) by EI nor the phosphorylation at Ser(46) by the ATP-dependent HPr kinase. The levels of HPr(Ser-P) in glucose-grown cells of the parental and mutant Ga3.78 were virtually the same. However, mutant cells growing on glucose produced two- to threefold less HPr(Ser-P)(His approximately P) than the wild-type strain, while the levels of free HPr and HPr(His approximately P) were increased 18- and 3-fold, 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 and 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 on the repression of alpha-galactosidase, beta-galactosidase, and galactokinase by glucose, but this mutation abolished diauxie by rendering 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 glucose over non-PTS sugars resulted mainly from inhibition of the catabolism of these secondary energy sources via a HPr-dependent mechanism. This mechanism was activated following glucose but not lactose metabolism, and it did not involve HPr(Ser-P) as the only regulatory molecule.
Plamondon
P
P
Groupe de Recherche en Ecologie Buccale, Département de Biochimie, Faculté des Sciences et de Génie and Faculté de Médecine Dentaire, Université Laval, Cité Universitaire, Québec, Québec, Canada G1K 7P4.
Brochu
D
D
Thomas
S
S
Fradette
J
J
Gauthier
L
L
Vaillancourt
K
K
Buckley
N
N
Frenette
M
M
Vadeboncoeur
C
C
D000469:Alkaline Phosphatase / Q000378:metabolism; D001426:Bacterial Proteins; D015153:Blotting, Western; D050260:Carbohydrate Metabolism; D004734:Energy Metabolism; D005632:Fructose / Q000378:metabolism; D005690:Galactose / Q000378:metabolism; D015964:Gene Expression Regulation, Bacterial; D005947:Glucose / Q000378:metabolism*; D007785:Lactose / Q000378:metabolism; D008358:Mannose / Q000378:metabolism; D008553:Melibiose / Q000378:metabolism; D008715:Methionine; D010641:Phenotype; D010731:Phosphoenolpyruvate Sugar Phosphotransferase System / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D010766:Phosphorylation; D017354:Point Mutation; D013291:Streptococcus / Q000235:genetics* / Q000254:growth & development / Q000378:metabolism; D014633:Valine
D001426:Bacterial Proteins; D005632:Fructose; D008553:Melibiose; D008715:Methionine; D010731:Phosphoenolpyruvate Sugar Phosphotransferase System; C030182:phosphocarrier protein HPr; D000469:Alkaline Phosphatase; D014633:Valine; D005947:Glucose; D007785:Lactose; D008358:Mannose; D005690:Galactose