Effect of growth rate and pH on intracellular levels and activities of the components of the phosphoenolpyruvate: sugar phosphotransferase system in Streptococcus mutans Ingbritt.

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RESUMO

The growth of Streptococcus mutants Ingbritt in continuous culture at low pH or high growth rates repressed the biosynthesis of the components of the phosphoenolpyruvate:sugar phosphotransferase system (PTS). The cellular concentrations of the soluble components HPr, enzyme I (EI), and EIII for mannose (IIIman) and EII activity for glucose, mannose, 2-deoxyglucose (2DG), and fructose were determined in membrane preparations from cells grown at pHs from 8.0 to 5.0 and at dilution (D) or growth rates from 0.1 to 1.0 h-1. The cellular levels of HPr and EI varied less than threefold under all of the growth conditions tested. On the other hand, EII activity in membranes from cells grown at D = 0.1 h-1 was repressed by growth at pHs below 8.0, with cells grown at pH 5.0 completely devoid of EII activity. In addition, cells grown at D = 0.5 and 1.0 h-1 exhibited little PTS activity for glucose, mannose, and 2DG and twofold-lower activity for fructose. These activities were stimulated by the addition of a membrane-free cytoplasmic fraction, and this activating activity was shown to be due to the presence of IIIman. Estimation of the cellular content of IIIman indicated that the synthesis of this factor was repressed by growth above and below pH 7.0 and was particularly sensitive to growth at high rates. These results indicate that with S. mutans Ingbritt, both pH and growth rate regulate the genes for the synthesis of EIIs involved in the phosphorylation of glucose, mannose, 2DG, and fructose and the gene for the formation of IIIman.

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