Co-induction of beta-galactosidase and the lactose-P-enolpyruvate phosphotransferase system in Streptococcus salivarius and Streptococcus mutans

Abstract

The addition of lactose, galactose, or isopropyl-.beta.-D-thiogalactoside (IPTG) to glucose-grown cells of S. salivarius 25975 resulted in the co-induction of both the lactose-P[phospho]-enolpyruvate phosphotransferase system (lactose-PTS) and .beta.-galactosidase, with the latter the predominant metabolic system. With various strains of S. mutans and S. sanguis 10556, the lactose-PTS was the major metabolic pathway with .beta.-galactosidase induced either to low or negligible levels. In all cases, induction of the lactose-PTS resulted in the concomitant induction of 6-P-.beta.-galactosidase. The induction by lactose of both the lactose-PTS and .beta.-galactosidase in all strains was repressed by glucose and other catabolites, notably, fructose. Induction of .beta.-galactosidase in S. salivarius 25975 by IPTG was relatively resistant to glucose repression. Induction experiments with IPTG and lactose suggested that a cellular metabolite of lactose metabolism was a repressor of enzyme activity. Exogenous c[cyclic]AMP reversed the transient repression by glucose of .beta.-galactosidase induction in cells of S. salivarius 25975 receiving lactose, provided the cells were grown with small amounts of toluene to overcome the permeability barrier to this nucleotide. cAMP was unable to overcome the permanent repression of .beta.-galactosidase activity to a significant extent under these conditions.