Control of lactose transport, beta-galactosidase activity, and glycolysis by CcpA in Streptococcus thermophilus: Evidence for carbon catabolite repression by a non-phosphoenolpyruvate-dependent phosphotransferase system sugar

Streptococcus thermophilus, unlike many other gram-positive bacteria, prefe rs lactose over glucose as the primary carbon and energy source. Moreover, lactose is not taken up by a phosphoenolpyruvate-dependent phosphotransfera se system (PTS) but by the dedicated transporter LacS. In this paper we sho w that CcpA plays a crucial role in the fine-tuning of lactose transport, b eta-galactosidase (LacZ) activity, and glycolysis to yield optimal glycolyt ic flux and growth rate. A catabolite-responsive element (cre) was identifi ed in the promoter of the lacSZ operon, indicating a possible role for regu lation by CcpA. Transcriptional analysis showed a sevenfold relief of repre ssion in the absence of a functional CcpA when cells were grown on lactose. This CcpA-mediated repression of lacSZ transcription did not occur in wild -type cells during growth on galactose, taken up by the same LacS transport system. Lactose transport during fermentation was increased significantly in strains carrying a disrupted ccpA gene. Moreover, a ccpA disruption stra in was found to release substantial amounts of glucose into the medium when grown on lactose. Transcriptional analysis of the ldh gene showed that exp ression was induced twofold during growth on lactose compared to glucose or galactose, in a CcpA-dependent manner. A reduced rate of glycolysis concom itant with an increased lactose transport rate could explain the observed e xpulsion of glucose in a ccpA disruption mutant. We propose that CcpA in S. thermophilus acts as a catabolic regulator during growth on the preferred non-PTS sugar lactose. In contrast to other bacteria, S. thermophilus posse sses an overcapacity for lactose uptake that is repressed by CcpA to match the rate-limiting glycolytic flux.