Journal of Bacteriology, November 2000, p. 5982-5989, Vol. 182, No. 21
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
-Galactosidase Activity, and
Glycolysis by CcpA in Streptococcus thermophilus: Evidence
for Carbon Catabolite Repression by a Non-Phosphoenolpyruvate-Dependent
Phosphotransferase System Sugar
andWageningen Centre for Food Sciences, NIZO Food Research, Department of Flavour and Natural Ingredients, 6710 BA Ede, The Netherlands
Received 12 April 2000/Accepted 1 August 2000
Streptococcus thermophilus, unlike many other
gram-positive bacteria, prefers lactose over glucose as the primary
carbon and energy source. Moreover, lactose is not taken up by a
phosphoenolpyruvate-dependent phosphotransferase system (PTS) but by
the dedicated transporter LacS. In this paper we show that CcpA plays a
crucial role in the fine-tuning of lactose transport,
-galactosidase
(LacZ) activity, and glycolysis to yield optimal glycolytic flux and
growth rate. A catabolite-responsive element (cre) was
identified in the promoter of the lacSZ operon, indicating
a possible role for regulation by CcpA. Transcriptional analysis showed
a sevenfold relief of repression 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 strain was found to release substantial
amounts of glucose into the medium when grown on lactose.
Transcriptional analysis of the ldh gene showed that
expression was induced twofold during growth on lactose compared to
glucose or galactose, in a CcpA-dependent manner. A reduced rate of
glycolysis concomitant with an increased lactose transport rate could
explain the observed expulsion 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 possesses an overcapacity for lactose uptake that is
repressed by CcpA to match the rate-limiting glycolytic flux.
Present address: Molecular Genetics, Groningen Biomolecular
Sciences and Biotechnology Institute, University of Groningen, 9750 AA
Haren, The Netherlands.
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