Galactose metabolism by Streptococcus mutans

Abstract

The galK gene, encoding galactokinase of the Leloir pathway, was insertionally inactivated in Streptococcus mutans UA159. The galK knockout strain displayed only marginal growth on galactose, but growth on glucose or lactose was not affected. In strain UA159, the sugar phosphotransferase system (PTS) for lactose and the PTS for galactose were induced by growth in lactose and galactose, although galactose PTS activity was very low, suggesting that S. mutans does not have a galactose-specific PTS and that the lactose PTS may transport galactose, albeit poorly. To determine if the galactose growth defect of the galK mutant could be overcome by enhancing lactose PTS activity, the gene encoding a putative repressor of the operon for lactose PTS and phospho-beta-galactosidase, lacR, was insertionally inactivated. A galK and lacR mutant still could not grow on galactose, although the strain had constitutively elevated lactose PTS activity. The glucose PTS activity of lacR mutants grown in glucose was lower than in the wild-type strain, revealing an influence of LacR or the lactose PTS on the regulation of the glucose PTS. Mutation of the lacA gene of the tagatose pathway caused impaired growth in lactose and galactose, suggesting that galactose can only be efficiently utilized when both the Leloir and tagatose pathways are functional. A mutation of the permease in the multiple sugar metabolism operon did not affect growth on galactose. Thus, the galactose permease of S. mutans is not present in the gal, lac, or msm operons.

FIG. 1.

The arrangement of the gal and msm operons in S. mutans UA159. The location of the insertion of a nonpolar Km (NP kan) cassette within the galK gene to generate the strain JAM2 is indicated.

FIG. 2.

RT-PCR analysis of galK and lacF expression in the wild-type strain UA159, JAM2, and JAM18 grown in glucose (G), lactose (L) or, when possible, galactose (Gal). Panel A shows the expression of galK in UA159 grown in G, L, or Gal. Panel B indicates that the lacF gene is induced or derepressed by lactose or galactose for UA159 and by lactose for JAM2. Panel C shows the expression of lacF in UA159 and JAM18, a galK and lacR mutant strain.

FIG. 3.

Sugar-specific PTS activity of UA159, JAM2, JAM18, and JAM19 grown in glucose (A), lactose (B), or galactose (C). The values are the means ± standard deviations of results from at least three independent experiments.

FIG. 3.

Sugar-specific PTS activity of UA159, JAM2, JAM18, and JAM19 grown in glucose (A), lactose (B), or galactose (C). The values are the means ± standard deviations of results from at least three independent experiments.

FIG. 3.

Sugar-specific PTS activity of UA159, JAM2, JAM18, and JAM19 grown in glucose (A), lactose (B), or galactose (C). The values are the means ± standard deviations of results from at least three independent experiments.