Glucose kinase-dependent catabolite repression in Staphylococcus xylosus.

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RESUMO

By transposon Tn917 mutagenesis, 16 mutants of Staphylococcus xylosus were isolated that showed higher levels of beta-galactosidase activity in the presence of glucose than the wild-type strain. The transposons were found to reside in three adjacent locations in the genome of S. xylosus. The nucleotide sequence of the chromosomal fragment affected by the Tn917 insertions yielded an open reading frame encoding a protein with a size of 328 amino acids with a high level of similarity to glucose kinase from Streptomyces coelicolor. Weaker similarity was also found to bacterial fructokinases and xylose repressors of gram-positive bacteria. The gene was designated glkA. Immediately downstream of glkA, two open reading frames were present whose deduced gene products showed no obvious similarity to known proteins. Measurements of catabolic enzyme activities in the mutant strains grown in the presence or absence of sugars established the pleiotropic nature of the mutations. Besides beta-galactosidase activity, which had been used to detect the mutants, six other tested enzymes were partially relieved from repression by glucose. Reduction of fructose-mediated catabolite repression was observed for some of the enzyme activities. Glucose transport and ATP-dependent phosphorylation of HPr, the phosphocarrier of the phosphoenolpyruvate:carbohydrate phosphotransferase system involved in catabolite repression in gram-positive bacteria, were not affected. The cloned glkA gene fully restored catabolite repression in the mutant strains in trans. Loss of GlkA function is thus responsible for the partial relief from catabolite repression. Glucose kinase activity in the mutants reached about 75% of the wild-type level, indicating the presence of another enzyme in S. xylosus. However, the cloned gene complemented an Escherichia coli strain in glucose kinase. Therefore, the glkA gene encodes a glucose kinase that participates in catabolite repression in S. xylosus.

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