1887

Abstract

The ability of () to grow on lactose strongly depends on the formation of an extracellular glycoside hydrolase (GH) family 35 -galactosidase, encoded by the gene. Previous studies, using batch or transfer cultures of pregrown cells, had shown that is induced by lactose and -galactose, but to a lesser extent by galactitol. To test whether the induction level is influenced by the different growth rates attainable on these carbon sources, expression was compared in carbon-limited chemostat cultivations at defined dilution (=specific growth) rates. The data showed that expression by lactose, -galactose and galactitol positively correlated with the dilution rate, and that galactitol and -galactose induced the highest activities of -galactosidase at comparable growth rates. To know more about the actual inducer for -galactosidase formation, its expression in strains impaired in the first steps of the two -galactose-degrading pathways was compared. Induction by -galactose and galactitol was still found in strains deleted in the galactokinase-encoding gene , which is responsible for the first step of the Leloir pathway of -galactose catabolism. However, in a strain deleted in the aldose/-xylose reductase gene , which performs the reduction of -galactose to galactitol in a recently identified second pathway, induction by -galactose, but not by galactitol, was impaired. On the other hand, induction by -galactose and galactitol was not affected in an -arabinitol 4-dehydrogenase ()-deleted strain which is impaired in the subsequent step of galactitol degradation. These results indicate that galactitol is the actual inducer of Bga1 formation during growth on -galactose in .

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2007-02-01
2024-07-21
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