MODULATION OF BACILLUS-SUBTILIS CATABOLITE REPRESSION BY TRANSITION-STATE REGULATORY PROTEIN ABRB

The first enzyme of the Bacillus subtilis histidine-degradative (hut) pathway, histidase, was expressed at higher levels during the onset of the stationary growth phase in nutrient sporulation medium in early-b locked sporulation mutants (spo0A) than in wild-type strains. Histidas e expression was also elevated in spo0A mutant cultures compared with wild-type cultures during the logarithmic growth phase in minimal medi um containing slowly metabolized carbon sources. Histidase expression was not derepressed in spo0A abrB mutant cultures under these growth c onditions, suggesting that the AbrB protein is responsible for the der epression of histidase synthesis seen in spo0A mutant cultures. spo0A mutants contain higher levels of the AbrB protein than do wild-type st rains because the Spo0A protein represses AbrB expression. A direct co rrelation between the levels of abrB transcription and histidase expre ssion was found in spo0A mutant cultures. The hutO(CR2) operator, whic h is required for wild-type regulation of hut expression by catabolite repression, was also required for AbrB-dependent derepression of hut expression in spo0A mutants. Purified AbrB protein bound to the hutO(C R2) operator in vitro, suggesting that AbrB protein alters hut express ion by competing with the hut catabolite repressor protein for binding to the hutO(CR2) site. During the logarithmic growth phase in media c ontaining slowly metabolized carbon sources, the expression of several other enzymes subject to catabolite repression was elevated in spo0A mutants but not in spo0A abrB mutants. This suggests that the AbrB pro tein acts as a global modulator of catabolite repression during carbon -limited growth.