CATABOLITE REPRESSION OF THE BACILLUS-SUBTILIS XYL OPERON INVOLVES A CIS-ELEMENT FUNCTIONAL IN THE CONTEXT OF AN UNRELATED SEQUENCE, AND GLUCOSE EXERTS ADDITIONAL XYLR-DEPENDENT REPRESSION

Catabolite repression (CR) of xylose utilization by Bacillus subtilis involves a 14-bp cis-acting element (CRE) located in the translated re gion of the gene encoding xylose isomerase (xylA). Mutations of CRE ma king it more similar to a previously proposed consensus element lead t o increased CR exerted by glucose, fructose, and glycerol. Fusion of C RE to an unrelated, constitutive promoter confers CR to beta-galactosi dase expression directed by that promoter. This result demonstrates th at CRE can function independently of sequence contest and suggests tha t it is indeed a generally active cis element for CR. In contrast to t he other carbon sources studied here, glucose leads to an additional r epression of xylA expression, which is independent of CRE and is not f ound when CRE is fused to the unrelated promoter. This repression requ ires a functional xylR encoding Xyl repressor and is dependent on the concentrations of glucose and the inducer xylose in the culture broth. Potential mechanisms for this glucose-specific repression are discuss ed.