The Bacillus subtilis xyl operon encoding enzymes for xylose utilization is repressed in the absence of xylose and in the presence of glucose. Transcriptional fusions of spoVG-lacZ to this operon show regulation of beta-galactosidase expression by glucose, indicating that glucose repression operates at the level of transcription. A similar result is obtained when glucose is replaced by glycerol, thus defining a general catabolite repression mechanism. A deletion of xylR, which encodes the xylose-sensitive repressor of the operon, does not affect glucose repression. The cis element mediating glucose repression was identified by Bal31 deletion analysis. It is confined to a 34 bp segment located at position +125 downstream of the xyl promoter in the coding sequence for xylose isomerase. Cloning of this segment in the opposite orientation leads to reduced catabolite repression. The homology of this element to various proposed consensus sequences for catabolite repression in B. subtilis is discussed.

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