Multiple regulation involved in the expression of the uxuR regulatory gene in Escherichia coli K-12.

We have isolated a strain carrying a fusion of the beta-galactosidase structural gene to the promoter of the uxuR regulatory gene with the aid of the Casadaban Mud (Aprlac) phage. Analysis of mutants with deletions that were derived from the uxuR::Mud1 insertion strain confirmed the counterclockwise transcription direction of the uxuR gene. The uxuR-lacZ fusion strain was also used to examine the regulation of expression from the uxuR promoter. It was observed that an increase in the copy number of the uxuR gene results in an increased repression of beta-galactosidase synthesis. Overproduction of the exuR repressor also caused a decrease of the beta-galactosidase level. In all cases, the repression of beta-galactosidase synthesis was accompanied by a stronger repression of uxuB gene product synthesis. These results indicate that the expression of the uxuR gene is repressed by its own product but also by the exuR repressor. The different types of regulation of the two uxu operons are thus identical.