The uidR regulatory gene of Escherichia coli codes for a repressor molecule that negatively controls the expression of the uidA gene. The uidR gene was fused in front of the lacZ gene in vitro on plasmid cloning vectors developed by Casadaban et al. [J. Bacteriol. 143 (1980) 971-980. The transcriptional direction of uidR was deduced from the restriction pattern and the phenotypic properties of the uidR-lacZ fusion plasmids. The gene is transcribed counterclockwise on the standard E. coli map, as is the uidA gene. The uidR-lacZ fusions were also used to examine the regulation of expression from the uidR promoter. It was observed that the uidR gene expression is repressed by its own product and is sensitive to catabolite control. The uidR-lacZ-encoded proteins of various sizes were isolated but attempts to obtain hybrid molecules possessing, in a single polypeptide, both the beta-galactosidase and UidR repressor activities were unsuccessful.