Interchangeability of repressors for the control of the uxu and uid operons in E. coli K12.

The uidA and uxuAB operons are each under the dual control of two repressor molecules: the uidR and uxuR encoded repressors negatively control the uidA operon whereas the uxuAB operon is regulated by the uxuR and exuR gene products. Plasmids overproducing the regulatory molecules encoded by exuR, uxuR, or uidR were used to investigate the regulation of these two operons. Large amounts of either exuR or uxuR repressor caused a complete repression of the uxuAB operon in exuR and uxuR double-deleted mutants, suggesting that the two repressors, when they are overproduced, are totally interchangeable for the control of the uxuAB operon. In contrast, the UxuR molecule has no effect on the other exu regulon operons controlled by the exuR gene product. The uidR and uxuR gene products appear to be partially interchangeable for the regulation of the uidA gene since addition of multicopy plasmids bearing uidR+, in uxuR deleted mutant strains, only partially suppresses the derepression of the uidA gene. Inversely, multicopies of uidR weakly reduced the synthesis of the uxuB gene product in uxuR derepressed mutants. The restrictions placed by these phenomena on the formulation of the mechanism of cooperation between the two repressor molecules for repressing the uxuAB and uidA operons are discussed.