This study examined the regulation of the uxu operon and uxuR regulatory gene in Escherichia coli K12 using lac gene fusions. Researchers created gene fusions between the lac reporter genes and various genes of the hexuronate degradation system using transposon-based insertion methods. They found that β-galactosidase expression from uxu-lac fusions was induced by glucuronate and fructuronate (the natural substrates) and was sensitive to catabolite repression by glucose. Importantly, they isolated a uxuR-lac fusion strain that allowed study of the regulatory gene itself. When wild-type uxuR was present, expression of the uxuR-lac fusion was partially repressed, and this repression was relieved by adding inducer. These results demonstrated that the uxuR gene undergoes autogenous regulation—its product represses its own synthesis. The repression appeared to be weak compared to other regulatory genes, suggesting this autoregulation may help maintain appropriate repressor protein concentrations under varying growth conditions.

Key findings

• β-galactosidase from uxu-lac fusions was inducible by glucuronate and fructuronate and sensitive to glucose catabolite repression
• Fructuronate acts as a true inducer of the uxu operon, being required for induction even when not further metabolized
• The uxuR regulatory gene exhibits weak autogenous repression—its product represses its own expression at approximately 50% efficiency
• The uxuR repressor has relatively low affinity for its own operator compared to regulatory genes like exuR
• The uxu operon is controlled by both uxuR and exuR regulatory genes