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Gene fusions between the lac structural genes and various genes of the hexuronate system of Escherichia coli K12 were isolated by the technique of Casadaban. Mud(Apr lac) and λplac-Mu insertion mutants were constructed in which the lac genes were fused to the regulatory region of the uxu operon. In all the uxu-lac fusion strains, β-galactosidase expression was shown to be inducible by the natural inducers of the uxu operon (glucuronate and fructuronate) and sensitive to catabolite repression by glucose. In addition we isolated a Mud(Apr lac) fusion where the lac genes were fused to the uxuR regulatory gene. In this fusion the synthesis of β-galactosidase reflects the regulation of the uxuR gene. In the presence of a wild-type uxuR allele, partial repression of β-galactosidase expression was found; the repression was removed when inducer was added. This result indicates that while the uxuR gene is subject to autogenous control, the uxuR repressor may have only a low affinity for its own operator.
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