Use of uxu-lac fusion strains to study the regulation of the uxuAB operon in Escherichia coli K12.

In the hexuronate system of Escherichia coli, the uxuAB operon is negatively controlled by the UxuR and ExuR repressors. A Mudlac phage was used, by the method of Casadaban, to construct strains where a truncated lacZ was fused to uxuB. From these fusion strains, deletions of various lengths extending into the uxu region were created in vivo by selecting temperature-insensitive mutants. Operator-constitutive mutations were also selected for in such strains and their preliminary analysis is presented. Large amounts of either ExuR or UxuR repressor caused a strong decrease of the constitutive expression of the uxuAB operon in the operator mutants. The implications of this repression for the presence of one or two operator sites in the uxuAB operon are discussed.