Characterization of the operator sites of the exu regulon in Escherichia coli K-12 by operator-constitutive mutations and repressor titration.

In Escherichia coli, the exu regulon of the hexuronate system involves the three exuT, uxaCA and uxaB operons and is under the negative control of the exuR regulatory gene product. The technique developed by Casadaban, Chou and Cohen was employed to construct two plasmids containing operon fusions in which the lactose genes were fused to the uxaCA and exuT operons. These fusions were transferred into the chromosome by a reciprocal recombination event, and the resulting strains were used for isolation of mutants defective in repression. Two types of operator-constitutive mutants were obtained: one specific for the uxaCA operon expression and the other affecting the exuT gene expression. This genetic evidence confirms that these two operons which are divergently transcribed each possess their own operator site.--The derepressed expression of the two exuT-lac and uxaCA-lac operons and the uxaB gene was also examined upon introduction of plasmids bearing various operators of the exu regulon. The results of testing exuR repressor titration by multiple copies of the exu operators allowed us to show a gradation in the affinity degrees for the three exu operators: uxaBo has the strongest affinity for the exuR repressor and uxaCo the weakest, although that of exuTo seems to be just slightly greater. This gradation may play a role in the control of the exu regulon expression.