Isolation of fusions between the lac genes and several genes of the exu regulon: analysis of their regulation, determination of the transcription direction of the uxaC-uxaA operon, in Escherichia coli K-12.

Gene fusions between the lac structural genes and different genes of the hexuronate system were isolated by the two methods described by Casadaban (1976, 1979). Mud (Apr lac) mutants which have the lac genes fused to the regulatory region of exuT, uxaC, uxaA and uxaB genes were constructed. Separately, the lac genes carried by a lambda plac-Mu hybrid phage were placed into a Mucts prophage inserted into uxaC gene and fusions were obtained, by selection of deletions putting the lac genes under the control of the uxaC regulatory elements. Induction of the lysogen led to the isolation of a lambda transducing phage bearing this uxaC-lac fusion. In all the fusion strains, beta-galactosidase expression was shown to be inducible by the natural inducers of hexuronate system enzymes, sensitive to catabolite repression and under the negative control of the exuR regulatory gene. The mutants with Mu insertion were used to establish the direction of transcription of the uxaC-uxaA operon, which is counter-clockwise on the circular genetic map of Escherichia coli (from uxaC to uxaA).