GENETIC-CHARACTERIZATION OF A RHIZOBIUM-MELILOTI LACTOSE UTILIZATION LOCUS

We identified several linked genes of a lactose regulon in Rhizobium m eliloti. These were lacZ, the structural gene for B-galactosidase; lac R, the lactose repressor gene; and two genes encoding proteins of unkn own function, lacW and lacX. Insertion mutants in lacW and lacZ belong ed to a single genetic complementation group, and lacW appeared to lie upstream of lacZ in an operon. Expression of lacZ, lacW and lacX was repressed by lacR, and expression of lacZ and lacW was derepressed by lactose. lacZ was not required for induction of lacW by lactose, sugge sting that lactose itself, rather than a processed form of lactose, ma y be the actual inducer molecule. Expression of all three genes was re pressed by succinate, and the lacR independence of this repression sho wed that inducer exclusion could not be the sole mechanism. This patte rn of lac gene organization and regulation differs in several ways fro m that observed in enteric bacteria.