Regulation of ß-Galactosidase Synthesis in Wild Type and in a Succinate-Resistant Mutant of Rhizobium meliloti

The synthesis o f /?-galactosidase in Rhizobium meliloti W U60 was found to be inducible by lactose and its non-metabolizable analogue, isopropyl-ß-D-thiogalactoside (IPTG). In contrast to Escherichia coli, galactose and melibiose were very weak inducers o f this enzyme in R. meliloti. The maximum level o f /3-galactosidase in this bacterium is 2% o f that in fully induced E. coli. In addition to glucose, the induced synthesis o f this enzyme in R. meliloti was repressed by galactose, glycerol, and succinate. In comparison to E. coli, addition o f cyclic AMP to the growth medium o f R. meliloti did not alleviate the repressive effect o f the above compounds on ßgalactosidase synthesis. High concentrations o f sodium succinate (100 m M ) were inhibitory to the growth o f R. meliloti. Spontaneous succinate-resistant mutants could be isolated at low frequency. In contrast to the wild type parent, in a succinate-resistant mutant, the synthesis o f /J-galactosidase was not repressed by succinate.