A Rhizobium meliloti mutant, lacking a functional γ-aminobutyrate (GABA) bypass, is defective in glutamate catabolism and symbiotic nitrogen fixation

Abstract A Rhizobium meliloti mutant, CMF1 2:38, was isolated which was specifically defective in the degradation of glutamate as sole carbon and nitrogen source. Biochemical analysis of CMF1 2:38 revealed a reduction in succinic semialdehyde dehydrogenase (SSDH) activity, the third enzyme of the γ-aminobutyrate (GABA) bypass. Evidence is presented which suggests that the Tn 5-induced mutation in CMF1 2:38 exists in a regulatory gene governing the expression of both NAD and NADP-linked SSDH activity. CMF1 2:38 nodulated alfalfa plants, but was reduced in its nitrogen fixation activity and biomass accumulating ability relative to the wild-type strain. The results presented in this study indicate that the GABA bypass is a major mechanism of glutamate degradation in R. meliloti CMF1 and that glutamate catabolism via this pathway may play an important role in the symbiotic nitrogen fixation process.