NAD(+)-dependent malic enzyme of Rhizobium meliloti is required for symbiotic nitrogen fixation.
DEAE-cellulose chromatography of extracts of free-living Rhizobium meliloti cells revealed separate NAD(+)-dependent and NADP(+)-dependent malic enzyme activities. The NAD+ malic enzyme exhibited more activity with NAD+ as cofactor, but also showed some activity with NADP+. The NADP+ malic enzyme only showed activity when NADP+ was supplied as cofactor. Three independent transposon-induced mutants of R. meliloti which lacked NAD+ malic enzyme activity (dme-) but retained NADP+ malic enzyme activity were isolated. In an otherwise wild-type background, the dme mutations did not alter the carbon utilization phenotype; however, nodules induced by these mutants failed to fix N2. Structurally, these nodules appeared to develop like wild-type nodules up to the stage where N2-fixation would normally begin. These results support the proposal that NAD+ malic enzyme, together with pyruvate dehydrogenase, functions in the generation of acetyl-CoA required for TCA cycle function in N2-fixing bacteroids which metabolize C4-dicarboxylic acids supplied by the plant.
Department of Biology, McMaster University, Hamilton, Ontario, Canada.
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D001426:Bacterial Proteins / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D002848:Chromatography, DEAE-Cellulose; D004251:DNA Transposable Elements; D008291:Malate Dehydrogenase / Q000145:classification / Q000235:genetics / Q000302:isolation & purification / Q000502:physiology*; D000455:Medicago sativa / Q000382:microbiology; D016254:Mutagenesis, Insertional; D009243:NAD / Q000502:physiology*; D009249:NADP / Q000502:physiology; D009586:Nitrogen Fixation; D016962:Sinorhizobium meliloti / Q000201:enzymology / Q000235:genetics / Q000502:physiology*; D013559:Symbiosis
D001426:Bacterial Proteins; D004251:DNA Transposable Elements; D009243:NAD; D009249:NADP; D008291:Malate Dehydrogenase