NAD<sup>+</sup> ‐dependent malic enzyme of <i>Rhizobium meliloti</i> is required for symbiotic nitrogen fixation

SummaryDEAE‐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 NADP+ 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.

NAD⁺ ‐dependent malic enzyme of Rhizobium meliloti is required for symbiotic nitrogen fixation Journal Articles