The NAD+-dependent malic enzyme (DME) and the NADP+-dependent malic enzyme (TME) of
Sinorhizobium melilotiare representatives of a distinct class of malic enzymes that contain a 440-amino-acid N-terminal region homologous to other malic enzymes and a 330-amino-acid C-terminal region with similarity to phosphotransacetylase enzymes (PTA). We have shown previously that dmemutants of S. melilotifail to fix N2 (Fix−) in alfalfa root nodules, whereas tmemutants are unimpaired in their N2-fixing ability (Fix+). Here we report that the amount of DME protein in bacteroids is 10 times greater than that of TME. We therefore investigated whether increased TME activity in nodules would allow TME to function in place of DME. The tmegene was placed under the control of the dmepromoter, and despite elevated levels of TME within bacteroids, no symbiotic nitrogen fixation occurred in dmemutant strains. Conversely, expression of dmefrom the tmepromoter resulted in a large reduction in DME activity and symbiotic N2 fixation. Hence, TME cannot replace the symbiotic requirement for DME. In further experiments we investigated the DME PTA-like domain and showed that it is not required for N2 fixation. Thus, expression of a DME C-terminal deletion derivative or the Escherichia coliNAD+-dependent malic enzyme ( sfcA), both of which lack the PTA-like region, restored wild-type N2 fixation to a dmemutant. Our results have defined the symbiotic requirements for malic enzyme and raise the possibility that a constant high ratio of NADPH + H+ to NADP in nitrogen-fixing bacteroids prevents TME from functioning in N2-fixing bacteroids.