Hybrid Structures of Malic Enzymes from Rhizobium meliloti

C4 dicarboxylic acids such as succinate and malate are believed to be the major carbon sources utilized by Rhizobium species during the symbiotic association with their plant hosts. These carbon sources are oxidized via the TCA cycle to produce the necessary ATP and reducing power for conversion of atmospheric nitrogen to ammonia. Since oxaloacetate and acetyl-CoA are essential for the formation of citrate to maintain the TCA cycle, a method must exist within the bacteroid to produce these essential components from the nutrients donated by the plant host. Malic enzymes convert malate to pyruvate while pyruvate dehydrogenase converts pyruvate to acetyl-CoA. Previous studies in this laboratory have shown that Rhizobium meliloti contains two distinct malic enzymes, one dependent on NADP+ (TME) (Driscoll, Finan, 1996) while the second preferentially uses NAD+ (DME) (Driscoll, Finan, 1993) as co-factor. Mutants lacking these proteins have been produced and the resulting symbiotic phenotypes show that dme is necessary for nitrogen fixation while tme is not. The dme and tme genes (Driscoll, Finan, 1997) were sequenced, and the DME and TME proteins were overproduced and purified to homogeneity from an Escherichia coli background in order to study their structure and enzymatic characteristics.