Regulation of the mdh-sucCDAB operon in Rhizobium leguminosarum

The malate dehydrogenase gene, mdh, from Rhizobium leguminosarum encodes a protein with a molecular weight of 33 590 that associates as a homotetramer. It is a lactate dehydrogenase-like malate dehydrogenase that most closely resembles the protein from the colonial green alga Botryococcus braunii. Malate dehydrogenase from R. leguminosarum has a Km of 74 μM and a Vmax of 822 μmol min-1 mg-1 protein for oxaloacetate, while the Km for malate is 3.6 mM and the Vmax 62 μmol min-1 mg-1 protein. Downstream of mdh are sucCDAB, which encode succinyl-CoA synthetase (sucCD) and the El and E2 components of the α-ketoglutarate dehydrogenase complex (sucAB). Complementation and LacZ fusion analysis indicates that there is a common promoter for the mdh-suc genes. Mutation of downstream genes in the mdh-suc operon increases transcription 7-fold from the mdh promoter. The transcriptional coupling of mdh and sucCDAB is likely to be important in the regulation of carbon and nitrogen metabolism in bacteroids.