Regulation and expression of the mdh-sucCDAB operon of Sinorhizobium meliloti

The genes encoding malate dehydrogenase (mdh), succinyl-CoA synthetase (sucCD), and subunits of 2-oxoglutarate dehydrogenase (sucAB) constitute an operon in the order mdh-sucCDAB in Sinorhizobium meliloti. Regulation of the operon was studied using beta-galactosidase gene fusions. Expression of the operon was assayed in response to the carbon source provided, and over the growth of the culture. A promoter upstream of the mdh gene was identified, and although the promoter was active in S. meliloti it was not expressed in Escherichia coli. It was demonstrated that the role of 2-oxoglutarate dehydrogenase (OGD) is minimal in symbiosis, as nodules with no OGD activity formed nodules able to fix nitrogen. Alfalfa plants inoculated with strains of S. meliloti carrying extra-chromosomal copies of the mdh gene did not show any increase in shoot dry weight compared to plants inoculated with the wild-type strain.