The disruption of a gene encoding a putative arylesterase impairs pyruvatedehydrogenase complex activity and nitrogen fixation in Sinorhizobium meliloti

Nitrogen-fixing Sinorhizobium meliloti cells depend upon dicarboxylic acids as carbon and energy sources. The metabolism of these intermediate compoun ds of the trichloroacetic acid cycle is dependent upon the availability of acetyl-coenzyme A (CoA), In bacteroids, the combined activities of malic en zymes and pyruvate dehydrogenase (PDH) have been proposed to be responsible for the anaplerotic synthesis of acetyl-CoA. We obtained a S. meliloti mut ant strain, PD3, in which a Tn5 insertion led to a significant decrease in the overall PDH activity. The genetic characterization of this mutant revea led that the transposon is located at the 3' end of a gene (ada) encoding a putative arylesterase. The mutant PD3 is deficient in nitrogen fixation, w hich strengthens the physiological importance of PDH activity ill the symbi osis of S. meliloti with alfalfa plants.