CLONING, SEQUENCING, AND OXYGEN REGULATION OF THE RHODOBACTER-CAPSULATUS ALPHA-KETOGLUTARATE DEHYDROGENASE OPERON

The Rhodobacter capsulatus sucA, sucB, acid lpd genes, which encode th e alpha-ketoglutarate dehydrogenase (E1o), the dihydrolipoamide succin yltransferase (E2o), and the dihydrolipoamide dehydrogenase (E3) compo nents of the alpha-ketoglutarate dehydrogenase complex (KGD), respecti vely, were cloned, sequenced, and used for regulatory analyses. The KG D enzymatic activity was greater in cells grown under aerobic, respira tory growth conditions than under anaerobic, photosynthetic conditions . Similarly, the sucA gene was transcribed differentially, leading to a greater accumulation of sucA mRNAs under respiratory growth conditio ns than under photosynthetic conditions, although differential rates o f mRNA decay could also contribute to the different amounts of sucA mR NAs under these two growth conditions. The sucA promoter was located a bout 4 kb upstream of the 5' end of the sucA gene, and transcripts gre ater than 9.5 kb hybridized to a sucA probe, suggesting the presence o f an operon that produces a polycistronic mRNA. Thus, these genes seem to be expressed as an unstable primary transcript, and we speculate t hat posttranscriptional processes control the stoichiometry of KGD pro teins.