CHIMERIC STRUCTURE OF THE NAD(P)(- AND NADP(+)-DEPENDENT MALIC ENZYMES OF RHIZOBIUM (SINORHIZOBIUM) MELILOTI())

Malic enzymes catalyze the oxidative decarboxylation of malate to pyru vate in conjunction with the reduction of a nicotinamide cofactor. We determined the DNA sequence and transcriptional start sites of the gen es encoding the diphosphopyridine nucleotide-dependent malic enzyme (D ME, EC 1.1.1.39) and the triphosphopyridine nucleotide-dependent malic enzyme (TME, EC 1.1.1.40) of Rhizobium (Sinorhizobium) meliloti. The predicted DME and TME proteins contain 770 and 764 amino acids, respec tively, and are approximately 320 amino acids larger than previously c haracterized prokaryotic malic enzymes. The increased size of DME and TME resides in the C-terminal extensions which are similar in sequence to phosphotransacetylase enzymes (EC 2.3.1.8). Modified DME and TME p roteins which lack this C-terminal region retain malic enzyme activity but are unable to oligomerize into the native state. Data base search es have revealed that similar chimeric malic enzymes were uniquely pre sent in Gram-negative bacteria. Thus DME and TME appear to be members of a new class of melic enzyme characterized by the presence of a phos photransacetylase-like domain at the C terminus of the protein.