Chimeric structure of the NAD(P)+- and NADP+-dependent malic enzymes of Rhizobium (Sinorhizobium) meliloti

Abstract

Malic enzymes catalyze the oxidative decarboxylation of malate to pyruvate in conjunction with the reduction of a nicotinamide cofactor. We determined the DNA sequence and transcriptional start sites of the genes encoding the diphosphopyridine nucleotide-dependent malic enzyme (DME, 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, respectively, and are approximately 320 amino acids larger than previously characterized 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 proteins which lack this C-terminal region retain malic enzyme activity but are unable to oligomerize into the native state. Data base searches have revealed that similar chimeric malic enzymes were uniquely present in Gram-negative bacteria. Thus DME and TME appear to be members of a new class of malic enzyme characterized by the presence of a phosphotransacetylase-like domain at the C terminus of the protein.