Tricarboxylic acid cycle and anaplerotic enzymes in rhizobia

Rhizobia are a diverse group of Gram-negative bacteria comprised of the genera Rhizobium, Bradyrhizobium, Mesorhizobium, Sinorhizobium and Azorhizobium. A unifying characteristic of the rhizobia is their capacity to reduce (fix) atmospheric nitrogen in symbiotic association with a compatible plant host. Symbiotic nitrogen fixation requires a substantial input of energy from the rhizobial symbiont. This review focuses on recent studies of rhizobial carbon metabolism which have demonstrated the importance of a functional tricarboxylic acid (TCA) cycle in allowing rhizobia to efficiently colonize the plant host and/or develop an effective nitrogen fixing symbiosis. Several anaplerotic pathways have also been shown to maintain TCA cycle activity under specific conditions. Biochemical and physiological characterization of carbon metabolic mutants, along with the analysis of cloned genes and their corresponding gene products, have greatly advanced our understanding of the function of enzymes such as citrate synthase, oxoglutarate dehydrogenase, pyruvate carboxylase and malic enzymes. However, much remains to be learned about the control and function of these and other key metabolic enzymes in rhizobia.