Glutamate and Β-Aminobutyrate Metabolism in Isolated Rhizobium meliloti Bacteroids. R. W. Miller. Plant Research Centre, Agriculture Canada, Ottawa, Ontario, Canada, K1A OC6. D. G. McRae(2), and K. Joy(2). (2)Department of Biology and Institute of Biochemistry, Carleton University, Ottawa, Ontario, Canada, K1S 5B6.. MPMI 4:37-45. Accepted 23 August 1990. Copyright 1991 Department of Agriculture, Government of Canada.

Amino acid levels of free-living Rhizobium meliloti cells were compared with those of undifferentiated nodule bacteria and mature bacteroids isolated and purified from alfalfa nodules. Glutamate and glutamine levels declined markedly during differentiation while alanine, glycine, and aspartate levels increased in bacteroids. Bacteroids purified anaerobically contained up to 40 nmoles of Β-aminobutyric acid (Gaba) per milligram of protein, while free-living cells lacked this compound. Anaerobiosis caused bacteroid Gaba levels to increase over those observed in bacteroids exposed to oxygen. Comparison of Gaba levels in the cytoplasm of bacteroids isolated anaerobically from several host-microsymbiont combinations showed that the R. meliloti-alfalfa symbiosis produced the highest levels of Gaba under these conditions. When succinate, fumarate, malate, or oxaloacetate was incubated with intact bacteroids under argon-oxygen or nitrogen-oxygen mixtures, glutamate levels increased threefold to nearly 30 nmoles per milligram of protein, while Gaba decreased to one fourth of the anaerobic level (40 nmoles/mg of protein). If oxygen was removed from respiring bacteroids by flushing with argon, the changes in glutamate and Gaba were reversed. Studies with the pyridoxal phosphate enzyme inhibitor, amino(oxy)acetate, indicated that Gaba was metabolized by at least two different pathways. One of these pathways involved transamination, and one required succinate plus oxygen and was independent of transamination. Enzymes necessary for the assimilation of ammonia by the glutamate and alanine dehydrogenase systems and for the formation of Gaba and succinic semialdehyde from glutamate had high activities in bacteroids. Transamination from glutamate, alanine, and Gaba was probably responsible for distribution of assimilated nitrogen within the bacteroids. Ammonia resulting from nitrogen fixation was found to be equilibrated between the bacteroids and cytosolic compartments exterior to the bacteroids within the time of bacteroid isolation.