Osmoregulation in Rhizobium meliloti: characterization of enzymes involved in glutamate synthesis.

Rhizobium meliloti, like many bacteria, accumulates elevated levels of glutamate when osmotically stressed. The biochemical basis for this increase in glutamate production was investigated. Enzymes involved in glutamate synthesis, including glutamine synthetase, glutamate synthase, and glutamic dehydrogenase, were characterized in dialyzed crude cell-free extracts. A transaminase activity, which uses branched chain amino acids for the amination of 2-ketoglutaric acid, was also characterized. With the exception of glutamic dehydrogenase, the specific activity of the enzymes did not vary more than 4-fold in response to the available source of nitrogen or supplemental glutamate. Glutamic dehydrogenase activity was 13-fold greater when cells grew with 10 mM than when cells grew with 0.5 mM . Glutamate synthase was repressed 2-fold when cells grew with supplemental glutamate. Conversely, this enzyme was derepressed 2× when cells grew with 0.5 mM or nitrate. Growing cells in minimal defined medium with 400 mM N...