Activity of nitrogenase and glutamine synthetase in relation to availability of oxygen in continuous cultures of a strain of cowpea Rhizobium sp. supplied with excess ammonium.

In samples from nitrogen-fixing continuous cultures of strain CB756 of the cowpea type rhizobia (Rhizobium sp.), newly fixed NH+4 is in equiblibrium with the medium, from where it is assimilated by the glutamine synthetase/glutamate synthase pathway. In samples from steady state cultures with different degrees of oxygen-limitation, nitrogenase activity was positively correlated with the biosynthetic of glutamine synthetase in cell free extracts. Also, activities in biosynthetic assays were positively correlated with activities in gamma-glutamyl transferase assays containing 60 mM Mg2+. Relative adenylylation of glutamine synthetase was conveniently measured in cell free extracts as the ratio of gamma-glutamyl transferase activities without and with addition of 60 mM Mg2+. Automatic control of oxygen supply was used to facilitate the study of transitions between steady-state continuous cultures with high and low nitrogenase activities. Adenylylation of glutamine synthetase and repression of nitrogenase activity in the presence of excess NH+4, were masked when oxygen strongly limited culture yield. Partial relief of the limitation in cultures supplied with 10 mM NH+4 produced early decline in nitrogenase activity and increase in relative adenylylation of glutamine synthetase. Decreased oxygen supply produced a rapid decline in relative adenylylation, followed by increased nitrogenase activity, supporting the concept that control of nitrogenase synthesis is modulated by glutamine synthetase adenylylation in these bacteria.