Glutamine synthetase and control of nitrogen fixation in Rhizobium

Rhizobium , a bacterium which reduces dinitrogen to ammonia in symbiotic association with leguminous plants, ‘fixes’ nitrogen while living free 1–4 . Unfortunately, free-living fixation seems to occur only at the end of exponential growth, and no condition is known in which bacterial growth is dependent on reduction of dinitrogen by nitrogenase (EC 1.7.99.2). Therefore a molecular biological analysis must be indirect. In the unrelated Klebsiella pneumoniae , where fixation does occur in culture, nitrogenase expression is regulated by the enzyme glutamine synthetase (GS, EC 6.3.1.2). By analogy, we have isolated GS auxotrophs in Rhizobium and screened for nitrogenase activity. We chose Rhizobium cowpea 32H1 because it has the highest reported nitrogenase activity, both in culture and in nodules 5 . We found that a glutamine-dependent auxotroph of Rhizobium cowpea 32H1, deficient in GS activity, is deficient in nitrogenase activity as well. Therefore GS seems to be involved in the control of nitrogenase expression by Rhizobium .