Regulation of nitrogen fixation. Nitrogenase-derepressed mutants of Klebsiella pneumoniae

Abstract

1. A new procedure is described for selecting nitrogenase-derepressed mutants based on the method of Brenchley et al. (Brenchley, J. E., Prival, M. J. and Magasanik, B. (1973) J. Biol. Chem. 248, 6122–6128) for isolating histidase-constitutive mutants of a non-N 2-fixing bacterium. 2. Nitrogenase levels of the new mutants in the presence of NH 4 + were as high as 100% of the nitrogenase activity detected in the absence of NH 4 +. 3. Biochemical characterization of these nitrogen fixation ( nif) derepressed mutants reveals that they fall into three classes. Three mutants (strains SK-24, 28 and 29), requiring glutamate for growth, synthesize nitrogenase and glutamine synthetase constitutively (in the presence of NH 4 +). A second class of mutants (strains SK-27 and 37) requiring glutamine for growth produces derepressed levels of nitrogenase activity and synthesized catalytically inactive glutamine synthetase protein, as determined immunologically. A third class of glutamine-requiring, nitrogenase-derepressed mutants (strain SK-25 and 26) synthesizes neither a catalytically active glutamine synthetase enzyme nor an immunologically cross-reactive glutamine synthetase protein. 4. F-prime complementation analysis reveals that the mutant strains SK-25, 26, 27, 37 map in a segment of the Klebsiella chromosome corresponding to the region coding for glutamine synthetase. Since the mutant strains SK-27 and SK-37 produce inactive glutamine synthetase protein, it is concluded that these mutations map within the glutamine synthetase structural gene.