Regulation of Nitrogen Fixation in Klebsiella pneumoniae : Evidence for a Role of Glutamine Synthetase as a Regulator of Nitrogenase Synthesis

Abstract

Mutations causing constitutive synthesis of glutamine synthetase (GlnC − phenotype) were transferred from Klebsiella aerogenes into Klebsiella pneumoniae by P1-mediated transduction. Such GlnC − strains of K. pneumoniae have constitutive levels of glutamine synthetase. Two of three GlnC − strains of K. pneumoniae studied, each containing independently isolated mutations that confer the GlnC − phenotype, continue to synthesize nitrogenase in the presence of NH 4 + . One strain, KP5069, produces 30% as much nitrogenase when grown in the presence of 15 mM NH 4 + as in its absence. The GlnC − phenotype allows the synthesis of nitrogenase to continue under conditions that completely repress nitrogenase synthesis in the wild-type strain. Glutamine auxotrophs of K. pneumoniae , that do not produce catalytically active glutamine synthetase, are unable to synthesize nitrogenase during nitrogen limited growth. Complementation of K. pneumoniae Gln − strains by an Escherichia coli episome (F′133) simultaneously restores glutamine synthetase activity and the ability to synthesize nitrogenase. These results indicate a role for glutamine synthetase as a positive control element for nitrogen fixation in K. pneumoniae .