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

^a Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, and Department of Chemistry, University of California at San Diego, La Jolla California 92037

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 NH₄⁺. One strain, KP5069, produces 30% as much nitrogenase when grown in the presence of 15 mM NH₄⁺ 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.

¹ Present address: Department of Biology, Harvard University, Cambridge, Mass.

² Present address: Department of Biology, Temple University, Philadelphia, Pa. 19122.

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