Nitrogen fixation gene (nifL) involved in oxygen regulation of nitrogenase synthesis in K. peumoniae

Abstract

The enzyme complex nitrogenase, which reduces N₂ to NH₄⁺, involves two redox proteins, both irreversibly damaged by O₂ (ref. 1). Enzyme activity therefore requires anaerobic conditions, a source of reductant and a large amount of ATP (∼16 ATPs per N₂)^2,3. In both aerobic and facultative anaerobic N₂-fixing bacteria, nitrogenase synthesis is regulated by O₂ and NH₄⁺, but in the aerobes there are also processes to protect the enzyme from O₂ damage^4,5. The mechanisms of repression by O₂ and NH₄⁺ seem to be independent in the organisms so far examined^6–8. In the facultative anaerobe, Klebsiella pneumoniae, O₂ was shown to repress nitrogenase synthesis in an NH₄⁺-constitutive strain⁸. The fusion of the Escherichia coli lacZ gene into each transcriptional unit of the nitrogen fixation (nif) gene cluster in K. pneumoniae has facilitated studies with O₂, because expression from the various nif promoters results in an O₂-stable product (β-galactosidase). Notably, the nifHDK operon (the nitrogenase structural genes) was more sensitive to O₂ repression than the nifLA operon (regulatory genes)⁹. The characterization of mutants, reported here, indicates the involvement of a nif-regulatory gene product in the mechanism of O₂ control of nitrogenase synthesis.