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 2 redox proteins, both irreversibly damaged by O2. Enzyme activity thus requires anaerobic conditions, a source of reductant and a large amount of ATP (.apprx. 16 ATP/N2). In 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. The mechanisms of repression by O2 and NH4+ seem to be independent in the organisms so far examined. In the facultative anaerobe, K. pneumoniae, O2 repressed nitrogenase synthesis in an NH4+-constitutive strain. 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 (.beta.-galactosidase). Notably, the nifHDK operon (the nitrogenase structural genes) was more sensitive to O2 repression than the nifLA operon (regulatory genes). The characterization of mutants indicates the involvement of a nif-regulatory gene product in the mechanism of O2 control of nitrogenase synthesis.