Hill S, Kennedy C, Kavanagh E, Goldberg R B, Hanau R
Nature. 1981 Apr 2;290(5805):424-6. doi: 10.1038/290424a0.
The enzyme complex nitrogenase, which reduces N2 to NH+4, 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 (approximately 16 ATPs per N2). In both aerobic and facultative anaerobic N2-fixing bacteria, nitrogenase synthesis is regulated by O2 and NH+4, but in the aerobes there are also processes to protect the enzyme from O2 damage. The mechanisms of repression by O2 and NH+4 seem to be independent in the organisms so far examined. In the facultative anaerobe, Klebsiella pneumoniae, O2 was shown to repress nitrogenase synthesis in an NH+4-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, reported here, indicates the involvement of a nif-regulatory gene product in the mechanism of O2 control of nitrogenase synthesis.
将N₂还原为NH₄⁺的酶复合物固氮酶涉及两种氧化还原蛋白，二者都会被O₂不可逆地破坏（参考文献1）。因此，酶活性需要厌氧条件、还原剂来源和大量ATP（每还原一分子N₂大约需要16个ATP）。在需氧和兼性厌氧的固氮细菌中，固氮酶的合成受O₂和NH₄⁺的调控，但在需氧菌中也存在保护酶免受O₂破坏的机制。在目前所研究的生物体中，O₂和NH₄⁺的抑制机制似乎是相互独立的。在兼性厌氧菌肺炎克雷伯菌中，O₂被证明能抑制NH₄⁺组成型菌株中固氮酶的合成。将大肠杆菌lacZ基因融合到肺炎克雷伯菌固氮（nif）基因簇的每个转录单元中，便于进行O₂相关研究，因为来自各种nif启动子的表达会产生一种对O₂稳定的产物（β-半乳糖苷酶）。值得注意的是，nifHDK操纵子（固氮酶结构基因）比nifLA操纵子（调控基因）对O₂抑制更敏感。本文报道的突变体特征表明，一种nif调控基因产物参与了O₂对固氮酶合成的控制机制。
