Dixon R
Nitrogen Fixation Laboratory, John Innes Centre, Norwich NR4 7UH, UK.
Arch Microbiol. 1998 May;169(5):371-80. doi: 10.1007/s002030050585.
The high energetic requirements for nitrogen fixation and the extreme oxygen sensitivity of the nitrogenase enzyme impose physiological constraints on diazotrophy that necessitate stringent control of nitrogen fixation (nif) gene expression at the transcriptional level. In the gamma-subdivision of the Proteobacteria, this control is maintained by a regulatory complex comprising an enhancer-binding protein (NIFA), which activates transcription at sigmaN-dependent nif (nitrogen fixation) promoters, and a sensor protein (NIFL), which inhibits NIFA activity in response to fixed nitrogen and external concentrations of molecular oxygen. Inhibition of NIFA activity by NIFL apparently requires stoichiometric amounts of the two proteins, implying direct protein-protein interaction rather than catalytic modulation of NIFA activity. NIFL contains FAD as a prosthetic group and is a novel type of flavoprotein in which the oxidation state of the bound flavin acts as a molecular switch to control transcriptional activation by NIFA. The FAD-binding domain of NIFL contains a motif common to a large family of redox sensory proteins. In addition to its ability to act as a redox sensor, the activity of NIFL is also responsive to adenosine nucleotides, particularly ADP, suggesting that formation of the inhibitory complex might be regulated by the ATP/ ADP ratio. Proposed mechanisms for the inhibition of NIFA activity by NIFL are beginning to emerge.
固氮对能量的高需求以及固氮酶对氧气的极端敏感性给固氮作用带来了生理限制，这使得在转录水平上必须严格控制固氮（nif）基因的表达。在变形菌门的γ-亚纲中，这种控制由一个调控复合物维持，该复合物包括一个增强子结合蛋白（NIFA），它在依赖σN的nif（固氮）启动子处激活转录，以及一个传感蛋白（NIFL），它在响应固定氮和分子氧的外部浓度时抑制NIFA的活性。NIFL对NIFA活性的抑制显然需要这两种蛋白的化学计量比，这意味着是直接的蛋白质-蛋白质相互作用，而不是对NIFA活性的催化调节。NIFL含有FAD作为辅基，是一种新型黄素蛋白，其中结合黄素的氧化态作为分子开关来控制NIFA的转录激活。NIFL的FAD结合结构域包含一大类氧化还原传感蛋白共有的基序。除了能够作为氧化还原传感器外，NIFL的活性还对腺苷核苷酸，特别是ADP有反应，这表明抑制复合物的形成可能受ATP/ADP比值的调节。NIFL抑制NIFA活性的推测机制已开始出现。
