The mechanism of oxygen sensing by the global transcriptional regulator FNR

细菌几乎存在于地球上的每一个环境中，包括一些非常恶劣的环境，以至于许多其他形式的生命无法生存。这一成功至少部分归功于细菌适应环境变化的能力，而这种适应植根于它们对外部和内部线索做出反应而改变基因表达模式的能力。许多细菌监测的一个关键环境参数是氧气浓度。我们特别感兴趣的是细菌大肠杆菌(E.coli)。它的显著特性之一是在有氧和无氧的情况下都能茁壮成长。要做到这一点，它必须极大地改变自己的新陈代谢，但这会产生后果，因为与有氧气存在时相比，没有氧气，能量守恒和生长的潜力是有限的。为了测试是否存在氧气，大肠杆菌使用了一种名为FNR的蛋白质，它起到了氧气传感器的作用。它有一种特殊的辅助因子，称为铁-硫簇，它与氧气发生反应，关闭FNR。在“关闭状态”(没有铁-硫簇)，FNR不能与DNA结合来激活在无氧生长过程中使用的基因的表达。当没有氧气时，铁硫仍然与FNR结合，因此蛋白质可以与DNA结合，并激活在没有氧气的情况下生长所需的基因的表达。在过去的几年里，我们研究了氧与FNR铁硫簇合物的反应。这些研究揭示了反应的复杂生物化学，以及这是如何使FNR蛋白成为一种极其敏感的氧传感器的。然而，我们的研究提出了更多的问题。我们现在建议通过确定氧与FNR铁硫簇反应的分子编排以及这如何导致DNA结合性质的改变来解决其中的一些问题。我们还将调查细胞如何管理潜在的毒性反应产物，以及为什么一些细菌有不止一种FNR蛋白；是否不同的FNR蛋白已经进化成在不同的氧浓度范围内工作？这项工作在几个层面上都很重要。它将使我们更多地了解一个基本的生物过程，这个过程是许多细菌病原体的毒力特性的基础，如沙门氏菌和大肠杆菌。它将提供新的洞察力和对在厌氧世界中进化的一类古老蛋白质的更深层次的理解。它对于信号感知和信号转导如何在无处不在的细菌基因调控家族中联系在一起具有更广泛的意义。最后，它提供了设计分子开关的可能性，这种开关将对生物技术中使用的氧气水平的变化做出反应。

Bacteria inhabit almost every environmental niche on Earth, including some that are so harsh that many other forms of life cannot survive. This success is at least in part due to the ability of bacteria to adapt to changes in environment, and this adaptation is rooted in their capacity to alter patterns of gene expression in response to external and internal cues. A key environmental parameter that is monitored by many bacteria is oxygen concentration. We are particularly interested in the bacterium Escherichia coli (E. coli). One of its remarkable properties is that it is able to thrive both in the presence and absence of oxygen. To do this it has to dramatically alter its metabolism, but this has consequences because without oxygen the potential for energy conservation and growth are limited compared to when oxygen is present. To test whether oxygen is present E. coli uses a protein called FNR, which acts as an oxygen sensor. It has a special co-factor called an iron-sulfur cluster that reacts with oxygen in a way that switches FNR off. In the 'off-state' (no iron-sulfur cluster) FNR cannot bind to DNA to activate expression of genes that are used during growth in the absence of oxygen. When there is no oxygen the iron-sulfur remains associated with FNR and so the protein can bind to DNA and activate expression of genes that are needed for growth in the absence of oxygen. For the last few years we have studied the reaction of oxygen with the FNR iron-sulfur cluster. These studies have revealed the complex biochemistry of the reaction and how this makes the FNR protein an exquisitely sensitive oxygen sensor. However, our studies have raised even more questions. We now propose to address some of these by determining the molecular choreography of the reaction of oxygen with the FNR iron-sulfur cluster and how this leads to altered DNA-binding properties. We will also investigate how the potentially toxic reaction products are managed by the cell, and why some bacteria have more than one FNR protein; could it be that different FNR proteins have evolved to operate in different ranges of oxygen concentration? The work is important at several levels. It will allow us to understand more about a basic biological process that is fundamental for the virulent properties of many bacterial pathogens, such as Salmonella and E. coli. It will provide new insight and a deeper understanding of an ancient class of proteins that evolved in an anaerobic world. It has wider significance for how signal perception and signal transduction are linked in a ubiquitous family of bacterial gene regulators. And finally, it offers possibilities of designing molecular switches that will respond to changes in oxygen levels for use in biotechnology.