A new pathway for iron-sulfur cluster repair

点击翻译按钮获取中文摘要

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. This involves the conversion of the cluster from one form (called the [4Fe-4S] form) to another (called the [2Fe-2S] form). In the 'off-state' FNR is in its [2Fe-2S] form and 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 cluster remains in the [4Fe-4S] form and 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. We recently showed that in its 'off-state' the [2Fe-2S] cluster is different from most other previously characterised [2Fe-2S] cluster cofactors and that this is a result of the fact that it is formed through oxidative conversion (which can be thought of as oxidative damage) of the [4Fe-4S] form. This results in an unusual modification of the protein in which sulfide from the initial [4Fe-4S] cluster is attached to the protein during the conversion reaction. This can be thought of as a form of stored sulfur and, remarkably, this can be used to repair the cluster back to its original [4Fe-4S] form when oxygen is no longer present. The unusual [2Fe-2S] cluster of FNR has also been discovered in other iron-sulfur cluster proteins, raising the possibility that repair of oxidatively damaged [4Fe-4S] clusters via this mechanism might be widespread.Using a wide range of approaches, we propose to investigate further the formation of the unusual [2Fe-2S] cluster and the repair of [4Fe-4S] cluster to determine the mechanism and the importance of this in vivo as a previously unrecognised pathway for repair of oxidatively damaged iron-sulfur clusters.