The Escherichia coli fumarate and nitrate reductase (FNR) regulator protein is an important transcriptional regulator that controls the expression of a large regulon of more than 100 genes in response to changes in oxygen availability. FNR is active when it acquires a [4Fe-4S]2+ cluster under anaerobic conditions. The presence of the [4Fe-4S]2+ cluster promotes protein dimerization and site-specific DNA binding, facilitating activation or repression of target promoters. Oxygen is sensed by the controlled disassembly of the [4Fe-4S]2+ cluster, ultimately resulting in inactive, monomeric, apo-FNR. The FNR [4Fe-4S]2+ cluster is also sensitive to nitric oxide, such that under anaerobic conditions the protein is inactivated by nitrosylation of the iron-sulfur cluster, yielding a mixture of monomeric and dimeric dinitrosyl-iron cysteine species. This chapter describes some of the methods used to produce active [4Fe-4S] FNR protein and investigates the reaction of the [4Fe-4S]2+ cluster with nitric oxide and oxygen in vitro.