FNR is a transcriptional regulator which controls the expression of target genes in response to anoxia in Escherichia coli. The mechanism by which FNR senses and responds to anaerobiosis is unknown but indirect evidence suggests that an iron cofactor is involved. Using KMnO4 as a probe for DNA melting at active promoters, footprinting studies have now shown that the ferrous iron chelator, ferrozine, inhibits open complex formation in vivo, and that FNR with a high iron-content is essential for open complex formation in vitro. Since open complex formation is an essential pre-requisite for transcription, it is concluded that transcriptional activation by FNR is mediated by a ferrous iron cofactor.
