Acts as a transcriptional regulator. Probably redox-responsive. The apo- but not holo-form probably binds DNA (By similarity). Binds 1 [4Fe-4S] cluster per subunit. Following nitrosylation of the [4Fe-4S] cluster binds 1 [4Fe-8(NO)] cluster per subunit. Constitutively expressed during all growth phases. Repressed by osmotic stress, not responsive to a number of other stimuli. The Fe-S cluster can be nitrosylated by nitric oxide (NO). Upon Fe-S cluster removal intramolecular disulfide bonds are formed. Belongs to the WhiB family.