Acts as a transcriptional regulator. Probably redox-responsive. The apo- but not holo-form probably binds DNA (By similarity). The apo-form functions as a chaperone, preventing aggregation or helping in correct refolding of a number of substrates; this activity does not require ATP or the ability to bind a Fe-S cluster. Chaperone activity is insensitive to the redox state of its cysteine residues. The apo-form has no protein disulfide reductase activity. The apo-form binds to its own promoter. Binds 1 [4Fe-4S] cluster per subunit. Contains 1 [2Fe-2S] cluster after reconstitution of overexpressed protein from E.coli. Following nitrosylation of the [4Fe-4S] cluster binds 1 [4Fe-8(NO)] cluster per subunit. May be phosphorylated, possibly on Ser-42. The cluster is degraded quickly in the presence of air. Upon cluster removal intramolecular disulfide bonds are formed. The Fe-S cluster can be nitrosylated by nitric oxide (NO). Fully oxidized recombinant protein tagged at both termini. Fully reduced recombinant protein tagged at both termini. Belongs to the WhiB family.