A redox-sensitive transcriptional regulator. Maintains intracellular redox homeostasis by regulating catabolic metabolism and polyketide biosynthesis. Regulates expression of the redox buffer ergothioneine (ERG). In concert with myothiol (MSH), another redox buffer, responds to low pH leading to acid resistance. Senses changes in the intracellular redox state and helps mediate a metabolic switchover to preferred in vivo carbon sources (fatty acids). The apo- but not holo-form probably binds DNA (By similarity). Plays a role in virulence (PubMed:11880648). Binds 1 [4Fe-4S] cluster per subunit. Following nitrosylation of the [4Fe-4S] cluster binds 1 [4Fe-8(NO)] cluster per subunit. The Fe-S cluster can be nitrosylated by nitric oxide (NO). Upon Fe-S cluster removal intramolecular disulfide bonds are formed. Not essential for growth in culture, however required for growth in vivo in guinea pig infections. Note strain ATCC 35723 is virulent whereas ATCC BAA-935 is not. Belongs to the WhiB family.