Acts as a transcriptional regulator. Probably redox-responsive. The apo- but not holo-form probably binds DNA (By similarity). Participates in maintaining a reduced cytoplasmic (MSH/MSSM) environment under normal growth conditions and directly or indirectly controls the concentration of mycothiol (MSH + MSSM). Binds 1 [4Fe-4S] cluster per subunit. Following nitrosylation of the [4Fe-4S] cluster binds 1 [4Fe-8(NO)] cluster per subunit. By a considerable number of antibiotics many of which perturb respiration, redox balance, or transmembrane ion flux, including erythromycin. Much stronger induction by erythromycin plus the reducing agent DTT but not erythromycin plus the oxidizing agent diamide. Positively regulates its own expression. The Fe-S cluster can be nitrosylated by nitric oxide (NO). Upon Fe-S cluster removal intramolecular disulfide bonds are formed. Not essential. Cells have a 5-X decrease in mycothiol reduced form (MSH)/ mycothiol oxidized disulfide form (MSSM), an indicator of a more oxidzed cytoplasm. Belongs to the WhiB family. Extended N-terminus.