Acts as a transcriptional regulator. Probably redox-responsive. The apo- but not holo-form probably binds DNA (By similarity). Complements a whiB2 disruption mutant in M.smegmatis (AC Q9S426). The apo-form functions as a chaperone, preventing aggregation or helping in correct refolding of a number of substrates. Binds 1 [4Fe-4S] cluster per subunit. Following nitrosylation of the [4Fe-4S] cluster binds 1 [4Fe-8(NO)] cluster per subunit. Steady during exponential growth (at protein level), decreases as cells age in stationary phase. About 2-fold induced by cell-wall active agents cycloserine, ethambutol and isoniazid, 5-fold by starvation. Repressed by heat shock and SDS. Slightly repressed in hypoxia and in macrophage and mouse infection, slightly induced by NO and cAMP. The Fe-S cluster can be nitrosylated by nitric oxide (NO). Upon Fe-S cluster removal intramolecular disulfide bonds are formed. Probably essential; depletion experiments (by expressing antisense RNA) show smaller colonies are formed, while individual cells are elongated and/or filamented. Colonies are glossy and spherical, cells are less viable. Belongs to the WhiB family. Extended N-terminus.