An redox-regulated anti-sigma factor for extracytoplasmic function (ECF) sigma factor SigH. ECF sigma factors are held in an inactive form by a cognate anti-sigma factor. RshA and some peptides derived from it inhibit the sigma factor activity of SigH. Probably releases SigH during oxidative stress. Binds 1 iron-sulfur cluster per subunit. As the iron-sulfur cluster is unstable, it is not clear from the contradictory experimental evidence whether it is 2Fe-2S, 4Fe-4S or something intermediate. Nor is it clear which other residues besides Cys-53 and Cys-56 are involved in metal binding (PubMed:22937074). Interacts (affinity=15 nM) 1:1 with cognate sigma factor SigH under reducing conditions; the complex is disrupted under oxiding conditions or as temperatures rise. Binding inhibits the interaction of SigH with the RNA polymerase catalytic core. By SigH, part of the sigH-rshA operon. Phosphorylated, probably by PknB. Phosphorylation decreases interaction with SigH, leading to increased SigH-mediated transcription. Belongs to the zinc-associated anti-sigma factor (ZAS) superfamily.