Chelates iron to the siroheme precursor. 2 H(+) + siroheme = Fe(2+) + sirohydrochlorin Porphyrin-containing compound metabolism; siroheme biosynthesis; siroheme from sirohydrochlorin: step 1/1. Up-regulated by sulfur starvation and repressed by cysteine. Also induced by O-acetyl-L-serine (OAS), a direct precursor of cysteine, maybe via inactivation of a putative transcriptional repressor of the cysH operon whose activity is controlled by the intracellular levels of OAS. Belongs to the CbiX family. SirB subfamily.