Chelates iron to the siroheme precursor. Catalyzes the last step of the siroheme biosynthesis. Unlike its counterparts in bacteria, contains an [Fe-S] cluster which is not involved directly in the enzymatic reaction, but may play regulatory role in iron, sulfur and tetrapyrrole metabolism (PubMed:15545265, PubMed:22414210). The [Fe-S] cluster is required for normal plant growth (PubMed:22414210). 2 H(+) + siroheme = Fe(2+) + sirohydrochlorin Binds 2 [4Fe-4S] clusters per dimer. The [4Fe-4S] cluster quickly oxidizes to a [2Fe-2S] form in the presence of oxygen. Porphyrin-containing compound metabolism; siroheme biosynthesis; siroheme from sirohydrochlorin: step 1/1. Homodimer. Post-germination growth arrest. Belongs to the CbiX family. SirB subfamily.