The Escherichia coli cysG gene encodes the multifunctional protein, siroheme synthase

Previously, the E. coli cysG gene product had been shown to sequentially methylate uro'gen III to produce precorrin‐2, hence it was given the trivial name uro'gen III methylase. We now report that in addition to methylase activity, the CysG protein catalyses both the NAD + dependent oxidation of precorrin‐2 to sirohydrochlorin, but also the insertion of iron into this oxidized intermediate, thereby producing siroheme. Thus CysG is a multifunctional protein solely responsible for siroheme synthesis from uro'gen III in E. coli , and accordingly is renamed siroheme synthase.