NirF is a periplasmic protein that binds d₁ heme as part of its essential role in d₁ heme biogenesis

The cytochrome cd₁ nitrite reductase from Paracoccus pantotrophus catalyses the one electron reduction of nitrite to nitric oxide using two heme cofactors. The site of nitrite reduction is the d₁ heme, which is synthesized under anaerobic conditions by using nirECFD-LGHJN gene products. In vivo studies with an unmarked deletion strain, ΔnirF, showed that this gene is essential for cd₁ assembly and consequently for denitrification, which was restored when the ΔnirF strain was complemented with wild-type, plasmid-borne, nirF. Removal of a signal sequence and deletion of a conserved N-terminal Gly-rich motif from the NirF coded on a plasmid resulted in loss of in vivo NirF activity. We demonstrate here that the product of the nirF gene is a periplasmic protein and, hence, must be involved in a late stage of the cofactor biosynthesis. In vitro studies with purified NirF established that it could bind d₁ heme. It is concluded that His41 of NirF, which aligns with His200 of the d₁ heme domain of cd₁, is essential both for this binding and for the production of d₁ heme; replacement of His41 by Ala, Cys, Lys and Met all gave nonfunctional proteins. Potential functions of NirF are discussed.