From genes to function: regulation, maturation, and evolution of cytochrome nitrite reductase in nitrate reduction to ammonium.

The dissimilatory nitrate reduction to ammonium pathway converts nitrate to ammonium, a vital reaction in the global nitrogen cycle. The second step of the pathway is performed by cytochrome nitrite reductase (NrfA), a soluble, periplasmic cytochrome responsible for the reduction of nitrite to ammonium. The pentaheme NrfA catalyzes this six-electron and eight-proton reduction of nitrite at a single active site with the help of its quinol oxidase partners. In this review, we discuss our current understanding of (i) the structure, homology, and evolution of both NrfA and its redox partners, (ii) the regulation of the operon, and (iii) the maturation of NrfA proteins via unique cytochrome maturation pathways.