nrfA

nrfA is the gene encoding the catalytic subunit of the periplasmic nitrite reductase NrfA, a multiheme c-type cytochrome that drives dissimilatory nitrate reduction to ammonium (DNRA) in many anaerobic and microaerophilic bacteria. In DNRA, nitrite is reduced to ammonium in the periplasm, with electrons supplied from membrane-associated electron carriers such as NrfH. The nrfA gene is typically part of a gene cluster that includes other components of the nitrite reduction system and related electron-transfer proteins.

Structural and biochemical features of NrfA include its localization in the periplasm and its status as a multiheme cytochrome. The protein coordinates several heme cofactors through characteristic CXXCH heme-binding motifs, enabling sequential electron transfer within the protein to reduce nitrite to ammonium. The enzyme accepts electrons from the external electron transport chain and uses protons from the cytoplasm to convert nitrite to ammonium, generally described by the overall reaction: nitrite + 6 electrons + 8 protons → ammonium + 2 water.

Genetic and ecological context: nrfA is found in a variety of nitrate-reducing bacteria, including members of the genera Shewanella, Escherichia, and related anaerobic lineages. Its presence is commonly used as a genetic marker for DNRA potential in environmental and metagenomic studies. Expression of nrfA is typically inducible under anaerobic or microaerophilic conditions with available nitrite or nitrate, linking nitrogen retention in ecosystems to microbial electron transport processes.

Functional role: by reducing nitrite to ammonium rather than releasing nitrogen as N2 via denitrification, NrfA contributes to nitrogen retention in ecosystems and influences the balance between DNRA and denitrification pathways in various environments.