<p>The nitrate reductase enzyme complex allows bacteria to use nitrate as an electron acceptor during anaerobic growth. The enzyme complex consists of a tetramer that has an alpha, beta and 2 gamma subunits. The alpha and beta subunits have catalytic activity and the gamma subunits attach the enzyme to the membrane and are b-type cytochromes that receive electrons from the quinone pool and transfers them to the beta subunit. The sequences in this family are the beta subunit for nitrate reductase I (narH) and nitrate reductase II (narY) for Gram-positive and Gram-negative bacteria. A few thermophiles and archaea also match the model. A number of the sequences in this set are experimentally characterised, these include: E.Coli NarH (<db_xref db="SWISSPROT" dbkey="P11349"/>) and NarY (<db_xref db="SWISSPROT" dbkey="P19318"/>) [<cite idref="PUB00008262"/>, <cite idref="PUB00009746"/>], <db_xref db="SWISSPROT" dbkey="P42176"/> from <taxon tax_id="1423">Bacillus subtilis</taxon>, and related proteins from <taxon tax_id="294">Pseudomonas fluorescens</taxon>, <taxon tax_id="266">Paracoccus denitrificans</taxon>, and <taxon tax_id="28252">Paracoccus halodenitrificans</taxon> (Halomonas halodenitrificans).</p>