The conversion of hydroxylamine to nitrite in nitrifying bacteria is catalysed by Hydroxylamine oxidoreductase (HAO). Hypothetical protein NE1300 from Nitrosomonas europaea seems to be a binding partner of HAO in vivo that may play a structural role in the ternary complex with cytochrome c554, the physiological electron acceptor of HAO. However, its exact function is not clear [1].

1: Structural studies of hydroxylamine oxidoreductase reveal a unique heme cofactor and a previously unidentified interaction partner. Cedervall P, Hooper AB, Wilmot CM; Biochemistry. 2013;52:6211-6218. PMID:23952581

2: Structural basis of biological NO generation by octaheme oxidoreductases. Maalcke WJ, Dietl A, Marritt SJ, Butt JN, Jetten MS, Keltjens JT, Barends TR, Kartal B; J Biol Chem. 2014;289:1228-1242. PMID:24302732