Molecular biology of cytochromes involved in ammonia oxidation and electron transport in the nitrifying bacterium Nitrosomonas europaea.

摘要

In this study, the molecular biology of proteins involved in ammonia oxidation and electron transport in a chemoautotrophic bacterium, Nitrosomonas europaea, was examined. N. europaea obtains energy by oxidizing ammonia to nitrite. Ammonia is oxidized to hydroxylamine by ammonia monoxygenase (AMO), and hydroxylamine is oxidized to nitrite in a reaction catalyzed by hydroxymine oxidoreductase (HAO), and, to a lesser degree, by cytochrome P460. Electrons produced by hydroxylame oxidation are transported by cytochrome c554 either back to AMO or through an electron transport chain to cytochrome c552 and a terminal oxidase.;The 3;The genes encoding HAO (hao) and cytochrome c554 (cycA) are in a gene cluster present in three genomic copies. The gene encoding a tetraheme c-cytochrome (cycB) is present in two copies of the gene cluster, in the same operon as cycA. Cytochrome c554 shares no sequence homology with any known protein, but cycB shares homology with several multiheme c-cytochromes in other bacteria which are involved in anaerobic respiratory chains using TMAO, nitrate, or nitrite as terminal electron acceptors. The product of cycB is cytochrome was shown to be the membrane bound, low potential, cytochrome ;The gene encoding cytochrome P460 (cyp) was cloned and sequenced. Although cytochrome P460 has spectral features similar to HAO, no homology exists between the primary structures of cytochrome P460 and HAO. Cytochrome P460 has a single c-heme binding motif, and a novel covalent linkage between the heme and a lysine residue. The cyp gene was expressed in E. coli, but neither periplasmic export nor heme attatchment were observed.;The gene, cyt, encoding cytochrome c552 was cloned and sequenced. The cytochrome has unique features not found in other members of the cytochrome c551 family, including a valine residue at position 65.