, 2007), as in the nitrite-oxidizing bacteria (Poly et al., 2008 and Starkenburg et al., 2006). However, neither of the predicted Nxr amino acid sequences (alpha subunit, BgP_0139; beta subunit, BgP_4784) has any obvious matches in the BOGUAY genome. Nitrite reduction to nitric oxide has been demonstrated for the aldehyde

oxidoreductase of Desulfovibrio gigas ( Maia and Moura, 2011); whether similar enzymes are involved in respiratory pathways is (to our knowledge) unknown. A variant of the dissimilatory nitrite reduction to ammonia (DNRA) pathway has been suggested more recently, with the characterization of nitrite-reducing octaheme cytochromes from the Gammaproteobacteria Thioalkalivibrio nitratireducens and Shewanella oneidensis. The purified T. nitratireducens protein is able to reduce nitrite, hydroxylamine, and sulfite in vitro. It has a CXXCK motif for the fourth heme-binding site, the counterpart of the first site in the pentaheme cytochromes PD0325901 nmr ( Tikhonova et al., 2006). The periplasmic S. oneidensis enzyme was initially described as an octaheme tetrathionite reductase (OTR Mowat et al., 2004), but was subsequently shown to also have nitrite reductase, hydroxylamine reductase, and thiosulfate oxidation activity in vitro ( Atkinson et al., 2007). Nitrite and hydroxylamine were reduced to ammonia, with no detectable intermediates.

Heme Epacadostat mw 2 of this cytochrome has an unusual lysine ligand (identified in the crystal structure) outside of the heme-binding motif, which has the standard “CXXCH” sequence. These proteins are now referred to as “ONR”, for

“octaheme cytochrome c reductase”. The BOGUAY genome contains a possible gene for an octaheme cytochrome of the S. oneidensis type (01341_2386), which encodes a lysine residue in the correct position to serve as a Heme 2 ligand and cysteine and lysine residues corresponding to those forming a thiocyanate–lysine intramolecular crosslink in S. oneidensis ( Fig. 3). It is predicted by IMG/ER to be periplasmic, with an N-terminal signal peptide and a C-terminal transmembrane helix. Related sequences are found in other Proteobacteria, a selection of which is included in the figure. The genomic neighborhood is consistent with a reductase function, including ORFs encoding (from upstream to downstream) a possible cytochrome cd1 (01341_2385), DOK2 a TorD-like chaperone (01341_2384), a molybdopterin oxidoreductase (01341_2383), an FeS cluster-containing hydrogenase (01341_2382), and a cytochrome b (01341_2381) with some similarity to genes annotated as encoding formate dehydrogenase cytochrome b556 subunits. Immediately downstream of these genes there is a pair of ORFs similar to cyanobacterial fdxN excision elements XisH and XisI, discussed elsewhere ( MacGregor et al., 2013a). Putative genes for both central components of the cNOR type of bacterial nitric oxide reductase were found interior to two separate contigs.