The unprecedented nos gene cluster of Wolinella succinogenes encodes a novel respiratory electron transfer pathway to cytochrome c nitrous oxide reductase

Abstract

The ε-proteobacterium Wolinella succinogenes grows anaerobically by respiratory nitrite ammonification but not by denitrification. Nevertheless, it is capable of N2O reduction to N2. Recently, the genome sequence of W. succinogenes revealed a nos gene cluster with intriguing features encoding a new type of N2O reductase. The predicted enzyme is similar to other N2O reductases exhibiting conservation of all residues ligating the two multinuclear copper centers but carries an unprecedented C-terminal monoheme cytochrome c domain. Notably, the N2O reductase pre-protein is synthesized with a Sec-dependent signal peptide, rather than the usually observed twin-arginine signal sequence, implying that the copper and heme cofactors are both incorporated in the periplasm. The nos gene cluster further consists of four adjacent open reading frames which are predicted to encode two monoheme c-type cytochromes as well as homologs of NapG and NapH. The latter proteins are thought to function in quinol oxidation coupled to cytochrome c reduction in electron transport to periplasmic nitrate reductase. While the accessory genes nosD, -F, -Y and -L are present in W. succinogenes, homologs of nosR and nosX are absent from the genome. We hypothesize that the nos gene cluster of W. succinogenes encodes a complete electron transport chain catalyzing N2O reduction by menaquinol, a pathway which might also be relevant to other bacteria.