Characterization of the NapGH quinol dehydrogenase complex involved in Wolinella succinogenes nitrate respiration

Nitrate respiration catalysed by the epsilon-proteobacterium Wolinella succinogenes relies on the NapAGHBFLD system that comprises periplasmic nitrate reductase (NapA) and various other Nap proteins required for electron transport from menaquinol to NapA or maturation of Nap components. The W. succinogenes Nap system is unusual as electron transfer to NapA was shown previously to depend on both subunits of the predicted menaquinol dehydrogenase complex NapGH but did not require a cytochrome c of the NapC/NrfH family. Nonetheless, minor residual growth by nitrate respiration was observed in napG and napH gene inactivation mutants. Here, the question is addressed whether alternative membrane-bound menaquinol dehydrogenases, like NrfH and NosGH, involved in nitrite or N2O reduction systems, are able to functionally replace NapGH. The phenotypes of various gene deletion mutants as well as strains expressing chimeric nap/nos operons demonstrate that NosH is able to donate electrons to the respiratory chain of nitrate respiration at a physiologically relevant rate, whereas NrfH and NosG are not. The iron-sulphur protein NapG was shown to form a complex with NapH in the membrane but was detected in the periplasmic cell fraction in the absence of NapH. Likewise, NosH is able to bind NapG. Each of the eight poly-cysteine motifs present in either NapG or NapH was shown to be essential for nitrate respiration. The NapG homologue NosG could not substitute for NapG, even after adjusting the cysteine spacing to that of NapG, implying that NapG and NosG are specific adapter proteins that channel electrons into either the Nap or Nos system. The current model on the structure and function of the NapGH menaquinol dehydrogenase complex is presented and the composition of the electron transport chains that deliver electrons to periplasmic reductases for either nitrate, nitrite or N2O is discussed.