Abstract
The periplasmic reduction of the electron acceptors nitrate (Em +420 mV) and trimethylamine-N-oxide (TMAO; Em +130 mV) by Nap and Tor reductases is widespread in Gram-negative bacteria and is usually considered to be driven by non-energy conserving quinol dehydrogenases. The EpsilonproteobacteriumCampylobacter jejuni can grow by nitrate and TMAO respiration and it has previously been assumed that these alternative pathways of electron transport are independent of the proton-motive menaquinol-cytochrome c reductase complex (QcrABC) that functions in oxygen-linked respiration. Here, we show that a qcrABC deletion mutant is completely deficient in oxygen-limited growth on both nitrate and TMAO and is unable to reduce these oxidants with physiological electron donors. As expected, the mutant grows normally on fumarate under oxygen-limited conditions. Thus, the periplasmic Nap and Tor reductases receive their electrons via QcrABC in C. jejuni, explaining the general absence of NapC and TorC quinol dehydrogenases in Epsilonproteobacteria. Moreover, the specific use of menaquinol (Em −75 mV) coupled with a Qcr complex to drive reduction of nitrate or TMAO against the proton-motive force allows the process to be electrogenic with a H+/2e− ratio of 2. The results have general implications for the role of Qcr complexes in bacterial oxygen-independent respiration and growth.
Highlights
The cytochrome bc[1] complex is a highly conserved proton-translocating, quinol-cytochrome c reductase (Qcr) that has a major role in oxygen-linked respiration in phylogenetically diverse prokaryotes[1]
Currently annotated as petABC, we propose that the qcrABC designation be used, as this more accurately reflects their function in this non-photosynthetic bacterium
The qcrABC genes were deleted from the chromosome of C. jejuni NCTC 11168 using allelic exchange via a plasmid containing upstream and downstream flanking regions for recombination and where the coding regions were replaced with a non-polar kanamycin resistance cassette with an outward reading promoter[25] (Fig. 1A)
Summary
The cytochrome bc[1] complex (referred to as Complex III in eukaryotic mitochondria) is a highly conserved proton-translocating, quinol-cytochrome c reductase (Qcr) that has a major role in oxygen-linked respiration in phylogenetically diverse prokaryotes[1]. Cells of WT, qcrABC and a ccoNOQP deletion strain[24], were grown in complex media under standard microaerobic conditions and the specific rate of oxygen consumption in cell suspensions compared, with formate as electron donor.
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