Roles of the twin-arginine translocase and associated chaperones in the biogenesis of the electron transport chains of the human pathogen Campylobacter jejuni.

The zoonotic pathogen Campylobacter jejuni NCTC 11168 uses a complex set of electron transport chains to ensure growth with a variety of electron donors and alternative electron acceptors, some of which are known to be important for host colonization. Many of the key redox proteins essential for electron transfer in this bacterium have N-terminal twin-arginine translocase (TAT) signal sequences that ensure their transport across the cytoplasmic membrane in a folded state. By comparisons of 2D gels of periplasmic extracts, gene fusions and specific enzyme assays in wild-type, tatC mutant and complemented strains, we experimentally verified the TAT dependence of 10 proteins with an N-terminal twin-arginine motif. NrfH, which has a TAT-like motif (LRRKILK), was functional in nitrite reduction in a tatC mutant, and was correctly rejected as a TAT substrate by the tatfind and TatP prediction programs. However, the hydrogenase subunit HydA is also rejected by tatfind, but was shown to be TAT-dependent experimentally. The YedY homologue Cj0379 is the only TAT translocated molybdoenzyme of unknown function in C. jejuni; we show that a cj0379c mutant is deficient in chicken colonization and has a nitrosative stress phenotype, suggestive of a possible role for Cj0379 in the reduction of reactive nitrogen species in the periplasm. Only two potential TAT chaperones, NapD and Cj1514, are encoded in the genome. Surprisingly, despite homology to TorD, Cj1514 was shown to be specifically required for the activity of formate dehydrogenase, not trimethylamine N-oxide reductase, and was designated FdhM.

A sulphite respiration system in the chemoheterotrophic human pathogen Campylobacter jejuni.

The ability to use sulphite as a respiratory electron donor is usually associated with free-living chemolithotrophic sulphur-oxidizing bacteria. However, this paper shows that the chemoheterotrophic human pathogen Campylobacter jejuni has the ability to respire sulphite, with oxygen uptake rates of 23 +/- 8 and 28 +/- 15 nmol O(2) min(-1) (mg cell protein)(-1) after the addition of 0.5 mM sodium sulphite or metabisulphite, respectively, to intact cells. The C. jejuni NCTC 11168 Cj0004c and Cj0005c genes encode a monohaem cytochrome c and molybdopterin oxidoreductase, respectively, homologous to the sulphite : cytochrome c oxidoreductase (SOR) of Starkeya novella. Western blots of C. jejuni periplasm probed with a SorA antibody demonstrated cross-reaction of a 45 kDa band, consistent with the size of Cj0005. The Cj0004c gene was inactivated by insertion of a kanamycin-resistance cassette. The resulting mutant showed wild-type rates of formate-dependent respiration but was unable to respire with sulphite or metabisulphite as electron donors. 2-Heptyl-4-hydroxyquinoline-N-oxide (HQNO), a cytochrome bc(1) complex inhibitor, did not affect sulphite respiration at concentrations up to 25 microM, whereas formate respiration (which occurs partly via a bc(1) dependent route) was inhibited 50%, thus suggesting that electrons from sulphite enter the respiratory chain after the bc(1) complex at the level of cytochrome c. Periplasmic extracts of wild-type C. jejuni 11168 showed a symmetrical absorption peak at 552 nm after the addition of sulphite, demonstrating the reduction of cytochrome c. No cytochrome c reduction was observed after addition of sulphite to periplasmic extracts of the Cj0004c mutant. A fractionation study confirmed that the majority of the SOR activity is located in the periplasm in C. jejuni, and this activity was partially purified by ion-exchange chromatography. The presence of a sulphite respiration system in C. jejuni is another example of the surprising diversity of the electron-transport chain in this small-genome pathogen. Sulphite respiration may be of importance for survival in environmental microaerobic niches and some foods, and may also provide a detoxification mechanism for this normally growth-inhibitory compound.