Hopper, Amanda Clare (2012)
Ph.D. thesis, University of Birmingham.
\(Neisseria gonorrhoeae\) is an obligate human pathogen that can use both oxygen and nitrite as electron acceptors, but its electron transfer chain has only been partially characterised. The gonococcus encodes one azurin, Laz, and eight c-type cytochromes. The aim of this project was to determine the functions of these redox proteins.
Single mutants lacking cytochromes c2, c4 and c5 reduced oxygen at 126%, 84% and 80% of the parental rate, respectively. It was not possible to construct a double mutant defective in both cytochromes c4 and c5, unless an ectopic copy of cytochrome c5 was present, the expression of which was induced by IPTG. It was concluded that cytochromes c4 and c5 form a bifurcated electron transfer pathway between the cytochrome bc1 complex and the cytochrome cbb3 oxidase.
Candidates for electron donors to the nitrite reductase, AniA, were Laz and cytochromes c2, c4 and c5. Mutants lacking various combinations of these redox proteins retained the ability to reduce nitrite, implicating the presence of further electron transfer pathways to AniA. Mutants defective in the third heme-binding domain of CcoP or the second domain of cytochrome c5 reduced nitrite at 52% and 39% of the parental rate, respectively. The double mutant still reduced nitrite, but at only 12% of the parental rate. It was concluded that the third heme group of CcoP and the second heme group of cytochrome c5 constitute the main electron transfer pathways to AniA, but a third pathway remains to be identified.
Previous research failed to demonstrate the presence of cytochrome c2 on an SDS-PAGE gel stained for covalently bound heme. This was shown to be due to the low constitutive level of expression of this protein. Although a precise role for cytochrome c2 could not be determined, the data presented suggest that the protein could function either as an electron donor to AniA, an electron donor to ScoI, or as a regulator of electron flux to the terminal reductases.
The combined data show that the plasticity of the gonococcal electron transfer chains allows this bacterium to respond rapidly to changes in terminal electron acceptor availability.
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