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has one cysteine residue (Cys99) and one histidine (His56) residue, it is
unlikely that a complex Fe-S cluster is present, although a simple metal
cofactor is possible. An intriguing idea is that the formation of an inter-
molecular disulphide bond between the Cys99 residues of each monomer
(
Fig. 4.7
B) can modulate transcriptional activation, especially since NssR
activity is abolished under anoxic conditions (
Avila-Ramirez et al., 2013
)
(oxygen influences cellular redox poise). Future investigations into the
cofactor/ligand-binding capabilities of NssR and the potential for redox
modulation of intermolecular disulphides may contribute towards the elu-
cidation of the molecular switch that initiates the response of
Campylobacter
to nitrosative stress.
7.4. NrfA of
Campylobacter
NrfA is a pentahaem cytochrome
c
nitrite reductase that catalyses the dissim-
ilatory reduction of nitrite to ammonia (
Pittman & Kelly, 2005; Sellars et al.,
2002
). A wide range of bacteria encode NrfA homologs, and the ability of
these proteins to reduce both nitrite and NO has been demonstrated
in vitro
(
Bamford et al., 2002; Costa et al., 1990
). In
E. coli
, NrfA catalyses the reduc-
tion of NO to ammonia under anaerobic conditions (
Costa et al., 1990;
Poock, Leach, Moir, Cole, & Richardson, 2002; van Wonderen, Burlat,
Richardson, Cheesman, & Butt, 2008
), and it has been suggested that NrfA
could play a role in anaerobic NO detoxification together with the
flavohaemoglobin Hmp and the flavorubredoxin NorV (
Poock et al., 2002
).
Pittman et al. explored the role of the constitutively expressed NrfA in the
protection of
C. jejuni
against nitrosative stress by testing a range of mutants
affected in key genes of the
nap
and
nrf
operons.Nitrite is produced fromnitrate
in stoichiometric quantities in oxygen-limited cultures of the
C. jejuni
parental
strain, although an
napA
-lacking strain failed to produce nitrite: this is consis-
tent with NapA being the sole Nap in
C. jejuni
. Likewise, consumption of
nitrite under the same conditions was dependent upon NrfA and the partner
electron donor NrfH. Under microaerobic conditions, an
nrfA
mutant was
hypersensitive to NO donors (spermine NONOate, GSNO and SNAP),
aerobic respiration was severely inhibited by NO and NO consumption
was diminished compared to the parental strain (
Pittman et al., 2007
). This
suggests that NrfA can offer a significant protection from nitrosative stress.
The level of protection attributable to NrfA and Cgb has not been
directly compared. Growth of an
nrfA
mutant in the presence of nitrite
was slower than the isogenic wild type, whereas growth of
cgb
or
nssR
