Biology Reference
In-Depth Information
mutants under the same conditions displayed a more severe growth inhibi-
tion (
Pittman et al., 2007
). For these reasons, it has been suggested that NrfA
might provide a constitutive defence against NO and RNS allowing
Campylobacter
to survive before Cgb induction is triggered (
Pittman et al.,
2007
). Interestingly, under oxygen-limited conditions (where NssR fails
to upregulate Cgb), loss of
nrfA
does not elicit sensitivity to GSNO
(
Avila-Ramirez et al., 2013
). This would seem to suggest that neither
Cgb nor NrfA play a role in resisting nitrosative stress under conditions
of oxygen limitation.
7.5. Induction of Cgb by nitrite and nitrate is independent
of NrfA
Reactive nitrogen compounds such as NO and N
2
O are produced during
growth with nitrite and nitrate as electron donors (
Corker & Poole, 2003;
Weiss, 2006
), and Hmp expression is induced by exposure to these com-
pounds in
E. coli
(
Poole et al., 1996
). Given that Cgb is also induced by
nitrite and nitrate (
Pittman et al., 2007
), a similar mechanism of transcrip-
tional activation is proposed for this globin, although the production of
NO or its reactive derivatives in
Campylobacter
remains undemonstrated.
The level of Cgb induction by nitrite and nitrate was similar in wild-type
and the
nrfA
strains. In contrast, in an
napA
mutant, Cgb expression occurred
only in the presence of nitrite implicating the reduction of nitrite as the
actual source of nitrosative stress (NO production perhaps) (
Pittman
et al., 2007
). However, since nitrite still induces Cgb in an
nrfA
-lacking
strain, this suggests that NrfA is not the source of nitrosative stress that
induces Cgb expression in
C. jejuni
. Furthermore, similar levels of nitrite-
mediated respiratory inhibition in the wild-type and the
nrfA
mutant suggest
an NrfA-independent mechanism for the generation of NO in this
microaerophilic microorganism.
The putative source of NO as a by-product of the nitrite reduction in
Campylobacter
remains elusive. However, it is well known that globin pro-
teins can produce NO from nitrite, which leads to an attractive hypothesis:
since Ctb is also upregulated in response to nitrosative stress, this truncated
globin may convert nitrite to NO, which is subsequently converted to
nitrate by Cgb. However, this is a speculative mechanism and remains to
be experimentally verified. Alternatively, it is possible that the molecular
switch for NssR activation could involve the binding of nitrite or nitrate
to NssR (
Smith et al., 2011
), rather than a direct interaction of the
