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to reveal the NssR regulon under oxygen-limited conditions, demonstrated
by transcriptomic analysis (
Avila-Ramirez et al., 2013
), can also be explained
by this hypothesis.
Several studies support the idea of interplay between the
C. jejuni
glo-
bins: (i) diminished viability in cultures of a
cgb
mutant (
Elvers et al.,
2004
) is reverted by mutation of the second globin, being comparable to
the parent strain or a
ctb
mutant (
Wainwright et al., 2005
); (ii) a double
mutant (
cgb
,
ctb
) is less sensitive to GSNO than a
cgb
mutant, suggesting a
partial suppression of the
cgb
phenotype by the absence of the second globin
(
Avila-Ramirez et al., 2013
); (iii) a
ctb
mutant shows a slower growth profile
compared to the parent strain during the stationary phase in microaerobic
cultures, whereas growth rates of the wild-type strain and a
cgb
,
ctb
double
mutant are similar (
Wainwright et al., 2005
).
9. CONCLUSIONS
Adaptations of
C. jejuni
to nitrosative stress conditions seem to be
linked to oxygen availability. Under microaerobic conditions, NO and
RNS trigger the induction of the NssR-dependent regulon that includes
the expression of the Cgb and Ctb globins, whereas in oxygen-limited envi-
ronments an inducible NssR-independent system offers protection. Even
though microaerobic resistance to NO and nitrosative stress agents is asso-
ciated with the presence of Cgb, the reaction intermediates during the Cgb-
mediated detoxification of NO are yet to be experimentally identified.
However, biophysical studies of the Cgb haem pocket support the conver-
sion of NO to nitrate. Indeed, much is known about the influence of
hydrogen-bonding networks upon the ligand-binding properties, and
how the haem environment is well suited to the conversion of NO to nitrate
via a dioxygenase or denitrosylase mechanism. However, these mechanisms
involve the oxidation of the haem cofactor, and a reductase system for the
regeneration of the ferrous haem has not yet been identified, although the
possibility remains that Cgb is reduced by endogenous small molecules
(e.g. glutathione).
The truncated globin Ctb is constitutively expressed at low levels and is
overexpressed under nitrosative stress conditions in an NssR-dependent
manner. However, Ctb fails to offer protection against NO or RNS.
A possible role for Ctb in promoting microaerobic growth and moderating
respiration in
C. jejuni
has been suggested, and biophysical and mechanistic
