Biology Reference
In-Depth Information
m
m
M and air saturation concen-
trations (
Wainwright et al., 2005
). This data suggest that Ctb could be a reg-
ulator of cellular oxygen consumption: given that high external oxygen
tensions are toxic and consumption of O
2
by Ctb might offer protection
during microaerophilic growth.
The studies discussed earlier suggest a possible role for Ctb in promoting
microaerobic growth and moderating respiration in
C. jejuni
. Nonetheless,
understanding the physiological relevance of these observations is not
straightforward. For instance, the
ctb
gene is upregulated under nitrosative
stress conditions, yet expression is not induced by variations in oxygen con-
centration or oxidative stress (
Wainwright et al., 2005
). Indeed, induction of
Ctb expression by GSNO and SNAP was demonstrated (by Western blot-
ting), but paraquat or peroxides did not influence Ctb production. It is pos-
sible that the differences in expression patterns between Cgb and Ctb have
physiological relevance; the expression of Cgb is strictly dependent upon
NssR (see
Section 7.1
) and, as far as we know, only occurs under nitrosative
stress conditions (
Elvers et al., 2005; Pittman et al., 2007
). However, Ctb is
constitutively expressed at low levels and induced under nitrosative stress
conditions (
Wainwright et al., 2005
), raising the question 'is the constitutive
level of Ctb enough to play a significant role in regulation of intracellular
oxygen tensions?' Furthermore, are higher levels of this globin needed to
play the same, or perhaps a different role, under nitrosative stress conditions?
Nonetheless, Ctb fails to offer growth protection in the presence of NO or
other RNS in
C. jejuni
(
Wainwright et al., 2005
). When
ctb
was expressed
under the control of an arabinose-inducible promoter in an
E. coli hmp
mutant, aerobic respiration was protected from NO inhibition compared
to cells carrying an empty vector. Interestingly, cells expressing Ctb
consumed NO at similar rates under aerobic and anaerobic conditions, while
the controls accumulated a significant concentration of NO (
Avila-Ramirez
et al., 2013
), perhaps suggesting a secondary role in NO detoxification in
Campylobacter
.
1
M but increases in the range between 1
6.2. Structural characterisation
The structure of dimeric cyanide-bound Ctb was solved via X-ray crystal-
lography to a resolution of 2.15
˚
(
Nardini et al., 2006
) and was the first class
III-truncated globin to be structurally characterised. This revealed a
2-over-2
-helical fold, as for all truncated globins characterised to date
(
Fig. 4.6
A). Previous structures of type I and II globins revealed conserved
Gly-Gly sequence motifs
a
located at
the AB inter-helical hinge and
