Biology Reference
In-Depth Information
6. CAMPYLOBACTER TRUNCATED GLOBIN, CTB:
FUNCTIONAL AND STRUCTURAL CHARACTERISATION
6.1. Functional characterisation
In 2004, Elvers et al. identified a second haemoglobin-like protein
(Cj0465c) in addition to the single-domain globin Cgb. This locus encodes
a truncated globin designated Ctb (
Campylobacter
-truncated globin) that is
constitutively expressed at low levels, and under nitrosative stress conditions
(GSNO and
S
-nitroso-
N
-acetylpenicillamine (SNAP)), expression is
increased in an NssR-dependent manner (see
Section 7
)(
Wainwright
et al., 2005
).
Ctb is a member of the truncated haemoglobin family (T family), a dis-
tinctive phylogenetic group characterised by the 2/2 Mb-fold with mem-
bers in archaea, eukaryotes and bacteria (
Vinogradov et al., 2013;
Wittenberg, Bolognesi, Wittenberg, & Guertin, 2002
). More specifically,
Ctb is classified within the poorly explored group III of the TrHbIII
(
Pesce et al., 2000; Vinogradov et al., 2005
). Although Ctb has been studied
for several years (
Bolli et al., 2008; Lu, Egawa, et al., 2007; Nardini et al.,
2006; Wainwright, Wang, Park, Yeh, & Poole, 2006
), its physiological
function remains unclear. Attempts to elucidate the role of this enigmatic
globin include testing its ability to improve cellular growth in micro-
aerophilic environments and its capacity to confer cellular protection against
toxic oxygen tensions and nitrosative stress conditions.
The ability of Vgb to improve microaerobic growth in
E. coli
and other
bacterial and eukaryotic species has been extensively documented (reviewed
by
Frey, Shepherd, Jokipii-Lukkari, Haggman, &Kallio, 2011
). Analysis of a
C. jejuni
strain that lacks
ctb
revealed a slower growth rate during the station-
ary phase in microaerobic conditions compared to the wild-type parental
strain (
Wainwright et al., 2005
), suggesting a role for Ctb in oxygen transfer.
However, growth under oxygen-limited conditions failed to show these dif-
ferences. The influence of Ctb upon oxygen consumption is equally inter-
esting: (i) respiration rates of the
ctb
mutant were 50% compared to wild
type; (ii) the
K
M
values for oxygen among the
ctb
,
cgb
and
cgb/ctb
mutants
and the wild type are all comparable, but the
V
max
determined by the deox-
ygenation of oxy-leghaemoglobin (
Contreras et al., 1999; D'mello et al.,
1994, 1995, 1996; Smith, Hill, & Anthony, 1990
) is greater for the
ctb
mutant than the others (
Wainwright et al., 2005
) and (iii) the oxygen con-
sumption rates of the
ctb
-lacking strain measured
in vivo
decreases below
