Biology Reference
In-Depth Information
holo-YybT is 23-fold lower and 12-fold higher than those of apo-YybT,
respectively (
Rao et al., 2011
). Thus, a catalytic efficiency (
k
cat
/
K
m
) for
holo-YybT is 276-fold lower than that of the apo-YybT. The catalytic
activity increases upon binding of NO to ferrous YybT by a threefold
increase. CO has a negligible effect on the catalytic activity. The redox
change in the haem also shows no effect on the enzymatic activity.
Based on these results,
Rao et al. (2011)
propose that YybT is a haem
sensor (a sensor protein for haem), not a haem-based sensor such as the pro-
teins described in the other sections. Haem binding to YybT would initiate
cellular response through the cyclic di-AMP signalling (
Tan et al., 2013
).
Though a TetR family haem sensor, HrtR from
Lactococcus lactis
, is well
characterised (
Sawai, Yamanaka, Sugimoto, Shiro, & Aono, 2012
), no
PAS family haem sensor is known to date. YybT is the first example of haem
sensors that adopt a PAS domain to sense a haem molecule. An
in vivo
study
for
S. aureus
suggests that YybT family proteins may respond to cellular haem
levels to regulate hemolysin secretion, which is required for acquiring nutri-
ents including haem iron from the host by lysing the host cells. The knock-
out of the SA0013 gene encoding YybT disrupts hemolysin secretion in
S. aureus
(
Burnside et al., 2010
). The physiological function of YybT is also
studied by disruption of the
yybT
gene in
L. lactis
.
L. lactis
mutant strain lac-
king YybT shows greater sensitivity towards haem treatment, which will be
caused by modulation of cyclic di-AMP concentration (
Tan et al., 2013
).
6. HAEM-CONTAINING PAS FAMILY FOR
CHEMOTAXIS REGULATION
6.1. Bacterial chemotaxis system
Bacterial chemotaxis has extensively been studied with enteric bacteria
(
Blair, 1995; Szurmant & Ordal, 2004
). The chemotaxis regulation system
consists of a signal transducer protein (sometimes called
m
ethyl-accepting
c
hemotaxis
p
rotein, MCP), CheA, CheW, CheY, and some other Che pro-
teins that are responsible for adaptation. MCP acts as a sensor for the external
signal and regulates the self-kinase activity of CheA that is a component of
the CheA/CheY signal transduction system (
Blair, 1995; Szurmant &Ordal,
2004
). CheA is the phosphodonor for the cognate response regulator CheY
that binds to the flagellar motor to control its rotational direction for the reg-
ulation of the swimming behaviour of bacteria (
Blair, 1995; Stock, Lukat, &
Stock, 1991; Szurmant & Ordal, 2004
).
