Biology Reference
In-Depth Information
behaviour changes. The organism moves along the gradient towards
(aerophilicity or positive aerotaxis) or away from (aerophobicity or negative
aerotaxis) the environmental area with higher O
2
presence.
The haem-based aerotaxis transducer (HemAT) are the only representa-
tives of this class.
2.1.1 The haem-based aerotaxis transducer
Several years ago, Hou and coworkers discovered the first GCSs, the haem-
based aerotaxis transducers (HemATs) in
Halobacterium salinarum
(HemAT-
Hs
),
B. subtilis
(HemAT-
Bs
), and
Bacillus halodurans
(HemAT-
Bh
)(
Hou,
Belisle, et al., 2001; Hou et al., 2000
).
Recently, cellular stoichiometry experiments performed on the chemo-
taxis signalling proteins of
B. subtilis
have been published (
Cannistraro,
Glekas, Rao, & Ordal, 2011
). From quantitative immunoblot studies, it is
clear that HemAT is the most expressed chemoreceptor in
B. subtilis
, cou-
nting 19,000
3900 copies per cell. Although HemAT-
Bs
lacks a trans-
membrane domain, investigation of cellular localization using fluorescent
fusion protein shows clustering of this sensor at the cell poles together with
other transmembrane chemoreceptors (
Cannistraro et al., 2011
).
Interestingly, next to the well-established aerotactic function, the charac-
terization of the twin-arginine protein translocation (Tat) pathway of
B. subtilis
, responsible for the transport of folded proteins to the extra-
cytoplasm, highlighted an alternative role for HemAT-
Bs
. Monteferrante
and colleagues published early this year proofs of HemAT participation in
the PhoD secretion under phosphate starvation conditions. Intriguingly,
HemAT colocalizes also with CsbC, a putative pentose transporter with
12 predicted transmembrane passes (
Monteferrante et al., 2013
).
The protein sequence analysis of
H. salinarum
,
B. subtilis
, and
B. halodurans
HemAT reveals an N-terminal globin domain (residues
1-184 in HemAT-
Hs
, 1-175 in HemAT-
Bs
, and 1-184 in HemAT-
Bh
)
with limited homology to the
Sperm whale
myoglobin (
Sw
Mb) and a coupled
C-terminal MCP domain (residues 222-489 in HemAT-
Hs
, 198-432 in
HemAT-
Bs
, and 222-439 in HemAT-
Bh
) that shares 30% identity with
the
E. coli
serine receptor Tsr (
Hou, Belisle, et al., 2001; Hou et al., 2000
).
Deletion and overexpression of
HemAT
genes influence the movement
of these organisms in response to an O
2
gradient, conclusively proving the
engagement of HemAT in the aerophobic response in
H. salinarum
and in
the aerophilic response in
B. subtilis
(
Hou et al., 2000
).
