Biology Reference
In-Depth Information
transcriptional regulator NsrR results from the submicromolar levels of NO
released from GSNO. In the aforementioned transcriptomic studies of
C. jejuni
, cultures were exposed to 250
m
M GSNO (
Monk et al., 2008
)
or 10
M NOC-7 (
Smith et al., 2011
). It is unlikely
that a concentration of NO under 2.5
m
M NOC-5 plus 10
m
M (putatively released from GSNO)
was responsible for the majority of the transcriptional changes, although
given the extremely high affinity of bacterial globins for NO, it seems plau-
sible that micromolar concentrations of NO should induce their expression.
Remarkable differences in the transcriptional response of
C. jejuni
to NO
elicited by variations in oxygen availability have been recently reported
(
Avila-Ramirez et al., 2013
). Genes induced by NOCs in microaerobic con-
ditions and oxygen-limited conditions are mutually exclusive. In oxygen-
limited conditions, only 11 genes show upregulation, and members of the
NssR regulon were absent. Notably, the
cj1585c
gene encoding a lactate
dehydrogenase, located next to
cgb
, was marginally upregulated under
oxygen-limited conditions but not in microaerobic cultures. This protein
has been proposed as the Cgb redox partner, although since
cj1585c
induc-
tion does not coincide with the expression of Cgb (the absence of Cgb in
oxygen-limited cultures under conditions of nitrosative stress has been
proved by immunoblotting), this now seems unlikely. This finding raises
a number of new questions related to the ability of this microorganism to
survive in the host, where variations in oxygen concentrations are expected.
m
7.2. NssR classification, targets and mechanistic insights
NssR belongs to branch E of the Crp-Fnr superfamily (
Korner et al., 2003;
Matsui, Tomita, & Kanai, 2013
) and is the only member of this branch to be
involved in NO regulation. Some members of the Crp-Fnr family do not
directly interact with the signal molecule but with an independent sensor
system that increases the concentration of the regulator (
Fischer, 1994
).
However, since the expression of NssR is only very modestly increased
under nitrosative stress conditions (
Elvers et al., 2005
), this suggests a role
for NssR as an NO sensor and regulator of the response.
In the
C. jejuni
genome, the
70
-controlled promoters
are not conserved (
Petersen, Larsen, Ussery, On, & Krogh, 2003
). Analysis
of promoters for
cgb
,
ctb
,
cj0761
,
cj0830
and
nssR
showed a typical
35 regions for
s
10 motif
and an Fnr-like binding motif upstream of the
10 region (
Elvers et al.,
2005
). The consensus sequence (TTAAC-N
4
-GTAA) is similar to the
suggested recognition sequences
for the regulator of virulence gene
