Biology Reference
In-Depth Information
expression PrfA in
Listeria monocytogenes
(TTAACA-N
2
-TGTTAA)
(
Korner et al., 2003
) and for the NO-sensing regulator Nnr from
Paraccocus
pantotrophus
(TTAAC-N4-GTCAA) (
Saunders, Ferguson, & Baker, 2000
).
Thus, the architecture of the four genes regulated by NssR is reminiscent of
a class II Fnr-dependent promoter (
Guest, Green, Irvine, & Spiro, 1996
).
The specificity of the NssR recognition for the Fnr-like motif in the
cgb
gene
(TTAACacaaGTCAA) was demonstrated by comparing the wild-type and
modified sequence (CTAACacaaGTCAG) in transcriptional fusions to
lacZ
(
Wosten, Boeve, Koot, van Nuenen, & van der Zeijst, 1998
): modification
of the motif prevented NssR-mediated transcriptional activation (
Elvers
et al., 2005
).
The
nssR
and the
ctb
genes are divergently transcribed sharing a putative
NssR-binding sequence in their promoter regions, implicating NssR in the
regulation of both
ctb
and
nssR
expression (
Elvers et al., 2005
). Certainly, the
modest upregulation of
nssR
in continuous cultures exposed to GSNO
(
Monk et al., 2008
) might be due to autoregulation. NssR binds to a specific
ctb
promoter region (
50 nM)
(
Smith et al., 2011
), in agreement with the low but constitutive expression
of Ctb in the absence of nitrosative conditions (
Elvers et al., 2005
). How-
ever, the presence of GSNO or NOCs in gel shift experiments did not show
an increase in the affinity of NssR for the
ctb
promoter (
Smith et al., 2011
).
This finding has significant implications since it is likely that NssR differs
from other well-characterised regulators within the Crp/Fnr family that dis-
play enhanced DNA binding in the presence of their related signal molecules
compared to the absence of the signal (
K
d(app)
in the nanomolar range and in
the micromolar range, respectively) (
Green, Scott, & Guest, 2001
). In this
context, it seems plausible that NssR binds permanently to the promoters of
genes that are members of the regulon and that conformational changes
induce the binding of the transcriptional machinery upon exposure to NO.
Nitrosylation of the sole cysteine or nitration of one of the several tyrosine
residues present in the structure of NssR has been suggested as possible
mechanisms (
Smith et al., 2011
). Nitration implies the production of per-
oxynitrite, a product of the reaction between NO and superoxide. Since
superoxide production has been demonstrated for other bacterial
haemoglobins (
Membrillo-Hernandez, Ioannidis, & Poole, 1996
), produc-
tion of this compound by the oxyferrous form of Ctb seems plausible. How-
ever, it has been shown in a strain lacking
ctb
that Cgb expression is elevated
under oxygen-limited conditions (
Smith et al., 2011
), conditions that are
inconsistent with superoxide production.
32 bp) in the absence of GSNO (
K
d(app)
