Biology Reference
In-Depth Information
response. A second regulon, the soxRS, is activated in
response to superoxide or nitric oxide. 10 Thirty genes
are induced in response to treatment with hydrogen
peroxide, including nine genes that are activated by
OxyR. 11 The bacteria then become resistant to subse-
quent doses of hydrogen peroxide that would otherwise
be lethal. 11 e 13 The mechanism by which OxyR regulates
expression of these nine genes was reported by Ames
and coworkers in 1990. 14 Although both oxidized and
reduced OxyR bind DNA, the two forms of the protein
recognize different promoter regions, with the oxidized
form acting as a positive regulator of expression of genes
including catalase and alkyl hydroperoxide reductase.
OxyR also negatively regulates its own expression.
Conversion between the two forms of OxyR was found
to be reversible and proposed to involve a conforma-
tional change in the structure of OxyR. 14
The structural change induced by oxidation of OxyR
was revealed more recently in a comparison of the
crystal structures of reduced and oxidized forms of the
regulatory domain of OxyR. In the reduced state, Cys
199 and Cys 208 are 17 ˚ apart. 15 Formation of an intra-
molecular disulfide bond between these two Cys resi-
dues results in a significant change in the structure of
the protein, as shown in Figure 11.2 . The mechanism
of disulfide bond formation is thought to involve oxida-
tion of Cys 199 to sulfenic acid, making it susceptible to
nucleophilic attack by Cys 208. The structural change
imparted by disulfide bond formation in turn affects
the oligomeric state of OxyR, which alters the spatial
positioning of the N-terminal DNA-binding domains
and thereby its preferred DNA-binding sites. 15 In this
way, OxyR activates transcription of the proteins that
help the bacteria adapt to oxidative stress induced by
hydrogen peroxide. Thus in this example, ROS mediate
a refolding of OxyR in order to effect a response.
FIGURE 11.2 The response of OxyR to hydrogen peroxide is
shown with ribbon representations of the reduced (top) and oxidized
(bottom) forms of the protein. In response to hydrogen peroxide,
a disulfide bond forms in OxyR, changing the structure of the protein
in this region, and conferring specificity for new promoter sites in E.
coli. The oxidized form of OxyR activates transcription of a number of
proteins that confer protection to the bacteria from hydrogen
peroxide. (Please refer to color plate section).
activity of OxyR, Curran and coworkers were investi-
gating redox regulation of AP-1(c-Jun/c-Fos). Both c-
Jun and c-Fos belong to a family of proteins referred to
as Activator protein-1 (AP-1), including structurally
and functionally related basic leucine zipper proteins
(bZIP) that intermix to form heterodimeric sequence-
specific DNA-binding proteins. 16,17 The AP-1 proteins
include primarily Jun proteins, c-Jun, JunB, and JunD,
Fos proteins, c-Fos, FosB, Fra-1 and Fra-2, and some
ATF family members, ATFa, ATF-2, and ATF-3. 16 AP-1
genes are induced in response to phorbol ester
tumor promoters, such as 12-O-tetradecanoylphorbol-
13-acetate (TPA) and also respond to environmental
changes including stress and radiation or to growth
factor signals. Cellular processes that are mediated by
AP-1 include proliferation, differentiation, apoptosis,
and transformation. 16
The discovery of a nuclear redox factor derives from
initial work characterizing the nuclear viral oncogene
v-jun that functions in cell growth and differentiation.
Tjian and coworkers reported in 1987 that v-jun shared
over 80% sequence identity with c-jun, which when
overexpressed was found to have properties identical
IDENTIFICATION OF A NUCLEAR
REDOX FACTOR
Three compelling lines of evidence support the role of
a nuclear redox factor in regulating the DNA-binding
activity of certain transcription factors. The first came
from the role of redox regulation in the DNA-binding
activity of AP-1. The second was a similar study con-
ducted for p53. And, finally, redox regulation was
shown to play an important role in the DNA-binding
activity of NF- k B.
Redox Regulation of the DNA-binding
Activity of AP-1 (c-Jun/c-Fos)
During this same time period that Ames and
coworkers were investigating the redox regulated
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