Anthrax (Bacillus anthracis), Molecular Biology of
321
pagA
and is cotranscribed with that gene.
Transcription from the
pagA
promoter
results in a monocistronic transcript con-
taining
pagA
alone, and in a bicistronic
transcript that encodes both
pagA
and
pagR
. Expression of
pagR
,aswith
pagA
,is
atxA
-dependent and CO
2
-enhanced. PagR
has sequence similarity to transcription
regulators, and it has been found that the
level of the
pagA
transcript, and PA protein,
is higher in a
pagR
mutant than in its par-
ent strain, indicating that PagR normally
represses
pagA
expression. Interestingly,
PagR also represses
atxA
expression; the
pagR
mutant
strain
produces
approxi-
mately twofold more
atxA
mRNA than
its parent strain. It is not believed that the
effects of
pagR
on
pagA
expression occur
through AtxA because AtxA levels are not
limiting for
pagA
expression.
Before
pagR
was discovered, a mutant
B. anthracis
strain
was
found
that
ex-
pressed the toxin genes in the absence
of CO
2
. An unidentiFed virulence regu-
lator was therefore proposed. This strain
was postulated to have a mutation in a
negative regulator of toxin gene expres-
sion, whose repressive effect is alleviated
in the presence of CO
2
, thereby allowing
AtxA to stimulate toxin expression. CO
2
has the opposite effect on
pagR
-regulated
toxin expression – it would be predicted to
decrease toxin gene expression because it
stimulates
pagR
expression.
pagR
,the
re
-
fore, cannot be the sole CO
2
-dependent
regulator of toxin gene expression because
CO
2
has the overall effect of increasing
toxin gene expression, rather than limit-
ing it. Does CO
2
turn off the proposed
repressor while at the same time increas-
ing repression by PagR so that toxin gene
expression does not get out of control? Un-
fortunately, several things about the CO
2
regulatory system are unknown, including
the effects of PagR on expression of
lef
and
cya
,theef
fec
tofCO
2
on toxin expression
in a
pagR
mutant, and the identity of the
gene mutated in the strain that produces
toxinintheabsenceofCO
2
. As such, more
researchneedstobedonebeforethereg
-
ulatory pathways that limit anthrax toxin
synthesis are fully understood.
4.3
AbrB
Another regulator of toxin expression is
abrB
,ano
r
tho
logto
abrB
from
Bacillus
subtilis
.The
B. subtilis
gene is a transition
state regulator, meaning it mediates the
transition of cells from one state to an-
other, such as from quiescence to active
growth. The
B. anthracis
ortholog is lo-
cated on the chromosome and appears to
work similarly. An
abrB
null mutant shows
increased expression of toxin gene–lacZ
reporter fusions, with the largest effect
seen for
pagA::lacZ
. Slightly elevated ex-
pression of an
atxA::lacZ
reporter was also
found in the
abrB
null mutant, although,
in the reciprocal experiment,
abrB
expres-
sion was found to be independent of
atxA
.
Thus, similar to
pagR, abrB
appears to
be a negative regulator of toxin gene and
atxA
expression. As with
pagR
, however,
the
abrB
-mediated regulation of toxin gene
expression is not believed to occur solely
through
abrB
’s effects on
atxA
expression
because AtxA levels in cells are not limiting
for toxin gene expression.
In
B. subtilis, abrB
is regulated by Spo0A.
The situation is similar in
B. anthracis
.
Normally, low levels of AbrB are found
in early log phase. These levels increase
to a maximum level in log phase then
drop back down to a low level as the
cells enter stationary phase. In contrast,
AbrB in an
spo0A
mutant persists at high
levels in stationary phase, suggesting that
abrB
expression is normally negatively
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