Environmental Engineering Reference
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genes are also present in
R. capsulatus
37B4 but in the absence of a genome
sequence this cannot be verified at present.
Two additional proteins, DorXY, involved in regulating the activity of the
R. sphaeroides
DMSO reductase have been described [
122
]. The genes encoding
these proteins are located upstream of the
dorSR
genes and appear to be conserved in
the available genomes of
R. sphaeroides
strains, however, our analyses indicate that
no homologues of DorY or DorX are found in the
R. capsulatus
SB1003 genome.
DorY is thought to be directly involved in the activation of DMSOR expression [
122
].
2.1.1.3 Regulation of Dimethylsulfoxide Reductase Expression
Despite the very different structures of the operons encoding the enzymes, the
expression of DMSOR is induced in the absence of oxygen in both
E. coli
and the
Rhodobacter
species
.
In the
Rhodobacter
species expression of the
dorCDA
structural genes is directly
controlled by the DorRS two-component system. DorR is an OmpR-type response
regulator and has been shown to bind the
dorR-dorCDA
intergenic region where
four binding sites (consensus pentanucleotide: 5
0
-TTC/AAC-3
0
[
123
]) have been
identified [
123
-
125
]. Deletion of the
dorR
gene leads to a loss of DMSO reductase
activity [
114
,
115
], and a similar effect was observed in
R. sphaeroides
for a
dorY
deletion strain [
122
]. While this indicates that DorR and, if present, DorY are the
main regulators of
dorCDA
expression, the expression of this operon is closely
linked to several other major regulatory circuits including light intensity, cellular
redox state, availability of molybdate, and others [
114
,
119
,
122
,
126
,
127
].
In some cases specific binding sites for particular regulators have been identified,
for example, it is known that fumarate nitrate reductase regulation (FNR) controls
dorCDA
expression by activating the
dorS
promoter, thus controlling the production
of the complex sensor kinase that is part of the DorSR two component system [
114
,
126
]. In other cases the links are not as obvious, e.g., reporter gene studies identified a
reduced activity of the
dorCDA
promoter in the presence of high light intensities, an
effect that may be linked to other work that observed a negative effect of the global
RegA regulator on
dorCDA
expression [
78
,
122
]. The RegAB two-component
system is a global regulatory system that is highly prevalent in Proteobacteria and
in
Rhodobacter
it is known to control processes as diverse as photosynthesis gene
expression, nitrogen metabolism, and redox homeostasis [
128
-
130
]. The RegB
sensor kinase can directly interact with molecules of the cellular quinone pool,
while RegA has been shown to affect gene regulation not only by DNA binding,
but also through direct interactions with other regulatory proteins [
131
-
133
].
In
E. coli
a dedicated regulator of the
dmsABC
operon such as the DorSR system
appears to be absent and instead the expression of the
dmsABC
operon is controlled
by two promoters, P1 and P2, located ~200 bp upstream of the
dmsA
gene that
interact with a variety of transcription factors. P1 is controlled by FNR and a nitrate
reductase regulator protein (NarL), while P2 is regulated by integration host factor
(IHF) and the molybdate-responsive ModE regulator [
134
,
135
]. In keeping with
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