Environmental Engineering Reference
In-Depth Information
enzyme systems have not been studied so far. This section briefly highlights some
DMSO reductase systems that are of interest due to their occurrence in particular
microorganisms and specific adaptations to the lifestyle of the source organisms.
This highlights the functional diversity of the DMSOR enzymes that has not been
explored in detail for most bacteria capable of DMSO reduction.
2.1.3.1 The
E. coli
Ynf Selenate Reductase
The
ynf
operon of
E. coli
has long been known to encode an enzyme related to the
DmsA DMSO reductase, but the enzyme had also been shown to be unable to
support growth of
E. coli
on DMSO in a DmsA-knockout background [
108
]. The
operon encoding this enzyme is unusual as there are two genes (
ynfEF
) encoding
catalytic subunits related to DmsA at the start of the
ynfEFGHI
operon. The
ynfGH
genes encode proteins related to DmsB and DmsC, respectively. The final gene in
the gene cluster,
ynfI
, has been renamed and is now known as
DmsD
after it was
recognized that it encodes a molecular chaperone that is required for the maturation
of the DmsABC DMSO reductase and presumably also the Ynf selenate reductase
[
104
,
105
]. Interestingly, the
ynf
promoter is activated by FNR and repressed by
NarL, both of which also control expression of the
dmsABC
promoter [
107
]. A
recent study into selenate reduction in
Salmonella typhimurium
and
E. coli
K12
identified
ynfEF
as putative selenate reductases, which explains the inability of the
ynf
gene locus to enable DMSO-based anaerobic respiration in a
dmsABC
knock-
out strain [
162
]
.
2.1.3.2 The
E. coli
TorZ S- and N-Oxide Reductase
Although it has been clearly shown that the DmsABC protein is the main DMSO
reductase in
E. coli
, a 'Dor-type' DMSO reductase has been identified in
E. coli
[
163
]. This protein, TorZ, is encoded by a two gene operon,
torYZ
(also known as
yecK bisZ
), in which the second gene encodes a cytochrome that is related to the
TorC/DorC cytochromes. Initially, TorZ was described as encoding an alternative
“minor” biotin sulfoxide reductase and had been designated 'BisZ' [
164
], but it was
later reclassified as a TMAO respiratory system, based on its structural similarity to
the well characterized TorA TMAO reductase, and kinetic studies that showed that
TorZ is capable of reducing both N- and S-oxides [
163
]. Sequence alignments
clearly show that TorZ contains residues equivalent to both Y114 and W116, the
residues that have been implicated in substrate specificity and the ability to reduce
S-oxides in the
Rhodobacter
DMSO reductases. The exact function of TorZ in
E. coli
metabolism is unknown at present, but TorZ-like enzymes are present in a
variety of Enterobacteriaceae and also Pasteurellaceae.
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