Environmental Engineering Reference
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ferred to agar growth medium supplemented with U(VI)-carbonate as electron
acceptor and incubated under microaerobic conditions. The resulting colonies
were examined for production of a brown precipitate (presumably U(IV)) on
their surface. Strains unable to produce the U(IV) precipitate were subsequently
tested for anaerobic growth in liquid medium supplemented with U(VI) as sole
electron acceptor. Strains displaying a U(VI) reduction-deficient phenotype on
the rapid plate assay were also unable to respire U(VI) in anaerobic liquid
growth medium. All Urr mutant strains also lacked the ability to respire NO
2
−
.
In particular, Urr mutant strain U14 retained the ability to respire all electron
acceptors except U(VI) and NO
2
−
. These results suggest that the electron trans-
port chains terminating with the reduction of NO
2
−
and U(VI) share common
respiratory components, possibly including the NO
2
−
reductase itself.
Three types of respiratory NO
2
−
reductases are found in bacteria: Cu-contai-
ning NirK, cytochrome
cd
1
-containing NirS and
c
-type cytochrome NrfA [52].
In the NO
3
−
reduction pathways of denitrifying bacteria such as
Alcaligenes
faecalis
and
Pseudomonas stutzeri
, NirK and NirS reduce NO
2
−
to nitric oxide
(NO), which is subsequently reduced to N
2
via a nitrous oxide (N
2
O) interme-
diate. In the NO
3
−
reduction pathways of
E. coli
and
Wolinella succinogenes
,
on the other hand, periplasmic NrfA catalyzes the 6-electron reduction of NO
2
−
to NH
3
[81]. The
S. oneidensis
MR-1 genome was scanned for putative NO
2
−
reductases via Basic Local Alignment Search Tool (BLAST) analysis. Only the
nrfA
homolog was found in the
S. oneidensis
genome: Locus SO3980 is 79%
similar (65% identical) to
nrfA
of
Escherichia coli
.
In
E. coli
, NrfA requires three intermediary components (NrfB-D) to receive
electrons from the menaquinone pool. NrfD is a putative inner-membrane
quinol oxidase that transfers electrons from the menaquinone pool to NrfC.
NrfC is a FeS protein that delivers electrons to the soluble
c
-type cytochrome
NrfB, which subsequently delivers electrons to NrfA [31].
W. succinogenes
,
in contrast, requires only one intermediary component: inner-membrane asso-
ciated NrfH (a NapC/NirT homolog; [80]), which displays quinol:NrfA ox-
idoreductase activity. Interestingly, the
S. oneidensis
genome contains ORFs
encoding proteins similar to both NrfB and NrfH. NrfB displays similarity to
MtrA and MtrD, the paralogous proteins involved in Fe(III) reduction [73].
In addition, CymA, a recently discovered NapC/NirT homolog also required
for NO
2
−
reduction [77] transfers electrons from the menaquinone pool to the
Fe(III) terminal reductase complexes in
S. oneidensis
[59, 60, 77] (Figs. 1-4).
The periplasmic location of the Nrf system in
E. coli
and
W. succinogenes
is
consistent with the observed subcellular location of U(IV) in U(VI)-respiring
Shewanella
. A working model for the U(VI) reduction pathway in
S. oneidensis
is displayed in Fig. 5.
