Environmental Engineering Reference
In-Depth Information
menaquinol and subsequently transfers electrons to downstream components
of the electron transport chains terminating with reduction of Fe(III), Mn(IV),
NO
3
−
and fumarate [54, 60, 77]. Menaquinol oxidation and concurrent reduc-
tion of CymA releases protons to the periplasmic space, thereby generating
a proton motive force. Electrons are transferred to one of four
c
-type hemes
within CymA, followed by inter-heme electron transfer (according to decreas-
ing heme redox potential) until a final transfer is made to subsequent electron
carriers in the periplasmic space [28].
cymA-
deficient mutants of
S. putrefaciens
are unable to reduce NO3-, Fe(III), Mn(IV) or fumarate as terminal electron
acceptor [59]. CymA is therefore an integral part of the
Shewanella
electron
transport system.
After CymA, the
Shewanella
electron transport system deviates from that
generally observed in gram-negative bacteria respiring on soluble electron ac-
ceptors. Fe(III) reduction activity is detected in wild-type OM fractions [57],
and is severely impaired in mutants lacking a variety of OM proteins, including
several multi-heme
c
-type cytochromes (see below). Electron transport from
CymA to solid Fe(III) oxides is postulated to proceed via an electron transport
chain spanning the periplasmic space and terminating on the outside face of
the OM [57]. Some of the most convincing evidence supporting this hypothe-
sis has been derived from genetic studies with
S. putrefaciens
[16, 17]. Initial
gene cloning studies have demonstrated that a 23.3 kb
S
.
putrefaciens
wild-
type DNA fragment confers Fe(III) and Mn(IV) reduction activity to a set of
Fe(III) and Mn(IV) reduction-deficient mutants. The smallest complementing
fragment contains one open reading frame (ORF) whose translated product dis-
plays 87% sequence similarity to
Aeromonas hydrophila
ExeE, a member of the
GspE family of proteins found in Type II protein secretion systems. GspE inser-
tional mutants (constructed by targeted replacement of wild-type
gspE
with an
insertionally inactivated
gspE
construct) are unable to respire anaerobically on
solid Fe(III) or solid Mn(IV), yet retain the ability to respire all other electron
acceptors. Nucleotide sequence analysis of regions flanking
gspE
reveal one
partial and two complete ORFs whose translated products display 55%-70%
sequence similarity to the GspD-G homologs of Type II protein secretion sys-
tems. A heme-containing Fe(III) reductase is present in the peripheral proteins
loosely attached to the outside face of the wild-type OM, yet is missing from
this location in the
gspE
mutants. Membrane fractionation studies with the
wild-type strain support this finding: the heme-containing Fe(III) reductase is
detected in the OM but not the IM or cytoplasmic fractions. These findings
provide the first genetic evidence linking anaerobic Fe(III) and Mn(IV) respi-
ration to Type II protein secretion and provide additional biochemical evidence
supporting OM localization of
Shewanella
Fe(III) reductases [16, 17].
A contiguous cluster of 12 Type II protein secretion genes (
gspC-N
ho-
mologs) has also been identified in the
S. oneidensis
MR-1 genome. A Type II
