Environmental Engineering Reference
In-Depth Information
protein secretion mutant of S. oneidensis (constructed by targeted replacement
of wild-type gspD , encoding the putative OM secretion of type II secretion,
with an insertionally-inactivated gspD construct), is unable to respire anaerobi-
cally on solid Fe(III) or solid Mn(IV), yet retains the ability to respire all other
electron acceptors, including soluble Fe(III) and AQDS [18]. As observed with
the Fe(III) respiration-deficient gspE insertional mutant of S. putrefaciens ,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 lo-
cation in the gspD insertional mutant. Type II protein secretion may therefore
be required for direct enzymatic reduction of solid Fe(III) and solid Mn(IV)
by all metal-reducing members of the genus Shewanella . The ability to respire
solid Fe(III) and solid Mn(IV) is rescued in the S. oneidensis gspD insertional
mutant by addition of AQDS, an indication that the AQDS electron shuttling
pathway (Mechanism No. 2; see below) may functionally replace the Type II
protein secretion-linked pathway for anaerobic respiration on solid Fe(III) and
Mn(IV) oxides [18].
The OM proteins involved in the terminal steps of electron transport to solid
Fe(III) remain unknown, yet most likely include several c -type cytochromes.
The S. oneidensis MR-1 genome encodes 42 predicted c -type cytochromes
[29, 55], including those in the mtrDEF-omcA-mtrCAB gene cluster which
may encode several Fe(III) terminal reductase components. MtrA and MtrD
[73] are decaheme c -type cytochromes that display 99% similarity to each
other, suggesting they may provide complementary function. MtrA and MtrD
are OM-associated, but are oriented toward the periplasm [73] and therefore not
in position to contact solid Fe(III) directly. MtrB is a putative beta-barrel protein
required for Fe(III) and Mn(IV) reduction activity [5, 62]. mtrB mutants are
completely abolished in Mn(IV) reduction activity and are severely impaired
(but not completely abolished) in Fe(III) reduction activity, yet retain the ability
to reduce all other electron acceptors [5]. MtrB is postulated to be involved in
OM localization of c -type cytochromes (e.g., OmcA, MtrC) involved in electron
transfer to Fe(III) and Mn(IV) [62]. MtrC (OmcB) is an OM decaheme, c -type
cytochrome required for both Fe(III) and Mn(IV) reduction. OmcA, on the
other hand, is an OM decaheme c -type cytochrome involved only in Mn(IV)
reduction [61]. omcA -deficient mutant strains reduce Mn(IV) at 45% wild-type
rates. Interestingly, mtrC overexpression in an omcA -deficient mutant restores
Mn(IV) reduction activity to greater than wild-type rates. MtrC overexpression
may therefore compensate for the absence of OmcA in the Shewanella Mn(IV)
reduction pathway, which suggests at least partial overlap in the roles of these
OM cytochromes in the Mn(IV) reduction pathway [63]. The functions of MtrC
and OmcA in solid Fe(III) oxide reduction remain unknown.
Lipopolysaccharides (LPS) and OM proteins may play a critical role in
establishing and maintaining contact with solid Fe(III) oxides. The ability of
Search WWH ::




Custom Search