Affiliation:
1. BioTechnology Institute, University of Minnesota—Twin Cities, St. Paul, Minnesota, USA
2. Department of Microbiology, University of Minnesota—Twin Cities, St. Paul, Minnesota, USA
Abstract
ABSTRACT
Shewanella oneidensis
strain MR-1 is widely studied for its ability to respire a diverse array of soluble and insoluble electron acceptors. The ability to breathe insoluble substrates is defined as extracellular electron transfer and can occur via direct contact or by electron shuttling in
S. oneidensis
. To determine the contribution of flavin electron shuttles in extracellular electron transfer, a transposon mutagenesis screen was performed with
S. oneidensis
to identify mutants unable to secrete flavins. A multidrug and toxin efflux transporter encoded by SO_0702 was identified and renamed
bfe
(
b
acterial
f
lavin adenine dinucleotide [FAD]
e
xporter) based on phenotypic characterization. Deletion of
bfe
resulted in a severe decrease in extracellular flavins, while overexpression of
bfe
increased the concentration of extracellular flavins. Strains lacking
bfe
had no defect in reduction of soluble Fe(III), but these strains were deficient in the rate of insoluble Fe(III) oxide reduction, which was alleviated by the addition of exogenous flavins. To test a different insoluble electron acceptor, graphite electrode bioreactors were set up to measure current produced by wild-type
S. oneidensis
and the Δ
bfe
mutant. With the same concentration of supplemented flavins, the two strains produced similar amounts of current. However, when exogenous flavins were not supplemented to bioreactors,
bfe
mutant strains produced significantly less current than the wild type. We have demonstrated that flavin electron shuttling accounts for ~75% of extracellular electron transfer to insoluble substrates by
S. oneidensis
and have identified the first FAD transporter in bacteria.
IMPORTANCE
Extracellular electron transfer by microbes is critical for the geochemical cycling of metals, bioremediation, and biocatalysis using electrodes. A controversy in the field was addressed by demonstrating that flavin electron shuttling, not direct electron transfer or nanowires, is the primary mechanism of extracellular electron transfer employed by the bacterium
Shewanella oneidensis
. We have identified a flavin adenine dinucleotide transporter conserved in all sequenced
Shewanella
species that facilitates export of flavin electron shuttles in
S. oneidensis
. Analysis of a strain that is unable to secrete flavins demonstrated that electron shuttling accounts for ~75% of the insoluble extracellular electron transfer capacity in
S. oneidensis
.
Publisher
American Society for Microbiology
Cited by
394 articles.
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