Studies with pure cultures of dissimilatory metal-reducing microorganisms have demonstrated that outer-surface
c-type cytochromes are important electron transfer agents for the reduction of metals, but previous environmental proteomic studies have typically not recovered cytochrome sequences from subsurface environments in which metal reduction is important. Gel-separation, heme-staining and mass spectrometry of proteins in groundwater from
in situ uranium bioremediation experiments identified a putative
c-type cytochrome, designated
Geobacter
subsurface
c-type
cytochrome
A (GscA), encoded within the genome of strain M18, a
Geobacter isolate previously recovered from the site. Homologs of GscA were identified in the genomes of other
Geobacter isolates in the phylogenetic cluster known as subsurface clade 1, which predominates in a diversity of Fe(III)-reducing subsurface environments. Most of the
gscA sequences recovered from groundwater genomic DNA clustered in a tight phylogenetic group closely related to strain M18. GscA was most abundant in groundwater samples in which
Geobacter sp. predominated. Expression of
gscA in a strain of
Geobacter sulfurreducens that lacked the gene for the
c-type cytochrome OmcS, thought to facilitate electron transfer from conductive pili to Fe(III) oxide, restored the capacity for Fe(III) oxide reduction. Atomic force microscopy provided evidence that GscA was associated with the pili. These results demonstrate that a
c-type cytochrome with an apparent function similar to that of OmcS is abundant when
Geobacter sp. are abundant in the subsurface, providing insight into the mechanisms for the growth of subsurface
Geobacter sp. on Fe(III) oxide and suggesting an approach for functional analysis of other
Geobacter proteins found in the subsurface.
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