Evaluation of outer-surface proteins in the reduction of iron(III) oxide in Geobacter sulfurreducens
Reduction of Fe(III) in the subsurface environment is of immense environmental significance due to its impact on remediation of toxic metals and organic pollutants. Iron is mostly present in these environments as insoluble Fe(III) oxide. Geobacter are the largest known group of Fe(III) reducing microorganisms in the subsurface environment. Previous studies to understand the mechanism of Fe(III) oxide reduction have demonstrated the need for direct contact. Current study was aimed at identifying the outer surface proteins that are involved in establishing contact, electron transfer and subsequent reduction of Fe(III) oxides.
Work presented here demonstrates novel function of type IV pili in Geobacter sulfurreducens in the reduction of insoluble Fe(III) oxides. It has been established by subsequent studies in the laboratory that G. sulfurreducens uses type IV pili for direct electron transfer to insoluble Fe(III). A homologue of pilA, the type IV pilin, gspG was identified as part of a type II secretion system involved specifically in the reduction of insoluble Fe(III), a unique mechanism for dissimilatory Fe(III) reduction. Cytochromes, OmcE and OmcS were identified in an attempt to identify outer surface proteins involved in Fe(III) reduciton. OmcS and its paralogue, OmcT are the first pair of c-type cytochromes identified that are expressed specifically for the reduction of Fe(III) oxides and are crucial for their reduction. OmcE was also demonstrated to be involved in the reduction of insoluble Fe(III). The studies also emphasize the importance of evaluating mechanisms for Fe(III) reduction with environmentally relevant Fe(III) oxide, rather than the more commonly utilized Fe(III) citrate, because additional electron transfer components are required for Fe(III) oxide reduction that are not required for Fe(III) citrate reduction.