Proteome of Geobacter sulfurreducens grown with Fe(III) oxide or Fe(III) citrate as the electron acceptor.

The mechanisms for Fe(III) oxide reduction in Geobacter species are of interest because Fe(III) oxides are the most abundant form of Fe(III) in many soils and sediments and Geobacter species are prevalent Fe(III)-reducing microorganisms in many of these environments. Protein abundance in G. sulfurreducens grown on poorly crystalline Fe(III) oxide or on soluble Fe(III) citrate was compared with a global accurate mass and time tag proteomic approach in order to identify proteins that might be specifically associated with Fe(III) oxide reduction. A total of 2991 proteins were detected in G. sulfurreducens grown with acetate as the electron donor and either Fe(III) oxide or soluble Fe(III) citrate as the electron acceptor, resulting in 86% recovery of the genes predicted to encode proteins. Of the total expressed proteins 76% were less abundant in Fe(III) oxide cultures than in Fe(III) citrate cultures, which is consistent with the overall slower rate of metabolism during growth with an insoluble electron acceptor. A total of 269 proteins were more abundant in Fe(III) oxide-grown cells than in cells grown on Fe(III) citrate. Most of these proteins were in the energy metabolism category: primarily electron transport proteins, including 13 c-type cytochromes and PilA, the structural protein for electrically conductive pili. Several of the cytochromes that were more abundant in Fe(III) oxide-grown cells were previously shown with genetic approaches to be essential for optimal Fe(III) oxide reduction. Other proteins that were more abundant during growth on Fe(III) oxide included transport and binding proteins, proteins involved in regulation and signal transduction, cell envelope proteins, and enzymes for amino acid and protein biosynthesis, among others. There were also a substantial number of proteins of unknown function that were more abundant during growth on Fe(III) oxide. These results indicate that electron transport to Fe(III) oxide requires additional and/or different proteins than electron transfer to soluble, chelated Fe(III) and suggest proteins whose functions should be further investigated in order to better understand the mechanisms of electron transfer to Fe(III) oxide in G. sulfurreducens.