Gene clusters encoding putative outer membrane electron conduits have specific roles during metal and electrode respiration in Geobacter sulfurreducens

Abstract

At least five gene clusters in the Geobacter sulfurreducens genome encode putative outer membrane 'electron conduits', which are redox active complexes containing a periplasmic multiheme c -cytochrome, integral outer membrane β-barrel, and outer membrane redox lipoprotein. Single gene-cluster deletion and all possible combinations of deletion mutants were constructed and grown with graphite electrodes poised at +0.24 V and -0.1 V vs. SHE, Fe(III)- and Mn(IV)-oxides, and soluble Fe(III)-citrate. Different gene clusters were necessary for reduction of each electron acceptor. For example, only the Δ extABCD cluster mutant had a severe growth defect on graphite electrodes at all redox potentials, but this mutation did not affect Fe(III)-oxide, Mn(IV)-oxide, or Fe(III)-citrate reduction. During metal oxide reduction, deletion of the previously described omcBC cluster caused defects, but deletion of additional components in the Δ omcBC background, such as extEFG , was necessary to produce defects greater than 50% compared to wild type. Deletion of all five gene clusters was required to abolish all metal reduction. Mutants containing only one cluster were able to reduce particular terminal electron acceptors better than wild type, suggesting routes for improvement by targeting substrate-specific electron transfer pathways. Our results show G. sulfurreducens utilizes different membrane conduits depending on the extracellular acceptor used.