OmcB, ac-Type Polyheme Cytochrome, Involved in Fe(III) Reduction inGeobacter sulfurreducens

Open Access
Publisher Website
Google Scholar
Abstract
Microorganisms in the familyGeobacteraceaeare the predominant Fe(III)-reducing microorganisms in a variety of subsurface environments in which Fe(III) reduction is an important process, but little is known about the mechanisms for electron transport to Fe(III) in these organisms. TheGeobacter sulfurreducensgenome was found to contain a 10-kb chromosomal duplication consisting of two tandem three-gene clusters. The last genes of the two clusters, designatedomcBandomcC, encode putative outer membrane polyhemec-type cytochromes which are 79% identical. The role of theomcBandomcCgenes in Fe(III) reduction inG. sulfurreducenswas investigated. OmcB and OmcC were both expressed during growth with acetate as the electron donor and either fumarate or Fe(III) as the electron acceptor. OmcB was ca. twofold more abundant under both conditions. DisruptingomcBoromcCby gene replacement had no impact on growth with fumarate. However, the OmcB-deficient mutant was greatly impaired in its ability to reduce Fe(III) both in cell suspensions and under growth conditions. In contrast, the ability of the OmcC-deficient mutant to reduce Fe(III) was similar to that of the wild type. WhenomcBwas reintroduced into the OmcB-deficient mutant, the capacity for Fe(III) reduction was restored in proportion to the level of OmcB production. These results indicate that OmcB, but not OmcC, has a major role in electron transport to Fe(III) and suggest that electron transport to the outer membrane is an important feature in Fe(III) reduction in this organism.