OmcF, a Putative c -Type Monoheme Outer Membrane Cytochrome Required for the Expression of Other Outer Membrane Cytochromes in Geobacter sulfurreducens

Abstract

Outer membrane cytochromes are often proposed as likely agents for electron transfer to extracellular electron acceptors, such as Fe(III). The omcF gene in the dissimilatory Fe(III)-reducing microorganism Geobacter sulfurreducens is predicted to code for a small outer membrane monoheme c-type cytochrome. An OmcF-deficient strain was constructed, and its ability to reduce and grow on Fe(III) citrate was found to be impaired. Following a prolonged lag phase (150 h), the OmcF-deficient strain developed the ability to grow in Fe(III) citrate medium with doubling times and yields that were ca. 145% and 70% of those of the wild type, respectively. Comparison of the c-type cytochrome contents of outer membrane-enriched fractions prepared from wild-type and OmcF-deficient cultures confirmed the outer membrane association of OmcF and revealed multiple changes in the cytochrome content of the OmcF-deficient strain. These changes included loss of expression of two previously characterized outer membrane cytochromes, OmcB and OmcC, and overexpression of a third previously characterized outer membrane cytochrome, OmcS, during growth on Fe(III) citrate. The omcB and omcC transcripts could not be detected in the OmcF-deficient mutant by either reverse transcriptase PCR or Northern blot analyses. Expression of the omcF gene in trans restored both the capacity of the OmcF-deficient mutant to reduce Fe(III) and wild-type levels of omcB and omcC mRNA and protein.Thus, elimination of OmcF may impair Fe(III) reduction by influencing expression of OmcB, which has previously been demonstrated to play a critical role in Fe(III) reduction.

Proteins involved in electron transfer to Fe(III) and Mn(IV) oxides by Geobacter sulfurreducens and Geobacter uraniireducens

“…In contrast to omcG and omcH, omcF was not differentially expressed during growth on metal oxides. Previous studies have shown that soluble Fe(III) reduction is impaired in the omcF deletion mutant strain (Kim et al, 2005). In this study, loss of OmcF significantly impaired growth on Fe(III) oxide but not Mn(IV) oxide (Table 5 and Fig.…”

“…A deletion mutant of omcA had no phenotype. Previous studies showed impaired reduction of soluble Fe(III) by a double mutant OmcF (GSU2432) is another monohaem outer membrane c-type cytochrome that is required for OmcB expression (Kim et al, 2005); deletion of the omcF gene impairs transcription of omcB. In contrast to omcG and omcH, omcF was not differentially expressed during growth on metal oxides.…”

Transcriptome of Geobacter uraniireducens growing in uranium-contaminated subsurface sediments

“…Some of the cytochromes embedded in the outer membrane, such as OmcB (Qian et al, 2007), may also be important for extracellular electron transfer (Leang et al, 2003;Leang and Lovley, 2005), whereas others, such as OmcF, OmcG and OmcH, appear to participate in Fe(III) reduction indirectly by influencing the expression of other outer-surface cytochromes (Kim et al, 2005(Kim et al, , 2006. Studies have also suggested that some of the periplasmic and inner-membrane c-type cytochromes are involved in the transfer of electrons derived from central metabolism to the outer membrane .…”

Involvement of Geobacter sulfurreducens SfrAB in acetate metabolism rather than intracellular, respiration-linked Fe(III) citrate reduction

“…Due to technical considerations, chelated forms of Fe(III), most often Fe(III) citrate and Fe(III) chelated with nitrilotriacetic acid (Fe(III)-NTA), are frequently used for laboratory studies addressing Fe(III) reduction by Geobacter species (Butler et al, 2004;Ding et al, 2006;Esteve-Nú ñez et al, 2005;Kim et al, 2005Kim et al, , 2006Leang et al, 2003;Lloyd et al, 2003;Methé et al, 2005;Nú ñez et al, 2004Nú ñez et al, , 2006). An understanding of the similarities and differences between the electron transport pathways to Fe(III) citrate and insoluble Fe(III) oxides is, therefore, crucial for determining the environmental relevance of these studies.…”