Role for Outer Membrane Cytochromes OmcA and OmcB of Shewanella putrefaciens MR-1 in Reduction of Manganese Dioxide

Abstract

Shewanella putrefaciens MR-1 can use a wide variety of terminal electron acceptors for anaerobic respiration, including certain insoluble manganese and iron oxides. To examine whether the outer membrane (OM) cytochromes of MR-1 play a role in Mn(IV) and Fe(III) reduction, mutants lacking the OM cytochrome OmcA or OmcB were isolated by gene replacement. Southern blotting and PCR confirmed replacement of the omcA and omcB genes, respectively, and reverse transcription-PCR analysis demonstrated loss of the respective mRNAs, whereas mRNAs for upstream and downstream genes were retained. The omcA mutant (OMCA1) resembled MR-1 in its growth on trimethylamine N-oxide (TMAO), dimethyl sulfoxide, nitrate, fumarate, thiosulfate, and tetrathionate and its reduction of nitrate, nitrite, ferric citrate, FeOOH, and anthraquinone-2,6-disulfonic acid. Similarly, the omcB mutant (OMCB1) grew on fumarate, nitrate, TMAO, and thiosulfate and reduced ferric citrate and FeOOH. However, OMCA1 and OMCB1 were 45 and 75% slower than MR-1, respectively, at reducing MnO 2 . OMCA1 lacked only OmcA. While OMCB1 lacked OmcB, other OM cytochromes were also missing or markedly depressed. The total cytochrome content of the OM of OMCB1 was less than 15% of that of MR-1. Western blots demonstrated that OMCB1 still synthesized OmcA, but most of it was localized in the cytoplasmic membrane and soluble fractions rather than in the OM. OMCB1 had therefore lost the ability to properly localize multiple OM cytochromes to the OM. Together, the results suggest that the OM cytochromes of MR-1 participate in the reduction of Mn(IV) but are not required for the reduction of Fe(III) or other electron acceptors.

Genome sequence of the dissimilatory metal ion–reducing bacterium Shewanella oneidensis

“…These include OmcA and OmcB, which are known to be involved in the reduction of extracellular MnO 2 (ref. 16). Diversification of the S. oneidensis decaheme cytochrome c proteins has allowed for a specialization of labor of these proteins in both cellular location (i.e., the periplasmic side of the cytoplasmic membrane, the periplasmic space, and the outer membrane) and specific electron-transporting role.…”

Characterization of Shewanella oneidensis MtrC: a cell-surface decaheme cytochrome involved in respiratory electron transport to extracellular electron acceptors

“…Electron transport proteins proposed to be involved in Fe(III) respiration in Shewanella oneidensis MR-1 include an inner-membrane periplasmic tetraheme quinol dehydrogenase (CymA) [8,12], two periplasmic decaheme cytochromes (MtrD and MtrA) [10,13], periplasmic tetraheme cytochrome (Stc) [14,15], the cytochrome domain of Fe(III)-induced flavocytochrome c 3 , [16] two putative outer-membrane b-barrel proteins (MtrE and MtrB) [17][18][19] and three outer-membrane decaheme cytochromes (MtrF, OmcA and MtrC) [11,18,20,21]. Although genetic studies have identified specific proteins that are involved in Fe(III) respiration in Shewanella sp.…”

“…OmcA and MtrC protein sequences both comprise ten putative cheme binding site motifs (CXXCH, where X is any residue) and a lipid-binding motif for anchoring to the outer membrane [11,22]. Because of their proposed location, these multiheme cytochromes are believed to function in Fe(III) respiration by mediating electron transfer across the microbe-mineral oxidant interface [5,10,13,18,22]. Significantly, a DmtrC strain of S. oneidensis MR-1 displayed a marked decrease in the ability to respire iron(III) oxides and iron(III) citrate, while retaining the ability to respire soluble terminal electron acceptors, including NO 3 À and NO 2 À [11,17].…”

Aerobic bioreduction of nickel(II) to elemental nickel with concomitant biomineralization

“…Although the specific mechanism of Ni(II) reduction in strain MBR was not well demonstrated, the TEM studies revealed that the final microbial pathway of Ni(II) reduction was intracellularly localized. Since the earlier studies of electron transfer in Geobacter species and Shewanella putrefaciens MR-1 Lovley et al 1993;Myers and Myers 2001), cytochromes involvement in electron transfer to Ni(II) has been speculated. Further studies on the potential role of external membrane proteins in electron transfer to Ni(II) by strain MBR are needed.…”