Metal reduction and protein secretion genes required for iodate reduction by Shewanella oneidensis

Abstract

The metal-reducing gammaproteobacterium Shewanella oneidensis reduces iodate (IO 3- ) as an anaerobic terminal electron acceptor. Microbial IO 3- electron transport pathways are postulated to terminate with nitrate (NO 3- ) reductase, which reduces IO 3- as an alternative electron acceptor. Recent studies with S. oneidensis, however, have demonstrated that NO 3- reductase is not involved in IO 3- reduction. The main objective of the present study was to determine the metal reduction and protein secretion genes required for IO 3- reduction by Shewanella oneidensis with lactate, formate, or H 2 as the electron donor. With all electron donors, the type I and type V protein secretion mutants retained wild-type IO 3- reduction activity, while the type II protein secretion mutant lacking the outer membrane secretin GspD was impaired in IO 3- reduction. Deletion mutants lacking the cyclic AMP receptor protein (CRP), cytochrome maturation permease CcmB, and inner membrane-tethered c-type cytochrome CymA were impaired in IO 3- reduction with all electron donors, while deletion mutants lacking c-type cytochrome MtrA and outer membrane β-barrel protein MtrB of the outer membrane MtrAB module were impaired in IO 3- reduction with only lactate as an electron donor. With all electron donors, mutants lacking the c-type cytochromes OmcA and MtrC of the metalreducing extracellular electron conduit MtrCAB retained wild-type IO 3- reduction activity. These findings indicate that IO 3- reduction by S. oneidensis involves electron donor-dependent metal reduction and protein secretion pathway components, including the outer membrane MtrAB module and type II protein secretion of an unidentified IO 3- reductase to the S. oneidensis outer membrane.