Identification of the gene encoding the sole physiological fumarate reductase in Shewanella oneidensis MR-1

Shewanella oneidensis MR-1 is a Gram-negative, nonfermentative rod with a complex electron transport system which facilitates its ability to use a variety of terminal electron acceptors, including fumarate, for anaerobic respiration. CMTn-3, a mutant isolated by transposon (TnphoA) mutagenesis, can no longer use fumarate as an electron acceptor; it lacks fumarate reductase activity as well as a 65-kDa soluble tetraheme flavocytochrome c. The sequence of the TnphoA-flanking genomic DNA of CMTn-3 did not align to those for fumarate reductase or related electron transport genes from other bacteria. Sequence analysis of the MR-1 genomic database demonstrated that an open reading frame encoding a 65-kDa tetraheme cytochrome c with sequence similarity to the fumarate reductase from S. frigidimarina NCIMB400 was found 8 kb away from the TnphoA-flanking genomic DNA of CMTn-3. PCR analysis demonstrated that a large deletion (>or=9.2 kb and