Anaerobic Respiration of Elemental Sulfur and Thiosulfate by Shewanella oneidensis MR-1 Requires psrA , a Homolog of the phsA Gene of Salmonella enterica Serovar Typhimurium LT2

ABSTRACT Shewanella oneidensis MR-1, a facultatively anaerobic gammaproteobacterium, respires a variety of anaerobic terminal electron acceptors, including the inorganic sulfur compounds sulfite (SO 3 2− ), thiosulfate (S 2 O 3 2− ), tetrathionate (S 4 O 6 2− ), and elemental sulfur (S 0 ). The molecular mechanism of anaerobic respiration of inorganic sulfur compounds by S. oneidensis , however, is poorly understood. In the present study, we identified a three-gene cluster in the S. oneidensis genome whose translated products displayed 59 to 73% amino acid similarity to the products of phsABC , a gene cluster required for S 0 and S 2 O 3 2− respiration by Salmonella enterica serovar Typhimurium LT2. Homologs of phsA (annotated as psrA ) were identified in the genomes of Shewanella strains that reduce S 0 and S 2 O 3 2− yet were missing from the genomes of Shewanella strains unable to reduce these electron acceptors. A new suicide vector was constructed and used to generate a markerless, in-frame deletion of psrA , the gene encoding the putative thiosulfate reductase. The psrA deletion mutant (PSRA1) retained expression of downstream genes psrB and psrC but was unable to respire S 0 or S 2 O 3 2− as the terminal electron acceptor. Based on these results, we postulate that PsrA functions as the main subunit of the S. oneidensis S 2 O 3 2− terminal reductase whose end products (sulfide [HS − ] or SO 3 2− ) participate in an intraspecies sulfur cycle that drives S 0 respiration.