CLONING AND CHARACTERIZATION OF A GENE-CLUSTER, PHSBCDEF, NECESSARY FOR THE PRODUCTION OF HYDROGEN-SULFIDE FROM THIOSULFATE BY SALMONELLA-TYPHIMURIUM

We cloned, by complementation of an H2S- mutant, a cluster of Salmonel la typhimurium genes, phsBCDEF, that appears to be essential for the a naerobic production of hydrogen sulfide from thiosulfate. Tn5 mutagene sis and ExoIII deletion analysis showed that approx. the entire region of a 3.3-kb subclone was necessary for H2S production. Subsequent seq uencing revealed the presence of five potential translationally couple d open reading frames (ORFs). Their putative protein products were con firmed by synthesis from a phage T7 expression system. Comparison of t he encoded sequences with previously determined sequences suggests tha t these genes constitute part of a thiosulfate-reducing operon coding for a membrane-associated electron transport chain which contains prot eins potentially capable of ligating iron-sulfur clusters and heme. Im mediately upstream from these genes, a region encoding the C-terminal portion of an ORF (OrfA) was identified that showed a high degree of s imilarity to some other anaerobic terminal reductases, polysulfide red uctase (PsrA) of Wolinella succinogenes and dimethylsulfoxide reductas e (DmsA), formate dehydrogenase (formate-hydrogene-lyase linked) (FdhF ) and nitrate reductase (NarG) of Escherichia coli.