The sequence of the dsr gene region of the phototrophic sulfur bacterium Chmmatium winosum D (DSMZ 1803 was determined to clarify the in wiwo role dissimilatory sulfite reductase are part of a gene cluster, dsrAB€FHCMK, that encodes four small, soluble proteins (DsrE, DsrF, DsrH and DsrC), a transmembrane protein (DsrM) with similarity to haem-bbinding polypeptides and a soluble protein (DsrK) resembling [4Fe4S]=cluster=containing heterodisulf ide reductase from methanogenic archaea. Northern hybridizations showed that expression of the dsr genes is increased by the presence of reduced sulfur compounds. The dsr genes are not only transcribed from a putative promoter upstream of dsrA but primary transcripts originating from (a) transcription start site(s) downstream of dsrB are also formed. Polar insertion mutations immediately upstream of dsrA, and in dsrB, dsrH and dsrM, led to an inability of the cells to oxidize intracellularly stored sulfur. The capability of the mutants to oxidize sulfide, thiosulfate and sulfite under photo1 ithoautotrophic conditions was unaltered. Photoorganoheterotrophic growth was also unaffected. 'Reverse' sulfite reductase and DsrEFHCMK are, therefore, not essential for oxidation of sulfide or thiosulfate, but are obligatory for sulfur oxidation. These results, together with the finding that the sulfur globules of C. vinosum are located in the extracytoplasmic space whilst the dsr gene products appear to be either cytoplasmic or membranebound led to the proposal of new models for the pathway of sulfur oxidation in this phototrophic sulfur bacterium.O f 'reverse' sirohaem sulfite reductase. The dsrAB genes encoding

The genes for adenosine-5'-phosphosulfate (APS) reductase, aprBA, and sirohaem sulf ite reductase, dsrAB, from the sulfur-oxidizing phototrophic bacterium Chmmatium winosum strain D (DSMZ 1803 were cloned and sequenced. Statistically significant sequence similarities and similar physicochemical properties suggest that the aprBA and dsrAB gene products from Chr. winosum are true homologues of their counterparts from the sulfatereducing chemotrophic archaeon Amhaeoglobus filgidus and the sulfatereducing chemotrophic bacterium Desulfowibrio wlgaris. Evidence for the proposed duplication of a common ancestor of the dsrAB genes is provided. Phylogenetic analyses revealed a greater evolutionary distance between the enzymes from Chr. winosum and D. vulgaris than between those from A. fulgidus and D. wlgaris. The data reported in this study are most consistent with the concept of common ancestral protogenotic genes both for dissimilatory sirohaem sulfite reductases and for APS reductases. The aprA gene was demonstrated to be a suitable DNA probe for the identification of apr genes from organisms of different phylogenetic positions. PCR primers and conditions for the amplification of apr homologous regions are described.

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