SEQUENCE-ANALYSIS OF THE PHS OPERON IN SALMONELLA-TYPHIMURIUM AND THECONTRIBUTION OF THIOSULFATE REDUCTION TO ANAEROBIC ENERGY-METABOLISM

The phs chromosomal locus of Salmonella typhimurium is essential for t he dissimilatory anaerobic reduction of thiosulfate to hydrogen sulfid e, Sequence analysis of the phs region revealed a functional operon wi th three open reading frames, designated phsA, phsB, and phsC, which e ncode peptides of 82.7, 21.3, and 28.5 kDa, respectively, The predicte d products of phsA and phsB exhibited significant homology with the ca talytic and electron transfer subunits of several other anaerobic moly bdoprotein oxidoreductases, including Escherichia coli dimethyl sulfox ide reductase, nitrate reductase, and formate dehydrogenase. Simultane ous comparison of PhsA to seven homologous molybdoproteins revealed nu merous similarities among all eight throughout the entire frame, hence , significant amino acid conservation among molybdoprotein oxidoreduct ases. Comparison of PhsB to six other homologous sequences revealed fo ur highly conserved iron-sulfur clusters, The predicted phsC product w as highly hydrophobic and similar in size to the hydrophobic subunits of the molybdoprotein oxidoreductases containing subunits homologous t o phsA and phsB. Thus, phsABC appears to encode thiosulfate reductase, Single-copy phs-lac translational fusions required both anaerobiosis and thiosulfate for full expression, whereas multicopy phs-lac transla tional fusions responded to either thiosulfate dr anaerobiosis, sugges ting that oxygen and thiosulfate control of phs involves negative regu lation. A possible role for thiosulfate reduction in anaerobic respira tion was examined, Thiosulfate did not significantly augment the final densities of anaerobic cultures grown on any of the 18 carbon sources tested, On the other hand, washed stationary-phase cells depleted of ATP were shown to synthesize small amounts of ATP on the addition of f ormate and thiosulfate, suggesting that thiosulfate reduction plays a unique role in anaerobic energy conservation by S. typhimurium.