SUMMARY: Whole organisms of oxidize thiosulphate to sulphate with the obligatory formation of tetrathionate as an intermediate. Oxidation of thiosulphate to tetrathionate shows an apparent of about 120 μ and is relatively insensitive to FCCP (carbonyl cyanide -trifluoromethoxyphenylhydrazone), HQNO (2-heptyl-4-hydroxyquinoline--oxide) or thiocyanate. Oxidation of tetrathionate to sulphate shows a of about 27 μ and is strongly inhibited by FCCP, HQNO, thiocyanate and gramicidin. Sulphite oxidation is also inhibited by FCCP and HQNO. Trithionate oxidation to sulphate occurred and showed unexplained dependence on the presence of sulphate ions. A H/O quotient of about 4 for proton translocation driven by substrate oxidation was seen for each of thiosulphate, tetrathionate and sulphite. ATP synthesis coupled to thiosulphate oxidation was completely abolished by FCCP. The results obtained are consistent with the oxidation of thiosulphate (and probably trithionate) to tetrathionate in the periplasm of the cell, with HQNO-insensitive electron transport to cytochrome , and with further oxidation of tetrathionate (and sulphite) to sulphate after FCCP-sensitive transport to the cytoplasmic side of the membrane. The latter oxidations involve HQNO-sensitive electron transport via cytochrome Inhibition of tetrathionate metabolism by thiocyanate and gramicidin would be consistent with tetrathionate transport by a SO/4H symport process. The proton translocation experiments indicate the mechanism of H extrusion to depend on electron transfer within the quinone/cytochromes segment of the respiratory chain, and does not involve a proton-pumping oxidase. The sulphur-compound-oxidizing system of is shown to be quite different from that previously described for


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