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The SHAM-sensitive alternative oxidase has been studied in Tetrahymena pyriformis strain ST as a function both of growth state and of chloramphenicol treatment. In low density cultures the total alternative oxidase activity, as revealed by SHAM titration in the presence of CN−, is equivalent to 17% of total respiration and is slightly utilized at all times. Stationary phase cells have somewhat less of the oxidase and it is not utilized even in CCCP-uncoupled cells. Respiration in chloramphenicol-treated cells is 100% CN−-resistant. Alternative oxidase activity is equivalent to 30-40% of this total and one third of it is active. The remaining 60% residual respiration is due to unknown oxidases. Following CCCP-uncoupling, a CN−-sensitive pathway is demonstrable and the alternative oxidase is fully utilized. The higher proportion of alternative oxidase in chloramphenicol-treated cells is brought about by its conservation at a time when the cytochrome chain is becoming non-functional. There is no large scale induction of the alternative oxidase.
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