SQR activity in A. halophytica. The assay mixture contained 10 mM potassium HEPES (pH 7.4), 10 mM MgCl2, 10 mM KCl, 30 μM PQ-1, and cells containing 3 μg of chlorophyll per ml. The reduction of PQ-1 was detected at A292 minus A266. Where indicated, the reaction was started by the injection of 60 μM Na2S. PQH2-1, reduced PQ-1.