FIG. 5.

SQR activity in membranes of E. coli expressing A. halophytica SQR (A) and kinetics of PQ-1 reduction and reoxidation in the presence of E. coli membranes (B). (A) Membranes of induced BL21(DE3)/pTAHsqr (60 μg of protein) were incubated under anaerobic conditions in an assay mixture containing 10 mM potassium HEPES (pH 7.4), 10 mM MgCl₂, 10 mM KCl, and 50 μM PQ-1. The reaction was started by the addition of 60 μM Na₂S. NQNO (1 μM) was added where indicated. (B) An assay mixture containing 60 μM H₂S was incubated under anaerobic conditions in either the presence (———) or the absence (–––) of noninduced E. coli membranes (60 μg of protein). Partially purified SQR obtained from O. limnetica thylakoids (40 μg of chlorophyll) was added where indicated. PQH₂-1, reduced PQ-1.