Fig. 4. Regulatory functions of four OPS regulatory subunits.

a The top curve shows a typical kinetics of chlorophyll fluorescence in WT Synechocystis 6803. After WT and five defective mutants of Synechocystis 6803 (ΔndhV, ΔndhS, ΔndhO, ΔndhL, and ΔndhI) were exposed to high light (200 µmol photons m⁻² s⁻¹), their short-term response was monitored by the subsequent change in the chlorophyll fluorescence level indicated in the red box region. Actinic light (AL; 620 nm; 45 µmol photons m⁻² s⁻¹) was switched on and off at the indicated time. b, c In vitro analysis of NdhV in Fd-dependent plastoquinone reduction. Thylakoid membranes were isolated from NdhV-defective mutant (ΔndhV) cells (b) and the M55 mutant (c) of Synechocystis 6803. The plastoquinone reduction activity was monitored by the chlorophyll fluorescence under illumination with actinic light (AL; 620 nm, 918 µmol photons m⁻² s⁻¹) and inhibited by the specific inhibitor rotenone. Purified Fd, NdhV alone, NDH-1LΔV complex mixed with NdhV or NdhM, and rotenone were added at the indicated time. NdhM or BSA was used as a negative control. d, e In vitro analysis of NdhO in Fd-dependent plastoquinone reduction. Thylakoid membranes were isolated from NdhO-defective mutant (ΔndhO) cells (d) and the M55 mutant (e) of Synechocystis 6803. Purified Fd, NdhO, NdhM, NDH-1L complex or NDH-1LΔO complex, and rotenone were added at the indicated time. NdhM or buffer as used as a negative control.