Figure 4.

PSI is photoinhibited in pgr5 cells adapted to phototrophic conditions at light intensities above 20 μmol photons m⁻² s⁻¹. Cells were acclimated to low-light conditions (TAP medium at 20 μmol photons m⁻² s⁻¹ illumination) before incubation in MIN medium for 24 h at given light intensities measuring P₇₀₀ oxidoreduction kinetics and protein accumulation. Under aerobic conditions, P₇₀₀ can be oxidized in the presence of DCMU. Using an actinic light intensity of 3,300 μmol photons m⁻² s⁻¹ gives access to the full amount of oxidizable P₇₀₀. A, P₇₀₀ kinetics for the wild-type (WT) line and pgr5 at two light intensities: 20 and 200 μmol photons m⁻² s⁻¹ grown cells. B, P₇₀₀ oxidation ratio (at a light intensity of 3,300 μmol photons m⁻² s⁻¹) for high-light-grown cells in the presence or absence of 1 mm methylviologen (MV) cannot restore the oxidizable portion of P₇₀₀ shown to be lost in A. Histograms show mean values ± sd (n = 3). C, PSAD is reduced by 50% in 200 μmol photons m⁻² s⁻¹-adapted pgr5 compared with the WT line. Western-blot analysis on extracted whole proteins runs on denaturing SDS-PAGE. WT line and pgr5 whole-cell extracts were loaded per chlorophyll basis (100% = 2.5 μg). Filters were incubated with antibodies against CF1, PSI subunit (PSAD), CEF protein (PGRL1), supercomplex protein (ANR1), the qE-related protein (LHCSR3), PSII subunit (D1), cyt f, NDH (Nda2), and mitochondrial protein (COXIIb). [See online article for color version of this figure.]