Covalent modification of chloroplast Photosystem II polypeptides by p-nitrothiophenol

Illumination of the chlorophyll a b light-harvesting complex in the presence of p-nitrothio[¹⁴C]phenol caused quenching of fluorescence emission at 685 nm (77 K) relative to 695 nm and covalent modification of light-harvesting complex polypeptides. Fluorescence quenching saturated with one p-nitrothiophenol bound per light-harvesting complex polypeptide (10-13 chlorophylls); 1 2 maximal quenching occurred with one p-nitrothiophenol bound per light-harvesting complex polypeptides (190-247 chlorophylls). This result provides direct evidence for excitation energy transfer between light-harvesting complex subunits which contain 4-6 polypeptides plus 40-78 chlorophylls per complex. Illumination of chloroplasts or Photosystem II (PS II) particles in the presence of p-nitrothio[¹⁴C]phenol caused inhibition of PS II activity and labeling of several polypeptides including those of 42-48 kilodaltons previously identified as PS II reaction center polypeptides. In chloroplasts, inhibition of oxygen evolution accelerated p-nitrothiophenol modification reactions; DCMU or donors to PS II decreased p-nitrothiophenol modification. These results are consistent with the hypothesis that accumulation of oxidizing equivalents on the donor side of PS II creates a 'reactive state' in which polypeptides of PS II are susceptible to p-nitrothiophenol modification.