Studies on the mechanism of photosystem II photoinhibition I. A two-step degradation of D1-protein.

The role of D1-protein in photoinhibition was examined. Photoinhibition of spinach thylakoids at 20°C caused considerable degradation of D1-protein and a parallel loss of variable fluorescence, QB-independent electron flow and QB-dependent electron flow. The breakdown of D1-protein as well as the loss of variable fluorescence and QB-independent electron flow were largely prevented when thylakoids were photoinhibited at 0°C. The QB-dependent electron flow markedly decreased under the same conditions. This inactivation may represent the primary event in photoinhibition and could be the result of some modification at the QB-site of D1-protein. Evidence for this comes from fluorescence relaxation kinetics following photoinhibition at 0°C which indicate a partial inactivation of QA (-)-reoxidation. These results support the idea of D1-protein breakdown during photoinhibition as a two step process consisting of an initial inactivation at the QB-site of the protein followed by its degradation. The latter is accompanied by the loss of PS II-reaction centre function.