Photoinhibition and Recovery in Oxygenic Photosynthesis: Mechanism of a Photosystem II Damage and Repair Cycle.

This Chapter provides highlights on the mechanism of a photosystem II (PS II) damage and repair cycle in chloroplasts. Photo-oxidative damage to the PS II reaction center is a phenomenon that occurs in every organism of oxygenic photosynthesis. Through the process of evolution, an elaborate repair mechanism was devised, one that rectifies this presumably unavoidable and irreversible photoinhibition and restores the PS II charge separation activity. The repair process entails several enzymatic reactions for the selective removal and replacement of the inactivated D1/32 kD reaction center protein (the chloroplast-encoded psbA gene product) from the massive (>1,000 kD) H2O-oxidizing and O2-evolving PS II holocomplex. This repair process is unique in the annals of biology; nothing analogous in complexity and specificity has been reported in other biological systems. Elucidation of the repair mechanism may reveal the occurrence of hitherto unknown regulatory and catalytic reactions for the selective in situ replacement of specific proteins from within multi-protein complexes. This may not only have significant applications in photosynthesis and agriculture but also in medicine and other fields. [ABSTRACT FROM AUTHOR]