Abstract
The extrinsic subunits of photosystem II (PSII) in the luminal side of thylakoids play crucial roles in optimizing the water-oxidizing activity. It is well known that the compositions of the PSII extrinsic proteins are significantly different among the photo-oxygenic organisms. In higher plants, PsbO, PsbP and PsbQ proteins are the major components of luminal extrinsic proteins. We have reported the crystal structure of PsbP from Nicotiana tabacum. In addition, our RNAi studies show that PsbQ is dispensable in higher plants, whereas PsbO and PsbP are required for the stability and regulation of PSII in vivo. These observations in higher plants are greatly different from those in cyanobacteria, which suggests that not only the composition, but also the nature and function of PSII extrinsic proteins have changed considerably during evolution. In this presentation, molecular evolution of PSII extrinsic proteins will be discussed from the structural and physiological standpoints.
