The effect of anionic phosphatidylglycerol (PG) on oxygen evolution in a photosystem Ⅱ(PSⅡ) particle depleted of Ca^2^+ designated as d~C~aPSⅡ in this work and the mechanism of their interaction were investigated. The major finding is the observation of a new role of PG in the PSⅡ function, that is, PG restores nearly the lost oxygen evolution in d~C~aPSⅡ particle owing to Ca^2^+ deficiency to the level in intact PSⅡ in which the same effect was achieved in our previous study. furthermore, there is a stimulation of oxygen- evolving activity in the d~C~aPSⅡ complexed with PG in the presence of exogenously added Ca^2^+, which PG enhances increasingly oxygen evolution with increasing Ca^2^+ concentration. It revealed that PG- induced oxygen evolution recovery results from resumption of normal structure in protein by PG effect, whereas the enhancement of oxygen evolution in d~C~aPSⅡ～PG complex subject to Ca^2^+ is ascribed to the optimization of such a structure due to coordination complex formation of Ca^2^+ ions with PG.

Key words: PHOSPHATIDYL GLYCEROL, CALCIUM ION, FOURIER TRANSFORM, INFRARED SPECTROPHOTOMETRY