Rebinding of the 33 kDalton Polypeptide of Photosystem II to the D-l/D-2 Sub-Core Complex

The photosystem II (PS-II) reaction center is composed of several polypeptides which contain carotenoids and chlorophyll. Three pigment-free extrinsic proteins, with masses 33, 24, 18 kDa, are involved in the oxygen evolving system [1]. One of the extrinsic proteins, a 33 kDa polypeptide, accelerates a dark step in the oxygen-evolving reaction [2] and preserves the binding of the Mn atoms to the oxygenevolving complex [3]. The 33 kDa protein is readily detached from the PS-II reaction center. The 33 kDa polypeptide is necessary to both preserve binding of 2 of the 4 Mn atoms to PS-II [3] and maintain the conformation of the Mn cluster. The 33 kDa polypeptide also accelerates the dark step in evolution; removal of 33 kDa retards the S3-S0 transition. Other proteins in the oxygen evolution process presumably function in light harvesting, regulatory and ion concentrating [4]. The object of these experiments is to determine if the 33 kDa protein will rebind to the D-l/D-2 subcore complex of photosystem II. Chlorophyll in the D-l/D-2 complex is used as an endogenous fluorescence probe to assay binding of the 33 kDa protein to the complex. Evidence for binding between D-l / D-2 and 33 kDa was obtained by observing the spectral changes induced upon addition of the 33 kDa polypeptide in: (1) room temperature, excitation spectra for chlorophyll fluorescence; (2) spectrum and yield of chlorophyll fluorescence; (3) low temperature, chlorophyll fluorescence spectra; (4) fluorescence yield as a function of the mole ratio of 33 kDa to D-l/D-2. The latter yields the stoichiometry, and evidence for specific binding between the