HETEROGENEITY AMONG PHOTOSYSTEM-I

Thylakoids from spinach were sonicated and separated by aqueous two-phase partitioning into two vesicle populations originating from grana (alpha-vesicles) and stroma lamellae (beta-vesicles), respectively, according to a procedure described earlier (Andreasson et al. (1988) Biochim. Biophys. Acta 936, 339-350). The functional antenna sizes of Photosystem I for these two vesicle populations were determined by measuring the kinetics of the photooxidation of P700. Under green light illumination, the antenna size of Photosystem I of the alpha-vesicles (PSI-alpha) was 40% larger (40% larger rate constant of photooxidation) than that of Photosystem I of the beta-vesicles (PSI-beta). By using actinic light, which is preferentially absorbed by chlorophyll b, this difference was accentuated (60%) indicating that the antenna of PSI-alpha contains relatively more chlorophyll b. The experiments did not support the possibility that this difference in functional antenna size was due to 'spill over' from Photosystem II. A model is presented which shows that PSI-alpha is located in the periphery of the grana and PSI-beta in the stroma lamellae.