FRACTIONATION OF THE THYLAKOID MEMBRANE FROM DUNALIELLA-SALINA - HETEROGENEITY IS FOUND IN PHOTOSYSTEM-I OVER A BROAD RANGE OF GROWTH IRRADIANCE

Thylakoids from the green alga, Dunaliella salina, were fragmented by sonication and the appressed grasna membranes separated from stroma la mellae by partitioning in aqueous two-phase systems. The concentration and antenna size of Photosystem I in the two membrane domains were de termined for cultures grown at three different light intensities. Alth ough the antenna size of both PS is decreased with increasing growth i rradiance, the antenna size of Photosystem I in the grana was approxim ately 25-30% greater than the antenna size of Photosystem I in the str oma lamellae. Counter-current distribution analysis of sonicated thyla koids revealed that the amount of stroma lamellae increased whereas th e amount of the stacked membranes decreased at higher growth irradianc e. The overall decrease in the antenna size of PS I, in D. salina, at higher light intensities can therefore be explained by the combined ef fect of a decrease in the antenna size of Photosystem I, both in the g rana and the stroma lamellae, and a relative increase in the amount of stroma lamellae which has smaller Photosystem I antennae than the Pho tosystem I centers found in the grana. Light-induced protein phosphory lation increased the relative amount of the stroma lamella fraction. T his is interpreted as a result of partial unstacking of the grana. It is suggested that this may be a mechanism for increasing the cyclic el ectron transport around Photosystem I.