Electron transport and fluorescence studies on the functional interaction between phospho-LHC II and Photosystem I in isolated stroma lamellae vesicles

The effects of the light harvesting chlorophyll a/b protein of Photosystem II (LHC II) phosphorylation on the functional antenna size of Photosystems (PS) I and II of intact thylakoids and isolated stroma lamellae vesicles were investigated in pea and spinach. In the stroma lamellae vesicles, which represent the non-appressed thylakoid regions where the presumed interplay between imported phospho-LHC II and PS I takes place, we have been able to quantify and compare both structural and functional changes due to phosphorylation. Fluorescence measurements showed a decreased F 685 - 695 emission in phosphorylated membranes, demonstrating a decreased antenna size of PS II. Concomitant an interaction between phospho-LHC II and PS I was demonstrated by an increased ability to excite fluorescence emission at 735 nm through chlorophyll b. PS II located in the stroma lamellae regions (PS II β ) was not affected. Electron transport assays using 650 nm excitation showed a 15% decrease in the apparent antenna size of PS II after phosphorylation. Under the same excitation conditions the apparent antenna size of PS I in phosphorylated stroma lamellae increased by 8%. However, since absorbance spectra of control and phosphorylated stroma lamellae normalized to equal P700 content revealed a 36% increased absorption capacity at 650 nm after phosphorylation, the attained increase in the apparent functional antennae size of PS I demonstrates a low efficiency of interaction between phospho-LHC II and PS I.