Photosystem II in different parts of the thylakoid membrane:: A functional comparison between different domains

The electron transport properties of photosystem II (PSTI) from five different domains of the thylakoid membrane were analyzed by flash-induced fluorescence kinetics. These domains are the entire grana, the grana con, the margins from the grana, the stroma lamellae, and the Y100 fraction (which represent more purified stroma lamellae). The two first fractions originate from appressed grana membranes and have PSII with a high proportion of O-2-evolving centers (80-90%) and efficient electron transport on the acceptor side. About 30% of the granal PSII centers were found in the margin fraction. Two-thirds of those PSII centers evolve O-2, but the electron transfer on the acceptor side is slowed. PSII from the stroma lamellae was less active. The fraction containing the entire stroma has only 43% O-2-evolving PSII centers and slow electron transfer on the acceptor side. In contrast, PSII centers of the Y100 fraction show no O-2 evolution and were unable to reduce QB. mash-induced fluorescence decay measurements in the presence of DCMU give information about the integrity of the donor side of PSII, We were able to distinguish between PSII centers with a functional Mn cluster and without any Mn cluster, and PSII centers which undergo photoactivation and have a partially assembled Mn cluster. From this analysis, we propose the existence of a PSII, activity gradient in the thylakoid membrane. The gradient is directed from the stroma lamellae, where the Mn cluster is absent or inactive, via the margins where photoactivation accelerates, to the grana core domain where PSII is fully photoactivated. The photoactivation process correlates to the PSII diffusion along the membrane and is initiated in the stroma lamellae while the final steps take place in the appressed regions of the grana core. The margin domain is seemingly very important in this process.