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 differ ent domains of the thylakoid membrane were analyzed by flash-induced fluore scence kinetics. These domains are the entire grana, the grana con, the mar gins from the grana, the stroma lamellae, and the Y100 fraction (which repr esent more purified stroma lamellae). The two first fractions originate fro m appressed grana membranes and have PSII with a high proportion of O-2-evo lving centers (80-90%) and efficient electron transport on the acceptor sid e. 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 t he 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 ce nters and slow electron transfer on the acceptor side. In contrast, PSII ce nters 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 gi ve information about the integrity of the donor side of PSII, We were able to distinguish between PSII centers with a functional Mn cluster and withou t any Mn cluster, and PSII centers which undergo photoactivation and have a partially assembled Mn cluster. From this analysis, we propose the existen ce of a PSII, activity gradient in the thylakoid membrane. The gradient is directed from the stroma lamellae, where the Mn cluster is absent or inacti ve, via the margins where photoactivation accelerates, to the grana core do main where PSII is fully photoactivated. The photoactivation process correl ates to the PSII diffusion along the membrane and is initiated in the strom a 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.