Differential distribution of pigment-protein complexes in the Thylakoid membranes of Synechocystis 6803

Abstract

Thylakoids in Synechocystis 6803, though apparently uniform in appearance in ultrastructure, were found to consist of segments which were functionally dissimilar and had distinct proteomes. These thylakoid segments can be isolated from Synechocystis 6803 by successive ultracentrifugation of cell free extracts at 40,000×g (40k segments), 90,000×g (90k segments) and 150,000×g (150k segments). Electron microscopy showed differences in their appearance. 40k segments looked feathery and fluffy, whereas the 90k and 150k thylakoid membrane segments appeared tiny and less fluffy. The absorption spectra showed heterogeneous distribution of pigment-protein complexes in the three types of segments. The photochemical activities of Photosystem I (PSI) and Photosystem II (PSII) showed unequal distributions in 40k, 90k and 150k segments which were substantiated with low temperature fluorescence measurements. The ratio of PSII/PSI fluorescence emission at 77K (λ(ex) = 435nm) was highest in 150k segments indicating higher PSII per unit PSI in these segments. The chlorophyll fluorescence lifetimes in the membranes, determined with a time-correlated single-photon counting technique, could be resolved in three components: τ(1) (=) <40ps, τ(2) (=) 425-900ps and τ(3) (=) 2.4-3.2ns. The percentage contribution of the fastest component (τ(1)) decreased in the order 40k > 90k > 150k segments whereas that of the other two components showed a reversed trend. These studies indicated differential distribution of pigment-protein complexes in the three membrane segments suggesting heterogeneity in the thylakoids of Synechocystis 6803.