THE SUPRAMOLECULAR STRUCTURE OF PHOTOSYSTEM-II - PHYCOBILISOME-COMPLEXES OF PORPHYRIDIUM-CRUENTUM

The structure and arrangement of phycobilisomes of the unicellular red alga Porphyridium cruentum is compared with the organization of the thylakoid freeze‐fracture particles in order to determine the relationship between phycobilisomes and photosystem II. The hemi‐ellipsoidal phycobilisomes, 20 nm thick, are predominantly organized into rows; their centre to centre periodicity is 30–40 nm, so that they are well separated by a gap of 10–20 nm. The phycobilisomes are cleaved by a central faint furrow, parallel to the long axis from top to base. The organization of the exoplasmic particles in rows is similar to the arrangement of the phycobilisomes so that a structural relationship between both systems, previously demonstrated in cyanobacteria, is evident. Within the rows, the 10 nm EF‐particles are grouped in tetrameric complexes separated by distances similar to those observed for phycobilisomes. We propose that the tetrameric EF‐particle complexes correspond to tetrameric photosystem II complexes which bind one hemi‐ellipsoidal phycobilisome on the stroma exposed surface of the thylakoid. A hypothetical model of this photosystem II‐phycobilisome complex is presented. 1990 Deutsche Botanische Gesellschaft/German Botanical Society