Ultrastructural Comparison of Cyanidium caldarium Wild Type and III-C Mutant Lacking Phycobilisomes

Abstract

C. caldarium wild type and III-C mutant lacking phycobilisomes were compared with respect to the ultrastructural organization of particles on the freeze-fractured thylakoid membrane. In the wild type, the particles on the exoplasmic fracture face were arranged in the same manner as that reported for the phycobilisomes on the membrane surface. The phycobilisomes constitute the major part of the photosystem II antenna and their absence in the III-C mutant was accompanied by a completely different arrangement of the particles on the exoplasmic fracture face. The density of these particles was almost 2 times higher in the mutant than in the wild type while that of the particles on the protoplasmic fracture face was about the same. The relative densities of the particles on the exoplasmic fracture face in the 2 organisms was consistent with the 2-fold higher photosystem II to photosystem I ratio in the mutant compared to the wild type as determined by measurements of the field indicating absorbance changes. These particles were 100 .ANG. in both organisms. It is concluded that the particles on the exoplasmic fracture face in Cyanidium are probably substructural units of the particles observed on the same surface in higher plants and green algae and attributed to photosystem II.