CHARACTERIZATION OF PHYCOCYANIN-DEFICIENT PHYCOBILISOMES FROM A PIGMENT MUTANT OF PORPHYRIDIUM SP. (RHODOPHYTA)

The structure and function of phycobilisomes in the rhodophyte Porphyridium sp. were investigated by comparing the properties of the wild type with a pigment mutant called C12. When grown under low light, cells of C12 were bright orange, while wild-type cells were deep red. The results obtained from a characterization of purified phycobilisomes of the mutant C12 led us to propose the existence in Porphyridium sp. phycobilisomes of two types of rods, some containing only phycoerythrin and others containing phycoerythrin bound to phycocyanin, which is in turn linked to the core by the linker L_RC. By studying the partitioning of phycobiliproteins between phycobilisomes and pools of free phycobiliproteins, we found that phycocyanin in the C12 mutant was only present in the pool of free proteins and that its specific linker, L_RC, was totally absent. Phycoerythrin was present in the free pool and in the purified phycobilisomes as well. One of the three specific phycoerythrin linkers γ was missing. In light of the fact that in the C12 mutant, the linker L_RC is absent and that there is no phycocyanin bound to the phycobilisomes, we propose that the rods in the mutant contain only phycoerythrin. These phycobilisomes are nevertheless functional and exhibit an efficient excitation transfer from phycoerythrin directly to allophycocyanin. Electron microscopy showed the purified phycobilisomes of C12 to be less dense than those of the wild type. This change was attributed to the disappearance of the rods containing the combination phycocyanin/phycoerythrin. Light still regulates phycobiliprotein synthesis in the mutant, as shown by the change in the color of the culture, which turned green-yellow when cells were shifted from low light to high light growth conditions. Light also regulates the structure of the phycobilisomes, which have fewer rods under high light growth conditions.