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

The structure and function of phycobilisomes in the rhodophyte Porphyr idium sp. were investigated ly comparing the properties of the wild ty pe 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 r esults obtained from a characterization of purified phycobilisomes of the mutant C12 led us to propose the existence in Porphyridium sp. phy cobilisomes of two types of rods, some containing only phycoerythrin a nd others containing phycoerythrin bound to phycocyanin, which is in t urn linked to the core by the linker L-RC. By studying the partitionin g of phycobiliproteins between phycobilisomes and pools of free phycob iliproteins, we found that phycocyanin in the C12 mutant was only pres ent in the pool of free proteins and that its specific linker, L-RC, w as totally absent. Phycoerythrin was present in the free pool and in t he purified phycobilisomes as well. One of the three specific phycoery thrin linkers gamma was missing. In light of the fact that in the C12 mutant, the linker L-RC is absent and that there is no phycocyanin bou nd to the phycobilisomes, we propose that the rods in the mutant conta in only phycoerythrin. These phycobilisomes are nevertheless functiona l and exhibit an efficient excitation transfer from phycoerythrin dire ctly to allophycocyanin. Electron microscopy showed the purified phyco bilisomes of C12 to be less dense than those of the wild type. This ch ange was attributed to the disappearance of the rods containing the co mbination phycocyanin/phycoerythrin. Light still regulates phycobilipr otein synthesis in the mutant, as shown by the change in the color of the culture, which turned green-yellow when cells were shifted from lo w light to high light growth conditions. Light also regulates the stru cture of the phycobilisomes, which have fewer rods under high light gr owth conditions.