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Articles

Light-Harvesting System of the Red Alga Gracilaria tikvahiae¹

II. Phycobilisome Characteristics of Pigment Mutants

Department of Biophysics and Theoretical Biology, The University of Chicago, Chicago, Illinois 60637, Department of Biology, Barnes Laboratory, The University of Chicago, Chicago, Illinois 60637, Atlantic Research Laboratory, National Research Council of Canada, Halifax, Nova Scotia, Canada

Phycobilisomes were isolated from wild type Gracilaria tikvahiae and a number of its genetically characterized Mendelian and non-Mendelian pigment mutants in which the principal lesions result in an increase or decrease in the accumulation of phycoerythrin. Both the size and phycoerythrin content of the phycobilisomes are proportional to the phycoerythrin content of the crude algal extracts. In most of the strains examined, the structure and function of the phycocyanin-allophycocyanin phycobilisome cores are the same as in wild type. The phycobilisome architecture is derived from wild type by the addition or removal of phycoerythrin. The same pattern is observed for the phycobilisome of mos² which contains a large excess of phycocyanin that is not bound to the phycobilisome. The single exception is a yellow, non-Mendelian mutant, NMY-1, which makes functional phycobilisomes composed of phycoerythrin and allophycocyanin with almost no phycocyanin. Characterization of the `linker' polypeptides of the phycobilisome indicates that a 29 kilodalton protein is required for the stable incorporation of phycocyanin into the phycobilisome. Evidence is provided for the requirement of nuclear and cytoplasmic genes in phycobilisome synthesis and assembly. The symmetry properties of the phycobilisome are considered and a structural model for the reaction center II-phycobilisome organization is presented.

³ R. S. A. was a Mellon Foundation Fellow during a portion of the research period. To whom reprint requests should be sent: Barnes Laboratory, 5630 S. Ingleside Avenue, Chicago, IL 60637.

¹ Supported by National Science Foundation grant PCM 78-10535, and in part by the Louis Block Foundation, The University of Chicago. National Research Council of Canada 22524.