Cytoplasmic and Chloroplast Synthesis of Phycobilisome Polypeptides

In vivo labeling of eukaryotic phycobilisomes in the presence of inhibitors of translation on 70S and 80S ribosomes demonstrates that some of the polypeptides of this light-harvesting complex are synthesized in the cytoplasm while others are synthesized in the chloroplast. The major pigmented polypeptides, the alpha and beta subunits of the biliproteins (molecular weights between 15,000 and 20,000) and the anchor protein (molecular weight about 90,000) are translated on 70S ribosomes. This suggests that these polypeptides are made within the algal chloroplast. Because the alpha and beta subunits comprise a group of closely related polypeptides, the genes encoding these polypeptides may reside in the plastid genome as a multigene family. Other prominent phycobilisome polypeptides, including a nonpigmented polypeptide that may be involved in maintaining the structural integrity of the complex, are synthesized on cytoplasmic ribosomes. Because the synthesis of phycobilisomes appears to require the expression of genes in two subcellular compartments, this system may be an excellent model for: (i) examining interaction between nuclear and plastid genomes: (ii) elucidating the molecular processes involved in the evolution of plastid genes: (iii) clarifying the events in the synthesis and assembly of macromolecular complexes in the chloroplast.