Synechocystis 6701 phycobilisomes consist of a core of three cylindrical elements in an equilateral array from which extend in a fanlike manner six rods, each made up of three to four stacked disks. Previous studies (see Gingrich, J. C., L. K. Blaha, and A. N. Glazer, 1982. J. Cell Biol. 92:261-268) have shown that the rods consist of four disk-shaped complexes of biliproteins with "linker" polypeptides of 27-, 33.5-, 31.5-, and 30.5-kdaltons, listed in order starting with the disk proximal to the core: phycocyanin (alpha beta)6-27 kdalton, phycocyanin (alpha beta)6-33.5 kdalton, phycoerythrin (alpha beta)6-31.5 kdalton, phycoerythrin (alpha beta)6-30.5 kdalton, where alpha beta is the monomer of the biliprotein. Phycoerythrin complexes of the 31.5- and 30.5-kdalton polypeptides were isolated in low salt. In 0.05 M K-phosphate-1 mM EDTA at pH 7.0, these complexes had the average composition (alpha beta)2-31.5 and (alpha beta)-30.5 kdalton polypeptide, respectively. Peptide mapping of purified 31.5- and 30.5-kdalton polypeptides showed that they differed significantly in primary structure. In 0.65 M Na-K-phosphate at pH 8, these phycoerythrin complexes formed rods of stacked disks of composition (alpha beta)6-31.5 or (alpha beta)6-30.5 kdaltons. For the (alpha beta)-30.5 kdalton complex, the yield of rod assemblies was variable and the self-association of free phycoerythrin to smaller aggregates was an important competing reaction. Complementation experiments were performed with incomplete phycobilisomes from Synechocystis 6701 mutant strain CM25. These phycobilisomes are totally lacking in phycoerythrin and the 31.5- and 30.5-kdalton polypeptides, but have no other apparent structural defects. In high phosphate at pH 8, the phycoerythrin-31.5-kdalton complex formed disk assemblies at the end of the rod substructures of CM25 phycobilisomes whereas no interaction with the phycoerythrin-30.5 kdalton complex was detected. In mixtures of both the phycoerythrin-31.5 and -30.5 kdalton complexes with CM25 phycobilisomes, both complexes were incorporated at the distal ends of the rod substructures. The efficiency of energy transfer from the added phycoerythrin in complemented phycobilisomes was approximately 96%. The results show that the ordered assembly of phycoerythrin complexes seen in phycobilisomes is reproduced in the in vitro assembly process.
Synechocystis 6701 phycobilisomes contain phycoerythrin, phycocyanin, and allophycocyanin in a molar ratio of approximately 2:2:1, and other polypeptides of 99-, 46-, 33.5-, 31.5-, 30.5-, and 27-kdaltons. Wild-type phycobilisomes consist of a core of three cylindrical elements in an equilateral array surrounded by a fanlike array of six rods each made up of 3-4 stacked disks. Twelve nitrosoguanidine-induced mutants were isolated which produced phycobilisomes containing between 0 and 53% of the wild-type level of phycoerythrin and grossly altered levels of the 30.5- and 31.5-kdalton polypeptides. Assembly defects in these mutant particles were shown to be limited to the phycoerythrin portions of the rod substructures of the phycobilisome. Quantitative analysis of phycobilisomes from wild-type and mutant cells, grown either in white light or chromatically adapted to red light, indicated a molar ratio of the 30.5- and 31.5-kdalton polypeptides to phycoerythrin of 1:6, i.e., one 30.5- or one 31.5-kdaltons polypeptide per (alpha beta)6 phycoerythrin hexamer. Presence of the phycoerythrin-31.5-kdalton complex in phycobilisomes did not require the presence of the 30.5-kdalton polypeptide. The converse situation was not observed. These and earlier studies (R. C. Williams, J. C. Gingrich, and A. N. Glazer. 1980. J. Cell Biol. 85:558-566) show that the average rod in wild type Synechocystis 6701 phycobilisomes consists of four stacked disk-shaped complexes: phycocyanin (alpha beta)6-27 kdalton, phycocyanin (alpha beta)6-33.5 kdalton, phycoerythrin (alpha beta)6-31.5 kdalton, and phycoerythrin-30.5 kdalton, listed in order starting with the disk proximal to the core.