Cyanobacterial phycobilisomes. Phycocyanin assembly in the rod substructures of anabaena variabilis phycobilisomes.

Phycocyanin complexes with "linker" polypeptides (Lundell, D. J., Williams, R. C., and Glazer, A. N. (1981) J. Biol. Chem. 256, 3580-3592) of 27 and 32.5 kilodaltons have been isolated from dissociated Anabaena variabilis phycobilisomes. In 0.05 M phosphate at pH 7.0, these "trimeric" complexes have the molar composition (alpha beta)3 . 27,000 and (alpha beta)3 . 32,500, where alpha and beta are the subunits of phycocyanin and 27,000 and 32,500 denote single copies of the linker polypeptides. The (alpha beta)3 . 27,000 and (alpha beta)3 . 32,500 complexes have lambda max at 638 and 629 nm and fluorescence emission maxima at 651 and 646 nm, respectively. In 0.6 M phosphate at pH 8.0, the (alpha beta)3 . 27,000 complex forms an (alpha beta)6 . 27,000 disc-shaped aggregate as seen in the electron microscope, whereas the (alpha beta)3 . 32,500 complex forms discs, (alpha beta)6 . 32,500, and stacked disc rods of varying lengths. The former material, containing the 27,000 polypeptide, when mixed with the (alpha beta)6 . 32,500 discs, limits their assembly into rods. The spectroscopic properties of the discs and rods assembled in vitro indicate that energy transfer in phycobilisome rod substructures proceeds from (alpha beta)6 . 32,500 discs to the (alpha beta)6 . 27,000 disc proximal to the core and thence to the core.