Molecular architecture of a light-harvesting antenna. In vitro assembly of the rod substructures of Synechococcus 6301 phycobilisomes.

The 75-, 33-, 30-, and 27-kilodalton polypeptide components ("linker polypeptides") of the phycobilisome of the unicellular cyanobacterium Synechococcus 6301 have been purified and characterized. In 0.6 M NaK phosphate buffer at pH 8, the 33-, 30-, and 27-kilodalton polypeptides assemble phycocyanin into ordered aggregates, whereas the 75-kilodalton polypeptide does not interact with phycocyanin. In the presence of the 33- and 30-kilodalton polypeptides, phycocyanin is assembled into hexameric discs and rods of stacked discs with the ultrastructural characteristics of the rod elements of intact phycobilisomes. Interaction of phycocyanin with only the 27-kilodalton polypeptide leads solely to the formation of discs that do not assemble into rods. Rods formed by interaction of phycocyanin with the 30- and 33-kilodalton polypeptides in the presence of the 27-kilodalton polypeptide are much shorter than those formed in its absence. This suggests that addition to growing rods of discs formed from phycocyanin and the 27-kilodalton polypeptide terminates rod assembly. Ordered structures formed upon interaction of phycocyanin with individual linker polypeptides contain at least 1 eq of linker polypeptide/(alpha beta)6 hexamer of phycocyanin. Complexes of phycocyanin with different linker polypeptides have distinctive spectroscopic properties that suggest a polar energy transfer along rod substructures toward the core. The linker polypeptides show no absorbance in the visible region of the spectrum. Peptide mapping shows that they are not related to each other by proteolytic processing.