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This report describes the properties of a relatively simple phycobilisome, Synechococcus 6301 (Anacystis nidulans). Morphology. -- Examination of wild type and mutant phycobilisomes by electron microscopy has shown them to have two morphologically differing substructures when seen in "face-view". There is a core consisting of two contiguous objects, disc-like in face-view projection, 115 A in diameter, and six rods, each composed of several stacked discs 60 A thick and 120 A in diameter, which radiate from the core in a hemidiscoidal arrangement. Each of the core components consists of four discs approximately 30 A thick. Rod substructures. -- Each of the discs in the rod substructure is a phycocyanin hexamer held together by interaction with a specific linker polypeptide, i. e., it has the composition (alpha beta)6 . X, where X is the linker polypeptide and alpha beta a phycocyanin monomer. The disc proximal to the core is an (alpha beta)6 . 27,000 complex. A small portion, Mr approximately 2,000,, of the Mr 27,000 polypeptide is essential to the attachment of this disc to the core. From studies of phycobilisomes from nitrogen-starved cells, and from mutants containing lowered amounts of phycocyanin relative to allophycocyanin, the second disc has been established to be an (alpha beta)6 . 33,000 complex. Either (alpha beta)6 . 33,000 or (alpha beta)6 . 30,000 complexes occupy the positions in the rods distal to the (alpha beta)6 .33,000 discs. Core substructure. -- Structural studies on the core and on core-rod junctions were greatly facilitated by the isolation of a mutant, strain AN112, which produces phycobilisomes with rods only one disc in length but with normal cores. Partial dissociation of these incomplete phycobilisomes under a variety of conditions, and separation and characterization of the resulting sub-complexes, has led to the determination of the composition of four distinct "trimeric" complexes, each of which is present in two copies per phycobilisome. These complexes, which account for the composition of the core, are as follows: (alpha beta)3AP . 10,500 with lambda maxF at 662 nm; (alpha beta)3AP with lambda maxF at 660 nm; (alpha 2AP alpha APB beta 3AP) . 10,500 with lambda maxF at 680 nm; where apha AP and alpha AFB are alpha subunits of allophycocyanin and allophycocyanin B, respectively, and beta AP is a subunit common to these two biliproteins; (alpha beta)2 AP . 18,300 . 40,000* . 11,000* with lambda maxF at 680 nm, where the Mr 40,000* and 11,000* polypeptides are derived from a Mr 75,000 polypeptide by tryptic digestion.(ABSTRACT TRUNCATED AT 400 WORDS)