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Phycobilisomes serve as the primary light-harvesting antennae for Photo system II in cyanobacteria and red algae. These supramolecular complexes are primarily composed of phycobiliproteins, a brilliantly colored family of water-soluble proteins bearing covalently attached, open-chain tetrapyrroles known as phycobilins. In addition, phycobilisomes also contain smaller amounts 'linker polypeptides,' most of which do not bear chromophores. These components are absolutely required for proper assembly and functional organization of the structure. Phycobilisomes are constructed from two main structural elements: a core substructure and peripheral rods that are arranged in a hemidiscoidal fashion around that core. The core of most hemidiscoidal phycobilisomes is composed of three cylindrical subassemblies. The peripheral rods radiate from the lateral surfaces of the core substructure which are not in contact with the thylakoid membrane. Absorbed light energy is transferred by very rapid, radiation-less downhill energy transfer from phycoerythrin or phycoerythrocyanin (if present) to C-phycocyanin and then to allophycocyanin species that act as the final energy transmitters from the phycobilisome to the Photo system II or (partially) Photosystem I reaction centers.This chapter focuses on important recent developments concerning the structure and function of phycobilisome architecture and their constituent phycobiliproteins. Studies with the phycobilisomes and phycobiliproteins of the cyanobacterium Mastigocladus laminosus as will be emphasized. During the last decade tremendous progress has been made in the molecular biology of cyanobacteria-through gene

Phycobilisomes, the major light-harvesting complexes of cyanobacteria are multimolecular structures made up of chromophoric proteins called phycobiliproteins and non chromophoric linker polypeptides. We report here the isolation and nucleotide sequence of the genes, cpeA and cpeB, which in Calothrix PCC 7601 encode the alpha and beta subunits of phycoerythrin, one of the major phycobiliproteins. In Calothrix PCC 7601, modulation of the polypeptide composition of the phycobilisomes occurs in response to changes of the light wavelength, a phenomenon known as complementary chromatic adaptation. Under green illumination, cells synthesize phycoerythrin and its two specifically associated linker polypeptides (LR35 and LR36), while under red illumination none of these proteins are detected. Using specific probes, a single transcript (1450 nucleotide long) corresponding to the cpe genes was detected but only in green-light-grown cells, establishing the occurrence of transcriptional regulation for the expression of this operon in response to light wavelength changes. The size of this transcript excludes the possibility that the phycoerythrin-associated LR35 and LR36 could be cotranscribed with the cpeA and cpeB genes.

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