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The cyanobacteria are photosynthetic procaryotes that employ a mechanism of photosynthesis which is essentially identical to the systems found in plant chloroplasts and the eukaryotic green algae. Cyanobacteria can drive photosynthesis with light energy from a broad region of the visible spectrum (500 - 650 nm wavelength) that is not available to plants and green algae, which are limited to the narrow band of light energy that is absorbed by chlorophyll (660-680 nm). The light-harvesting capacity of the cyanobacteria is a function of a complex protein structure that resides on the surface of the photosynthetic membrane in contact with the PSII chlorophyll reaction centers. This light-harvesting complex is called a phycobilisome and functions as a protein scaffold for a rigid array of chromophores that absorbs light energy and transfers it to chlorophyll. The chromophores are linear tetrapyrroles (the bilins) that are covalently attached to the biliproteins, which comprise 80 - 85% of the total phycobilisome mass. There are three major classes of spectrally distinct biliproteins [phycoerythrin (PE), (lambda) max equals 565 nm; phycocyanin (PC), (lambda) max equals 617 nm; and allophycocyanin (AP), (lambda) max equals 650 nm] and their spatial organization within the phycobilisome creates an array of donor and acceptor chromophores that is optimized for resonance energy transfer to chlorophyll on a picosecond timescale and at close to 100% efficiency. The cyanobacteria can exert control over the biliprotein composition of the phycobilisomes in response to both light quality and light quantity, and they do so primarily by light-responsive transcription control mechanisms. The biosynthesis and assembly of a phycobilisome is an interesting example of self-assembly in a complex protein system. A phycobilisome from Synechocystis sp. strain 6701 can contain 400 proteins derived from a repertoire of 16 different polypeptides that includes the (alpha) and (beta) subunits for each major biliprotein and the achromic linker proteins that mediate assembly throughout the structure. The biliprotein subunit structures all show an identical motif that is reflected by significant amino acid sequence similarities across the different classes. Since phycobilisomes can comprise up to 40% of the cyanobacterial dry mass, assembly of these complexes must occur in the presence of high localized concentrations of components that are very similar in structure. That phycobilisome assembly is an efficient process with no evidence of significant misassembly suggests that effective molecular recognition during phycobilisome biosynthesis is based upon the subtle differences between subunits of different class biliproteins. We are using a protein engineering approach to examine structural features that mediate molecular recognition in two of the earliest steps of phycobilisome assembly, the docking of (alpha) and (beta) subunits and the selective attachment of chromophores.
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