REGULATION OF GENES ENCODING PHYCOBILISOME POLYPEPTIDES

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Phycobilisomes are major light harvesting complexes in cyanobacteria and some eukaryotic algae. The complex includes both pigmented proteins (phycobiliproteins) and nonpigmented proteins (linker proteins). The phycobiliproteins in eukaryotic algae are synthesized in the chloroplast while the nonpigmented linker proteins are synthesized in the cytoplasm (and must be imported into the organelle). Since the subunits among the different phycobiliproteins are related, they are thought to represent a gene family localized to the plastid DNA. Recently, a gene encoding the Beta subunit of phycocyanin has been placed on the small single copy region (this region encodes at least one additional phycobiliprotein subunit) of the plastid genome of Cyanophora paradoxa. We propose to locate the other phycobiliprotein subunits (Alpha phycocyanin and Alpha and Beta allophycocyanin) on the plastid genome, determine whether these genes are transcribed separately or as a unit, and generate specific probes for identifying analogous genes in cyanobacteria. The synthesis of several phycobiliprotein subunits in cyanobacteria is regulated by light (complementary chromatic adaptation), and this regulation may be at the transcriptional level. The probes generated from the plastid genome of C. paradoxa will enable us to identify these light-regulated phycobiliprotein genes and analyze the molecular events involved in complementary chromatic adaptation. Experiments involving differential hybridizations of RNA isolated from cyanobacteria grown in different light qualities to clone banks of cyanobacterial DNA, and immunological techniques which would enable us to identify clones synthesizing phycobilisome polypeptides, would also aid in the isolation of light-regulated phycobilisome genes. Once these genes are obtained, we will examine their arrangement on the cyanobacterial genome (clustered or dispersed), characterize their transcription (especially in response to different light qualities) and sequence their 5' ends to establish features which might be essential for light-regulated transcription.