Biosynthesis and Assembly of Phycobiliproteins in Cyanobacteria

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Cyanobacteria are extremely important contributors to primary productivity in the world's oceans. In order to perform photosynthesis and fix carbon, cyanobacteria must absorb light. They achieve efficient light absorption utilizing light-harvesting complexes called phycobilisomes that contain brilliantly colored phycobiliproteins, which absorb light in the visible region of the light spectrum. The actual light-absorbing molecules are bilin chromophores, which are attached to each phycobiliprotein by enzymes called bilin lyases. Additionally, some marine cyanobacteria in the genus Synechococcus can harvest the blue light that dominates these environments due to the presence of a yellow chromophore called phycourobilin. The goal of this research is to determine how the yellow phycourobilin chromophore is synthesized and how all of the chromophores are attached to each phycobiliprotein in three species of Synechococcus (PCC 7002, WH8020, and WH8102). Several genes that encode potential enzymes in these biosynthetic processes have been identified in the genome of Synechococcus WH8020 and WH8102 by sequence similarity to other known bilin lyases. The role of each candidate gene in chromophore attachment will be evaluated by recreating the biosynthetic pathway for each phycobiliprotein inside Escherichia coli cells. In addition, candidate recombinant bilin lyase enzymes will be assayed for phycourobilin synthesis and attachment in vitro. In collaboration with a structural biologist, one of the bilin lyases will be purified and crystallized in order to determine its three-dimensional structure.Broader impacts: In addition to providing fundamental knowledge about photosynthetic light harvesting, this research will result in training two to four undergraduates per year along with two graduate students. Students will be recruited through mechanisms that encourage participation of underrepresented groups in science. The PI will continue to co-author publications with her high-school, undergraduate, MS, and PhD students. The PI has integrated her research with the curriculum of a Biochemistry and Molecular Biology lab course, reaching 20 students each year.