Date of Award
Schluchter, Wendy M.
Cyanobacteria are photosynthetic organisms that efficiently capture light by utilizing the light-harvesting complexes called phycobilisomes. In many cyanobacteria, phycobilisomes are composed of an allophycocyanin core with phycocyanin and phycoerythrin (PE) rods radiating from the core. These phycobiliproteins have multiple bilin chromophores, such as phycoerythrobilin (PEB), covalently attached to specific cysteine (Cys) residues for efficient photosynthetic light capture. Chromophore ligation on phycobiliprotein subunits occurs through bilin lyase catalyzed reactions.
This study mainly focuses on characterizing the roles of enzymes that are involved in the biosynthetic pathway of the phycobiliproteins within two cyanobacteria Thermosynechococcus elongatus and Fremyella diplosiphon. A combination of molecular and biochemical techniques were used to better understand the roles of these proteins in the post-translational modification and/or stability of phycobiliproteins. Using a heterologous plasmid coexpression system in E. coli, recombinant CpcS-III from T. elongatus was shown to ligate three different bilins to both subunits of allophycocyanin and to the beta subunit of phycocyanin, thus, acting as a bilin lyase. The crystal structure of CpcS-III was also solved, the first bilin lyase structure.
Next, the roles of three proteins from F. diplosiphon CpeY, CpeZ, and CpeF were analyzed using a combination of gene knock-out mutants and recombinant protein expression techniques. In the absence of cpeY, chromophorylation to the alpha subunit of PE at Cys-82 was reduced, coinciding with the recombinant data that CpeY is the lyase that attaches PEB to this site. Removing cpeZ from the genome resulted in the destabilization and reduced accumulation of PE, especially the beta subunit CpeB. Recombinant CpeZ was shown to act like a chaperone-like protein and increased the solubility and fluorescence of both recombinant and native CpeB by increasing the stability of the phycobiliprotein and/or by increasing the activities of other lyases. The deletion of cpeF resulted in a reduced-PE phenotype with the doubly attached PEB missing from CpeB at Cys-48/Cys-59. Recombinant CpeF was shown to ligate PEB to CpeB-Cys-48/Cys-59 in the presence of recombinant CpeS (lyase attaches PEB to CpeB-Cys-80) and CpeZ. CpeF also showed a chaperone-like function by stabilizing CpeB, but its main role appears to be as a bilin lyase.
Kronfel, Christina M., "Characterization of genes involved in phycobiliprotein biosynthesis in Fremyella diplosiphon and Thermosynechococcus elongatus" (2017). University of New Orleans Theses and Dissertations. 2333.