title: A photo-labile thioether linkage to phycoviolobilin provides the foundation for the blue/green photocycles in DXCF-cyanobacteriochromes.
abstract: The phytochrome superfamily encompasses a diverse collection of photochromic photoreceptors in plants and microorganisms that employ a covalently linked bilin cradled in a cGMP-phosphodiesterase/adenylyl-cyclase/FhlA (GAF) domain to detect light. Whereas most interconvert between red- and far-red-light-absorbing states, cyanobacteria also express variants called cyanobacteriochromes (CBCRs) that modify bilin absorption to collectively perceive the entire visible spectrum. Here, we present two X-ray crystallographic structures of the GAF domain from the blue/green photochromic CBCR PixJ from Thermosynechococcus elongatus. These structures confirm the hypothesis that CBCRs variably manipulate the chromophore π-conjugation system through isomerization and a second thioether linkage, in this case involving the bilin C10 carbon and Cys494 within a DXCF sequence characteristic of blue/green CBCRs. Biochemical studies support a mechanism for photoconversion whereby the second linkage ruptures on route to the green-light-absorbing state. Collectively, the TePixJ(GAF) models illustrate the remarkable structural and photochemical versatility among phytochromes and CBCRs in driving light perception.
authors.affiliation: Department of Genetics, 425-G Henry Mall, University of Wisconsin-Madison, Madison, WI 53706, USA.
authors.forename: E Sethe
authors.initials: ES
authors.lastname: Burgie
authors.affiliation: Department of Genetics, 425-G Henry Mall, University of Wisconsin-Madison, Madison, WI 53706, USA.
authors.forename: Joseph M
authors.initials: JM
authors.lastname: Walker
authors.affiliation: Department of Biochemistry, 455 Babcock Drive, University of Wisconsin-Madison, Madison, WI 53706, USA.
authors.forename: George N
authors.initials: GN
authors.lastname: Phillips
authors.affiliation: Department of Genetics, 425-G Henry Mall, University of Wisconsin-Madison, Madison, WI 53706, USA. Electronic address: vierstra@wisc.edu.
authors.forename: Richard D
authors.initials: RD
authors.lastname: Vierstra
mesh_terms: D020816:Amino Acid Motifs; D001426:Bacterial Proteins / Q000737:chemistry*; D018360:Crystallography, X-Ray; D000458:Cyanobacteria; D006207:Half-Life; D006860:Hydrogen Bonding; D007700:Kinetics; D008958:Models, Molecular; D010782:Photolysis; D020283:Photoreceptors, Microbial / Q000737:chemistry*; D010788:Photosynthesis; D052980:Phycobilins / Q000737:chemistry*; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D040681:Structural Homology, Protein; D013440:Sulfides / Q000737:chemistry*; D013816:Thermodynamics
chemical_list: D001426:Bacterial Proteins; D020283:Photoreceptors, Microbial; D052980:Phycobilins; D013440:Sulfides; C526232:phycoviolobilin