title: Cyanochromes are blue/green light photoreversible photoreceptors defined by a stable double cysteine linkage to a phycoviolobilin-type chromophore.
abstract: Phytochromes are a collection of bilin-containing photoreceptors that regulate a diverse array of processes in microorganisms and plants through photoconversion between two stable states, a red light-absorbing Pr form, and a far red light-absorbing Pfr form. Recently, a novel set of phytochrome-like chromoproteins was discovered in cyanobacteria, designated here as cyanochromes, that instead photoconvert between stable blue and green light-absorbing forms Pb and Pg, respectively. Here, we show that the distinctive absorption properties of cyanochromes are facilitated through the binding of phycocyanobilin via two stable cysteine-based thioether linkages within the cGMP phosphodiesterase/adenyl cyclase/FhlA domain. Absorption, resonance Raman and infrared spectroscopy, and molecular modeling of the Te-PixJ GAF (cGMP phosphodiesterase/adenyl cyclase/FhlA) domain assembled with phycocyanobilin are consistent with attachments to the C3(1) carbon of the ethylidene side chain and the C4 or C5 carbons in the A-B methine bridge to generate a double thioether-linked phycoviolobilin-type chromophore. These spectroscopic methods combined with NMR data show that the bilin is fully protonated in the Pb and Pg states and that numerous conformation changes occur during Pb --> Pg photoconversion. Also identified were a number of photochromically inactive mutants with strong yellow or red fluorescence that may be useful for fluorescence-based cell biological assays. Phylogenetic analyses detected cyanochromes capable of different signaling outputs in a wide range of cyanobacterial species. One unusual case is the Synechocystis cyanochrome Etr1 that also binds ethylene, suggesting that it works as a hybrid receptor to simultaneously integrate light and hormone signals.
authors: Department of Genetics, University of Wisconsin, Madison, WI 53706, USA.
authors: Andrew T
authors: AT
authors: Ulijasz
authors: Gabriel
authors: G
authors: Cornilescu
authors: David
authors: D
authors: von Stetten
authors: Claudia
authors: C
authors: Cornilescu
authors: Francisco
authors: F
authors: Velazquez Escobar
authors: Junrui
authors: J
authors: Zhang
authors: Robert J
authors: RJ
authors: Stankey
authors: Mario
authors: M
authors: Rivera
authors: Peter
authors: P
authors: Hildebrandt
authors: Richard D
authors: RD
authors: Vierstra
mesh_terms: D020418:Algal Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D017360:Arabidopsis / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D029681:Arabidopsis Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D000458:Cyanobacteria / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D003545:Cysteine / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D056890:Eukaryota / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D052980:Phycobilins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D010798:Phycocyanin / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D017434:Protein Structure, Tertiary / Q000502:physiology
chemical_list: D020418:Algal Proteins; D029681:Arabidopsis Proteins; D001426:Bacterial Proteins; D052980:Phycobilins; C526232:phycoviolobilin; D010798:Phycocyanin; C011069:phycocyanobilin; D003545:Cysteine