title: A rising tide of blue-absorbing biliprotein photoreceptors: characterization of seven such bilin-binding GAF domains in Nostoc sp. PCC7120.
abstract: Cyanobacteriochromes are photochromic sensory photoreceptors in cyanobacteria that are related to phytochromes but cover a much broader spectral range. Using a homology search, a group of putative blue-absorbing photoreceptors was identified in Nostoc sp. PCC 7120 that, in addition to the canonical chromophore-binding cysteine of cyanobacteriochromes, have a conserved extra cysteine in a DXCF motif. To assess their photochemical activities, putative chromophore-binding GAF domains were expressed in Escherichia coli together with the genes for phycocyanobilin biosynthesis. All except one covalently bound a chromophore and showed photoreversible photochromic responses, with absorption at approximately 420 nm for the 15Z states formed in the dark, and a variety of red-shifted absorption peaks in the 490-600 nm range for the 15E states formed after light activation. Under denaturing conditions, the covalently bound chromophores were identified as phycocyanobilin, phycoviolobilin or mixtures of both. The canonical cysteines and those of the DXCF motifs were mutated, singly or together. The canonical cysteine is responsible for stable covalent attachment of the bilin to the apo-protein at C3(1) . The second linkage from the cysteine in the DXCF motif, probably to C10 of the chromophore, yields blue-absorbing rubin-type 15Z chromophores, but is lost in most cases upon photoconversion to the 15E isomers of the chromophores, and also when denatured with acidic urea.
authors.affiliation: State Key Laboratory of Agricultural Microbiology, Huazhong Agricultural University, Wuhan, China.
authors.forename: Qiong
authors.initials: Q
authors.lastname: Ma
authors.forename: Huan-Huan
authors.initials: HH
authors.lastname: Hua
authors.forename: Yu
authors.initials: Y
authors.lastname: Chen
authors.forename: Bin-Bin
authors.initials: BB
authors.lastname: Liu
authors.forename: Anna Laura
authors.initials: AL
authors.lastname: Krämer
authors.forename: Hugo
authors.initials: H
authors.lastname: Scheer
authors.forename: Kai-Hong
authors.initials: KH
authors.lastname: Zhao
authors.forename: Ming
authors.initials: M
authors.lastname: Zhou
mesh_terms: D001654:Bile Pigments / Q000378:metabolism*; D015153:Blotting, Western; D003545:Cysteine / Q000378:metabolism*; D046937:Nostoc / Q000378:metabolism*; D010777:Photochemistry; D020283:Photoreceptors, Microbial / Q000378:metabolism*; D052979:Phycobiliproteins / Q000378:metabolism*; D011485:Protein Binding; D017434:Protein Structure, Tertiary
chemical_list: D001654:Bile Pigments; D020283:Photoreceptors, Microbial; D052979:Phycobiliproteins; D003545:Cysteine