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Structure. 2013 Jan 8;21(1):88-97. doi: 10.1016/j.str.2012.11.001. Epub 2012 Dec 6.

A photo-labile thioether linkage to phycoviolobilin provides the foundation for the blue/green photocycles in DXCF-cyanobacteriochromes.

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Department of Genetics, 425-G Henry Mall, University of Wisconsin-Madison, Madison, WI 53706, USA.
Department of Biochemistry, 455 Babcock Drive, University of Wisconsin-Madison, Madison, WI 53706, USA.
Department of Genetics, 425-G Henry Mall, University of Wisconsin-Madison, Madison, WI 53706, USA. Electronic address:


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.

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