Article

A second conserved GAF domain cysteine is required for the blue/green photoreversibility of cyanobacteriochrome Tlr0924 from Thermosynechococcus elongatus.

Section of Molecular and Cellular Biology, University of California, Davis, California 95616, USA.
Biochemistry (impact factor: 3.42). 08/2008; 47(27):7304-16. DOI:10.1021/bi800088t pp.7304-16
Source: PubMed

ABSTRACT Phytochromes are widely occurring red/far-red photoreceptors that utilize a linear tetrapyrrole (bilin) chromophore covalently bound within a knotted PAS-GAF domain pair. Cyanobacteria also contain more distant relatives of phytochromes that lack this knot, such as the phytochrome-related cyanobacteriochromes implicated to function as blue/green switchable photoreceptors. In this study, we characterize the cyanobacteriochrome Tlr0924 from the thermophilic cyanobacterium Thermosynechococcus elongatus. Full-length Tlr0924 exhibits blue/green photoconversion across a broad range of temperatures, including physiologically relevant temperatures for this organism. Spectroscopic characterization of Tlr0924 demonstrates that its green-absorbing state is in equilibrium with a labile, spectrally distinct blue-absorbing species. The photochemically generated blue-absorbing state is in equilibrium with another species absorbing at longer wavelengths, giving a total of 4 states. Cys499 is essential for this behavior, because mutagenesis of this residue results in red-absorbing mutant biliproteins. Characterization of the C 499D mutant protein by absorbance and CD spectroscopy supports the conclusion that its bilin chromophore adopts a similar conformation to the red-light-absorbing P r form of phytochrome. We propose a model photocycle in which Z/ E photoisomerization of the 15/16 bond modulates formation of a reversible thioether linkage between Cys499 and C10 of the chromophore, providing the basis for the blue/green switching of cyanobacteriochromes.

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Keywords

15/16 bond modulates formation
 
bilin chromophore
 
blue/green switchable photoreceptors
 
blue/green switching
 
CD spectroscopy
 
cyanobacteriochrome Tlr0924
 
Full-length Tlr0924 exhibits blue/green photoconversion
 
knotted PAS-GAF domain pair
 
model photocycle
 
physiologically relevant temperatures
 
phytochrome-related cyanobacteriochromes
 
red-absorbing mutant biliproteins
 
red-light-absorbing P r form
 
red/far-red photoreceptors
 
reversible thioether linkage
 
species absorbing
 
spectrally distinct blue-absorbing species
 
Spectroscopic characterization
 
thermophilic cyanobacterium Thermosynechococcus elongatus
 
Z/ E photoisomerization