A second conserved GAF domain cysteine is required for the blue/green photoreversibility of cyanobacteriochrome Tlr0924 from Thermosynechococcus elongatus.
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.
Section of Molecular and Cellular Biology, University of California, Davis, California 95616, USA.
Nathan C
NC
Rockwell
Stephanie Lane
SL
Njuguna
Laurel
L
Roberts
Elenor
E
Castillo
Victoria L
VL
Parson
Sunshine
S
Dwojak
J Clark
JC
Lagarias
Susan C
SC
Spiller
D001224:Aspartic Acid; D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism*; D002942:Circular Dichroism; D003116:Color; D017124:Conserved Sequence; D000458:Cyanobacteria / Q000378:metabolism* / Q000528:radiation effects*; D003545:Cysteine / Q000378:metabolism*; D008027:Light; D050505:Mutant Proteins / Q000737:chemistry / Q000378:metabolism; D010834:Phytochrome / Q000737:chemistry / Q000378:metabolism; D017434:Protein Structure, Tertiary; D011758:Pyrroles / Q000737:chemistry; D013329:Structure-Activity Relationship; D013440:Sulfides / Q000737:chemistry; D013696:Temperature
D001426:Bacterial Proteins; D050505:Mutant Proteins; D011758:Pyrroles; D013440:Sulfides; D010834:Phytochrome; D001224:Aspartic Acid; D003545:Cysteine