Comprehensive analysis of the green to blue photoconversion of full-length cyanobacteriochrome Tlr0924

Cyanobacteriochromes are members of the phytochrome superfamily of photoreceptors and are central in biological light activated signalling mechanisms. These photoreceptors are known to reversibly convert between two states in a photoinitiated process which involves a basic E/Z isomerisation of the bilin chromophore, and in certain cases the breakage of a thioether linkage to a conserved cysteine residue in the bulk protein structure. The exact details and timescales of the reactions involved in these photoconversions have not been conclusively shown. The cyanobacteriochrome Tlr0924 contains phycocyanobilin and phycoviolobilin chromophores, both of which photoconvert between two species, blue-absorbing and green-absorbing, and blue-absorbing and red-absorbing, respectively. Here we have followed the complete green to blue photoconversion process of the phycoviolobilin chromophore in the full-length form of Tlr0924 over timescales ranging from femtoseconds to seconds. Using a combination of time-resolved visible and mid-IR transient absorption spectroscopy and cryotrapping techniques we have shown that after photoisomerisation, which occurs with a lifetime of 3.6 ps, the phycoviolobilin twists, or distorts slightly with a lifetime of 5.3 μs. The final step, the formation of the thioether linkage with the protein occurs with a lifetime of 23.6 ms.