# Phytochromes

Cyanobacteriochrome Tlr0924

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Photoreceptors are proteins that with the help of embedded cofactors allow organisms to detect light. Interactions between the cofactor and the amino acids of the protein binding pocket, in addition to interactions between amino acids themselves, create a specific environment for the cofactor that tunes its spectral and functional properties. The photoreceptor’s ability to absorb visible light provides a convenient assay to study the cofactor-protein relationship. One type of plant photoreceptor is the red/far-red light absorbing phytochrome. Phytochromes use a bilin chromophore to absorb light and photoconvert between the red-absorbing and far-red-absorbing states. The protein detects these configurational changes and in response relocates to the nucleus where it can impact transcription. Amino acids lining the chromophore pocket position the chromophore in a stretched configuration and facilitate the photoconversion into a signaling state.

Plants use phytochromes in order to monitor the light conditions of their environment and adjust their metabolism accordingly. Related proteins with similar spectral properties have been found in cyanobacteria. However, previous studies have identified a class of cyanobacteriochromes that photoconvert upon the absorption of blue and green light instead of red and far-red light. One such example is Tlr0924 from the thermophilic cyanobacterium Thermosynechoccocus elongatus. The blue-absorbing state of Tlr0924, Pb, photoconverts to the green-absorbing state, Pg, upon excitation with blue light, which can convert back to Pb via the absorption of green light. In addition, studies have shown evidence of thermal equilibria in Tlr0924’s photocycle; something not seen in the red and far-red-absorbing cyanobacteriochromes and phytochromes.

The current model mechanism for Tlr0924 explains its spectral properties and the thermal equilibria with a reversible cysteine adduct formed between the apoprotein and the chromophore. This mechanism was based on the discovery of the two states, Pg’ and PbL. However, the photoconversion of the blue/green-absorbing cyanobacteriochrome has never been followed on a femtosecond time scale. The purpose of this study is to use ultrafast spectroscopy to elucidate intermediates in the photocycle and the time constants associated with their formation. Ultrafast spectroscopy uses femtosecond laser pulses to excite the system and then monitor its absorbance change after a variable time delay. An excitation pulse of the right wavelength, blue or green, will induce photoconversion and the change in absorbance following photoconversion can be tracked. This information will provide understanding of the impact of the cysteine residue and the source of these novel spectral properties.

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