Abstract
The family of phytochrome photoreceptors contains proteins with different domain architectures and spectral properties. Knotless phytochromes are one of the three main subgroups classified by their distinct lack of the PAS domain in their photosensory core module, which is in contrast to the canonical PAS-GAF-PHY array. Despite intensive research on the ultrafast photodynamics of phytochromes, little is known about the primary kinetics in knotless phytochromes. Here, we present the ultrafast Pr ⇆ Pfr photodynamics of SynCph2, the best-known knotless phytochrome. Our results show that the excited state lifetime of Pr* (~200 ps) is similar to bacteriophytochromes, but much longer than in most canonical phytochromes. We assign the slow Pr* kinetics to relaxation processes of the chromophore-binding pocket that controls the bilin chromophore’s isomerization step. The Pfr photoconversion dynamics starts with a faster excited state relaxation than in canonical phytochromes, but, despite the differences in the respective domain architectures, proceeds via similar ground state intermediate steps up to Meta-F. Based on our observations, we propose that the kinetic features and overall dynamics of the ultrafast photoreaction are determined to a great extent by the geometrical context (i.e., available space and flexibility) within the binding pocket, while the general reaction steps following the photoexcitation are most likely conserved among the red/far-red phytochromes.
Highlights
Phytochromes represent a superfamily of photosensory receptors that regulate photomorphogenesis, photoprotection, phototaxis and other biologically relevant processes in a variety of organisms [1,2]
We propose that the kinetic features and overall dynamics of the ultrafast photoreaction are determined to a great extent by the geometrical context within the binding pocket, while the general reaction steps following the photoexcitation are most likely conserved among the red/far-red phytochromes
Our work provides insight into the unexplored ultrafast dynamics of knotless phytochromes
Summary
Phytochromes represent a superfamily of photosensory receptors that regulate photomorphogenesis, photoprotection, phototaxis and other biologically relevant processes in a variety of organisms [1,2]. Their function is enabled by the Z ↔ E photoisomerization of a bilin chromophore embedded in the protein scaffold. In group I phytochromes, comprised of canonical plant and cyanobacterial phytochromes (e.g., PhyA and SynCph1) as well as bacteriophytochromes (e.g., Agp and DrBphP), the PSM consists of a PAS-GAF-PHY array where all three domains are necessary for photoconversion [7,8,9]. Group III consists of cyanobacteriochromes (CBCRs) where single GAF domains form the PSM [14]. While the phytochromes of Group I and II photoconvert almost exclusively between Pr (red-absorbing) and Pfr (far-red-absorbing) states, CBCRs exhibit significant spectral diversity covering the near-UV/visible/near-IR spectral range [13,15,16,17,18,19,20,21,22]
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