Primary and Secondary Photodynamics of the Violet/Orange
Dual-Cysteine NpF2164g3 Cyanobacteriochrome Domain from <i>Nostoc
punctiforme</i>
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Abstract
Cyanobacteriochromes
(CBCRs) are cyanobacterial photoreceptors
distantly related to phytochromes. Like phytochromes, CBCRs photointerconvert
between two photostates that accompany photoisomerization of their
bilin chromophores. While phytochromes typically exhibit red/far-red
photocycles, CBCR photocycles are much more diverse, spanning the
near-ultraviolet and the entire visible region. All CBCRs described
to date have a conserved Cys residue covalently attached to the linear
tetrapyrrole (bilin) chromophore; two CBCR subfamilies also exploit
a second thioether linkage to the chromophore for detection of near-ultraviolet
to blue light. Here, we present the photodynamic analysis of the insert-Cys
CBCR NpF2164g3, a representative of the second class of two-cysteine
CBCRs. Using broadband transient absorption pump–probe spectroscopy,
we characterize the primary (100 fs to 10 ns) and secondary (10 ns
to 1 ms) photodynamics in both directions, examining photodynamics
over nine decades of time. Primary isomerization dynamics occur on
a ∼10 ps time scale for both forward and reverse reactions.
In contrast to previous studies on Tlr0924, a representative of the
other class of two-cysteine CBCRs, formation and elimination of the
second linkage are slower than the 1 ms experimental range probed
here. These results extend our understanding of dual-cysteine CBCR
photocycles in the phytochrome superfamily