Biological Significance of Photoreceptor
Photocycle Length: VIVID Photocycle
Governs the Dynamic VIVID-White Collar
Complex Pool Mediating Photo-adaptation and Response to Changes in Light Intensity
Arko Dasgupta1, Chen-Hui Chen1, ChangHwan Lee2, Amy S. Gladfelter2, Jay C. Dunlap1,
Jennifer J. Loros1,3* 1 Department of Genetics, Geisel School of Medicine, Dartmouth, Hanover, New Hampshire, United States of America, 2 Department of Biological Sciences, Dartmouth, Hanover, New Hampshire, United States of
America, 3 Department of Biochemistry, Geisel School of Medicine, Dartmouth, Hanover, New Hampshire,
United States of America * Jennifer.J.Loros@dartmouth.edu
Most organisms on earth sense light through the use of chromophore-bearing photoreceptive proteins with distinct and characteristic photocycle lengths, yet the biological significance of this adduct decay length is neither understood nor has been tested. In the filamentous fungus Neurospora crassa VIVID (VVD) is a critical player in the process of photoadaptation, the attenuation of light-induced responses and the ability to maintain photosensitivity in response to changing light intensities. Detailed in vitro analysis of the photochemistry of the blue light sensing, FAD binding, LOV domain of VVD has revealed residues around the site of photo-adduct formation that influence the stability of the adduct state (light state), that is, altering the photocycle length. We have examined the biological significance of VVD photocycle length to photoadaptation and report that a double substitution mutant (vvdI74VI85V), previously shown to have a very fast light to dark state reversion in vitro, shows significantly reduced interaction with the White Collar Complex (WCC) resulting in a substantial photoadaptation defect. This reduced interaction impacts photoreceptor transcription factor WHITE COLLAR-1 (WC-1) protein stability when N. crassa is exposed to light: The fast-reverting mutant VVD is unable to form a dynamic VVD-WCC pool of the size required for photoadaptation as assayed both by attenuation of gene expression and the ability to respond to increasing light intensity. Additionally, transcription of the clock gene frequency (frq) is sensitive to changing light intensity in a wild-type strain but not in the fast photo-reversion mutant indicating that the establishment of this dynamic VVD-WCC pool is essential in general photobiology and circadian biology. Thus, VVD photocycle length appears sculpted to establish a VVD-WCC reservoir of sufficient size to sustain photoadaptation while maintaining sensitivity to changing light intensity. The great diversity in photocycle kinetics among photoreceptors may be viewed as reflecting adaptive
PLOSGenetics | DOI:10.1371/journal.pgen.1005215 May 15, 2015 1 / 23
Citation: Dasgupta A, Chen C-H, Lee C, Gladfelter
AS, Dunlap JC, Loros JJ (2015) Biological
Significance of Photoreceptor Photocycle Length:
VIVID Photocycle Governs the Dynamic VIVID-White
Collar Complex Pool Mediating Photo-adaptation and
Response to Changes in Light Intensity. PLoS Genet 11(5): e1005215. doi:10.1371/journal.pgen.1005215
Editor: Brian Crane, Cornell University, UNITED
Received: January 4, 2015
Accepted: April 13, 2015
Published: May 15, 2015
Copyright: © 2015 Dasgupta et al. This is an open access article distributed under the terms of the
Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability Statement: All relevant data are within the paper and its Supporting Information files.
Funding: This work was supported by grants from the National Institutes of Health to JJL (GM083336) and to JCD (GM34985). http://www.nigms.nih.gov/
Pages/default.aspx We acknowledge use of materials generated by P01 GM068087 from National Institutes of Health to JCD. http://www.nigms.nih.gov/Pages/ default.aspx We thank the Fungal Genetics Stock
Center for providing Neurospora knockout strains. http://www.fgsc.net/. The funders had no role in study responses to specific and salient tasks required by organisms to respond to different photic environments.
Sensing light from the environment using a variety of photoreceptors is of great adaptive significance for most eukaryotes. A key feature of photoreceptors is the photocycle length, the time taken to decay from the initial signaling light state back to the receptive dark state; however, the significance of photocycle length, or adduct decay length, has not been tested in a biological setting. The photocycle length is determined by the chemical environment of the active site where a photon absorbing chromophore forms an adduct with a conserved amino acid. There is clear evidence of evolutionary selection for a particular photocycle length even between photoreceptors containing the same prototypic light-sensing domains suggesting functional relevance. Using defined in vitromutations that change the photocycle length of the VIVID (VVD) protein over 4 orders of magnitude we were able to ascribe a pivotal role of the native photochemistry of the protein in its function as a photoreceptor in the light and circadian biology of Neurospora crassa. This study links in vitro photochemical studies with in vivo function and provides evidence that the true evolutionary and functional significance of native photochemistry of photoreceptors can be enhanced by studying photocycle mutants in their native systems.
Most organisms and nearly all eukaryotes respond to light in their environment, and do so through the use of proteins specially adapted to respond to light. Such photoreceptor proteins most often sense light through the use of prosthetic groups, chromophores, chosen by evolution for their ability to absorb light of particularly relevant wavelengths, flavins for UV-A and blue light, trans-p-coumaric acid for yellow, retinals for green, and tetrapyrroles for red and infrared . Absorption of light elicits photochemical changes in a chromophore resulting in conformational changes in the photoreceptor protein that initiate the intracellular signaling leading to a biological response, while at the same time leaving the photoreceptor itself unable to respond to a second light stimulus. In most cases, however, this loss-of-response is reversible through photochemistry [2,3] or via a photocycle in which thermal decay of the activated state restores the receptor to the ground (receptive) state. The kinetics of a particular photocycle is highly variable both among classes of photoreceptor domains and even within a class of photoreceptor domains. Although the general biochemistry of photoreception is well understood  and insights into the determinants of photocycle length are emerging as described below, much less is known regarding the functional and adaptive significance of the wide range of known photocycle lengths.