Arabidopsis cryptochrome 2 (CRY2) functions by the photoactivation mechanism distinct from the tryptophan (trp) triad-dependent photoreduction

Environmentally adaptive reshaping of plant photomorphogenesis by karrikin and strigolactone signaling

Coordinated morphogenic adaptation of growing plants is critical for their survival and propagation under fluctuating environments. Plant morphogenic responses to light and warm temperatures, termed photomorphogenesis and thermomorphogenesis, respectively, have been extensively studied in recent decades. During photomorphogenesis, plants actively reshape their growth and developmental patterns to cope with changes in light regimes. Accordingly, photomorphogenesis is closely associated with diverse growth hormonal cues. Notably, accumulating evidence indicates that light-directed morphogenesis is profoundly affected by two recently identified phytochemicals, karrikins (KARs) and strigolactones (SLs). KARs and SLs are structurally related butenolides acting as signaling molecules during a variety of developmental steps, including seed germination. Their receptors and signaling mediators have been identified, and associated working mechanisms have been explored using gene-deficient mutants in various plant species. Of particular interest is that the KAR and SL signaling pathways play important roles in environmental responses, among which their linkages with photomorphogenesis are most comprehensively studied during seedling establishment. In this review, we focus on how the phytochemical and light signals converge on the optimization of morphogenic fitness. We also discuss molecular mechanisms underlying the signaling crosstalks with an aim of developing potential ways to improve crop productivity under climate changes.

'Seeing' the electromagnetic spectrum: spotlight on the cryptochrome photocycle

Cryptochromes are widely dispersed flavoprotein photoreceptors that regulate numerous developmental responses to light in plants, as well as to stress and entrainment of the circadian clock in animals and humans. All cryptochromes are closely related to an ancient family of light-absorbing flavoenzymes known as photolyases, which use light as an energy source for DNA repair but themselves have no light sensing role. Here we review the means by which plant cryptochromes acquired a light sensing function. This transition involved subtle changes within the flavin binding pocket which gave rise to a visual photocycle consisting of light-inducible and dark-reversible flavin redox state transitions. In this photocycle, light first triggers flavin reduction from an initial dark-adapted resting state (FADox). The reduced state is the biologically active or 'lit' state, correlating with biological activity. Subsequently, the photoreduced flavin reoxidises back to the dark adapted or 'resting' state. Because the rate of reoxidation determines the lifetime of the signaling state, it significantly modulates biological activity. As a consequence of this redox photocycle Crys respond to both the wavelength and the intensity of light, but are in addition regulated by factors such as temperature, oxygen concentration, and cellular metabolites that alter rates of flavin reoxidation even independently of light. Mechanistically, flavin reduction is correlated with conformational change in the protein, which is thought to mediate biological activity through interaction with biological signaling partners. In addition, a second, entirely independent signaling mechanism arises from the cryptochrome photocycle in the form of reactive oxygen species (ROS). These are synthesized during flavin reoxidation, are known mediators of biotic and abiotic stress responses, and have been linked to Cry biological activity in plants and animals. Additional special properties arising from the cryptochrome photocycle include responsivity to electromagnetic fields and their applications in optogenetics. Finally, innovations in methodology such as the use of Nitrogen Vacancy (NV) diamond centers to follow cryptochrome magnetic field sensitivity in vivo are discussed, as well as the potential for a whole new technology of 'magneto-genetics' for future applications in synthetic biology and medicine.

Green means go: Green light promotes hypocotyl elongation via brassinosteroid signaling

Although many studies have elucidated the mechanisms by which different wavelengths of light (blue, red, far-red, or ultraviolet-B [UV-B]) regulate plant development, whether and how green light regulates plant development remains largely unknown. Previous studies reported that green light participates in regulating growth and development in land plants, but these studies have reported conflicting results, likely due to technical problems. For example, commercial green light-emitting diode light sources emit a little blue or red light. Here, using a pure green light source, we determined that unlike blue, red, far-red, or UV-B light, which inhibits hypocotyl elongation, green light promotes hypocotyl elongation in Arabidopsis thaliana and several other plants during the first 2-3 d after planting. Phytochromes, cryptochromes, and other known photoreceptors do not mediate green-light-promoted hypocotyl elongation, but the brassinosteroid (BR) signaling pathway is involved in this process. Green light promotes the DNA binding activity of BRI1-EMS-SUPPRESSOR 1 (BES1), a master transcription factor of the BR pathway, thus regulating gene transcription to promote hypocotyl elongation. Our results indicate that pure green light promotes elongation via BR signaling and acts as a shade signal to enable plants to adapt their development to a green-light-dominant environment under a canopy.

MKKK20 works as an upstream triple-kinase of MKK3-MPK6-MYC2 module in Arabidopsis seedling development

The mitogen-activated protein kinase (MAPK) cascade is involved in several signal transduction processes in eukaryotes. Here, we report a mechanistic function of MAP kinase kinase kinase 20 (MKKK20) in light signal transduction pathways. We show that MKKK20 acts as a negative regulator of photomorphogenic growth at various wavelengths of light. MKKK20 not only regulates the expression of light signaling pathway regulatory genes but also gets regulated by the same pathway genes. The atmyc2 mkkk20 double mutant analysis shows that MYC2 works downstream to MKKK20 in the regulation of photomorphogenic growth. MYC2 directly binds to the promoter of MKKK20 to modulate its expression. The protein-protein interaction study indicates that MKKK20 physically interacts with MYC2, and this interaction likely suppresses the MYC2-mediated promotion of MKKK20 expression. Further, the protein phosphorylation studies demonstrate that MKKK20 works as the upstream kinase of MKK3-MPK6-MYC2 module in photomorphogenesis.

Common evolutionary trajectory of short life-cycle in Brassicaceae ruderal weeds

Weed species are detrimental to crop yield. An understanding of how weeds originate and adapt to field environments is needed for successful crop management and reduction of herbicide use. Although early flowering is one of the weed trait syndromes that enable ruderal weeds to overcome frequent disturbances, the underlying genetic basis is poorly understood. Here, we establish Cardamine occulta as a model to study weed ruderality. By genome assembly and QTL mapping, we identify impairment of the vernalization response regulator gene FLC and a subsequent dominant mutation in the blue-light receptor gene CRY2 as genetic drivers for the establishment of short life cycle in ruderal weeds. Population genomics study further suggests that the mutations in these two genes enable individuals to overcome human disturbances through early deposition of seeds into the soil seed bank and quickly dominate local populations, thereby facilitating their spread in East China. Notably, functionally equivalent dominant mutations in CRY2 are shared by another weed species, Rorippa palustris, suggesting a common evolutionary trajectory of early flowering in ruderal weeds in Brassicaceae.

Rapid and reversible optogenetic silencing of synaptic transmission by clustering of synaptic vesicles

Acutely silencing specific neurons informs about their functional roles in circuits and behavior. Existing optogenetic silencers include ion pumps, channels, metabotropic receptors, and tools that damage the neurotransmitter release machinery. While the former hyperpolarize the cell, alter ionic gradients or cellular biochemistry, the latter allow only slow recovery, requiring de novo synthesis. Thus, tools combining fast activation and reversibility are needed. Here, we use light-evoked homo-oligomerization of cryptochrome CRY2 to silence synaptic transmission, by clustering synaptic vesicles (SVs). We benchmark this tool, optoSynC, in Caenorhabditis elegans, zebrafish, and murine hippocampal neurons. optoSynC clusters SVs, observable by electron microscopy. Locomotion silencing occurs with tau_on ~7.2 s and recovers with tau_off ~6.5 min after light-off. optoSynC can inhibit exocytosis for several hours, at very low light intensities, does not affect ion currents, biochemistry or synaptic proteins, and may further allow manipulating different SV pools and the transfer of SVs between them.

CRY2 interacts with CIS1 to regulate thermosensory flowering via FLM alternative splicing

Cryptochromes (CRYs) are evolutionarily conserved photolyase-like photoreceptors found in almost all species, including mammals. CRYs regulate transcription by modulating the activity of several transcription factors, but whether and how they affect pre-mRNA processing are unknown. Photoperiod and temperature are closely associated seasonal cues that influence reproductive timing in plants. CRYs mediate photoperiod-responsive floral initiation, but it is largely unknown whether and how they are also involved in thermosensory flowering. We establish here that blue light and CRY2 play critical roles in thermosensory flowering in Arabidopsis thaliana by regulating RNA alternative splicing (AS) to affect protein expression and development. CRY2 INTERACTING SPLICING FACTOR 1 (CIS1) interacts with CRY2 in a blue light-dependent manner and promotes CRY2-mediated thermosensory flowering. Blue light, CRYs, and CISs affect transcriptome-wide AS profiles, including those of FLOWERING LOCUS M (FLM), which is critical for temperature modulation of flowering. Moreover, CIS1 binds to the FLM pre-mRNA to regulate its AS, while CRY2 regulates the RNA-binding activity of CIS1. Thus, blue light regulates thermosensory flowering via a CRY2-CIS1-FLM signaling pathway that links flowering responses to both light and ambient temperature.

Engineering Light-Control in Biology

Unraveling the transformative power of optogenetics in biology requires sophisticated engineering for the creation and optimization of light-regulatable proteins. In addition, diverse strategies have been used for the tuning of these light-sensitive regulators. This review highlights different protein engineering and synthetic biology approaches, which might aid in the development and optimization of novel optogenetic proteins (Opto-proteins). Focusing on non-neuronal optogenetics, chromophore availability, general strategies for creating light-controllable functions, modification of the photosensitive domains and their fusion to effector domains, as well as tuning concepts for Opto-proteins are discussed. Thus, this review shall not serve as an encyclopedic summary of light-sensitive regulators but aims at discussing important aspects for the engineering of light-controllable proteins through selected examples.

Plant Cryptochromes Illuminated: A Spectroscopic Perspective on the Mechanism

Plant cryptochromes are central blue light receptors for the control of land plant and algal development including the circadian clock and the cell cycle. Cryptochromes share a photolyase homology region with about 500 amino acids and bind the chromophore flavin adenine dinucleotide. Characteristic for plant cryptochromes is a conserved aspartic acid close to flavin and an exceptionally long C-terminal extension. The mechanism of activation by excitation and reduction of the chromophore flavin adenine dinucleotide has been controversially discussed for many years. Various spectroscopic techniques have contributed to our understanding of plant cryptochromes by providing high time resolution, ambient conditions and even in-cell approaches. As a result, unifying and differing aspects of photoreaction and signal propagation have been revealed in comparison to members from other cryptochrome subfamilies. Here, we review the insight from spectroscopy on the flavin photoreaction in plant cryptochromes and present the current models on the signal propagation from flavin reduction to dissociation of the C-terminal extension.

Signaling, Deconstructed: Using Optogenetics to Dissect and Direct Information Flow in Biological Systems

Cells receive enormous amounts of information from their environment. How they act on this information-by migrating, expressing genes, or relaying signals to other cells-comprises much of the regulatory and self-organizational complexity found across biology. The "parts list" involved in cell signaling is generally well established, but how do these parts work together to decode signals and produce appropriate responses? This fundamental question is increasingly being addressed with optogenetic tools: light-sensitive proteins that enable biologists to manipulate the interaction, localization, and activity state of proteins with high spatial and temporal precision. In this review, we summarize how optogenetics is being used in the pursuit of an answer to this question, outlining the current suite of optogenetic tools available to the researcher and calling attention to studies that increase our understanding of and improve our ability to engineer biology.

Blue laser-induced selective vasorelaxation by the activation of NOSs

Phototherapy has been tried for treating cardiovascular diseases. In particular, ultraviolet and blue visible lights were suggested to be useful due to their nitric oxide (NO)-production ability in the skin. However, the effects of blue light on the arterial contractility are controversial. Here, we hypothesized that appropriate protocol of blue laser can induce selective vasorelaxation by activating vasodilating signaling molecules in arteries. Using organ chamber arterial mechanics, NO assay, Matrigel assay, and microarray, we showed that a 200-Hz, 300-μs, 445-nm pulsed-laser (total energy of 600 mJ; spot size 4 mm) induced selective vasorelaxation, without vasocontraction in rat mesenteric arteries. The laser stimulation increased NO production in the cord blood-endothelial progenitor cells (CB-EPCs). Both the laser-induced vasorelaxation and NO production were inhibited by a non-selective, pan-NO synthase inhibitor, L-NG-Nitro arginine methyl ester. Microarray study in CB-EPCs suggested up-regulation of cryptochrome (CRY)2 as well as NO synthase (NOS)1 and NOSTRIN (NOS trafficking) by the laser. In conclusion, this study suggests that the 445-nm blue puled-laser can induce vasorelaxation possibly via the CRY photoreceptors and NOSs activation. The blue laser-therapy would be useful for treating systemic hypertension as well as improving local blood flow depending on the area of irradiation.

Different response modes and cooperation modulations of blue-light receptors in photomorphogenesis

Cryptochromes photoreceptors, CRY1 and CRY2 in Arabidopsis, mediate blue light responses in plants and metazoa. The signalling interactions underlying photomorphogenesis of cryptochromes action have been extensively studied in experiment, expecting a systematical analysis of the dynamic mechanisms of photosensory signalling network from a global view. In this study, we developed a signalling network model to quantitatively investigate the different response modes and cooperation modulations on photomorphogenesis for CRY1 and CRY2 under blue light. The model shows that the different modes of time-dependent and fluence-rate-dependent phosphorylations for CRY1 and CRY2 are originated from their different phosphorylation rates and degradation rates. Our study indicates that, due to the strong association between blue-light inhibitor of cryptochromes (BIC) and CRY2, BIC negatively modulates CRY2 phosphorylation, which was confirmed by our experiment. The experiment also validated the model prediction that the time-dependent BIC-CRY1 and the fluence-rate-dependent BIC-CRY2 are both bell-shaped under blue light. Importantly, the model proposes that the COP1-SPA abundance can strongly inhibit the phosphorylation response of CRY2, resulting in the positive regulation of CRY2 phosphorylation by CRY1 through COP1-SPA. The model also predicts that the CRY1-HY5 axis, rather than CRY2-HY5 pathway, plays a dominant role in blue-light-dependent photomorphogenesis.

Cryptochromes: Photochemical and structural insight into magnetoreception

Cryptochromes (CRYs) function as blue light photoreceptors in diverse physiological processes in nearly all kingdoms of life. Over the past several decades, they have emerged as the most likely candidates for light-dependent magnetoreception in animals, however, a long history of conflicts between in vitro photochemistry and in vivo behavioral data complicate validation of CRYs as a magnetosensor. In this review, we highlight the origins of conflicts regarding CRY photochemistry and signal transduction, and identify recent data that provides clarity on potential mechanisms of signal transduction in magnetoreception. The review primarily focuses on examining differences in photochemistry and signal transduction in plant and animal CRYs, and identifies potential modes of convergent evolution within these independent lineages that may identify conserved signaling pathways.

Semi-In-Vivo Pull-Down Assay for Blue Light-Dependent Protein Interactions

Cryptochromes are photolyase-like blue-light receptors found in all major evolutionary lineages (Ahmad and Cashmore, Nature 366:162-166, 1993; Lin, Plant Physiol 110:1047, 1996; Cashmore, Cell 114:537-543, 2003; Partch and Sancar, Methods Enzymol 393:726-745, 2005). Arabidopsis cryptochrome 1 (CRY1) and cryptochrome 2 (CRY2) mediate primarily blue-light inhibition of hypocotyl elongation and photoperiodic control of floral initiation (Ahmad and Cashmore, Nature 366:162-166, 1993; Somers et al., Science, 282:1488-1490, 1998; Guo et al., Science 279 (5355):1360-1363, 1998; Yu et al., Arabidopsis Book 8:e0135, 2010). It has been proposed that phototransduction of cryptochromes involves the blue-light-dependent protein interactions, such as AtCRY2-CIB1 (CRYPTOCHROME-INTERACTING BASIC-HELIX-LOOP-HELIX 1), AtCRY1-PIF4 (PHYTOCHROME INTERACTING FACTOR 4) modules, sequentially mediate gene expression and plant growth (Liu et al., Science 322 (5907):1535-1539, 2008; Ma et al., Proc Natl Acad Sci U S A 113 (1):224-229, 2016; Wang et al., Science 354:343-347, 2016). Cryptochromes also showed blue light response in vitro when expressed in Sf9 insect cells using the baculovirus expression system, thus the wavelength-specific CRY2-CIB1 interaction can also be observed in Semi-in-vivo pull-down assay (Li et al., Proc Natl Acad Sci U S A 108 (51):20844-20849, 2011; Liu et al., EMBO Reports, 2018). Here, we describe the detailed process of blue light-dependent CRY2-CIB1 interaction in Semi-in-vivo conditions.

Express Arabidopsis Cryptochrome in Sf9 Insect Cells Using the Baculovirus Expression System

The Bac-to-Bac^® Baculovirus Expression System provides a rapid and efficient method to generate recombinant cryptochrome (CRY) proteins with chromophore flavin (FAD), which showed blue light response in vitro.

Tuning flavin environment to detect and control light-induced conformational switching in Drosophila cryptochrome

Light-induction of an anionic semiquinone (SQ) flavin radical in Drosophila cryptochrome (dCRY) alters the dCRY conformation to promote binding and degradation of the circadian clock protein Timeless (TIM). Specific peptide ligation with sortase A attaches a nitroxide spin-probe to the dCRY C-terminal tail (CTT) while avoiding deleterious side reactions. Pulse dipolar electron-spin resonance spectroscopy from the CTT nitroxide to the SQ shows that flavin photoreduction shifts the CTT ~1 nm and increases its motion, without causing full displacement from the protein. dCRY engineered to form the neutral SQ serves as a dark-state proxy to reveal that the CTT remains docked when the flavin ring is reduced but uncharged. Substitutions of flavin-proximal His378 promote CTT undocking in the dark or diminish undocking in the light, consistent with molecular dynamics simulations and TIM degradation activity. The His378 variants inform on recognition motifs for dCRY cellular turnover and strategies for developing optogenetic tools.

Chandrasekaran et al. engineered the Drosophila circadian clock protein Cryptochrome (dCRY) to form the neutral semiquinone, which serves as a dark-state proxy. They find that the C-terminal tail of dCRY remains docked when the flavin ring is reduced but uncharged. dCRY His378 variants provide insights into the recognition motifs for dCRY turnover and strategies for optogenetic tools.

Structural insights into photoactivation of plant Cryptochrome-2

Cryptochromes (CRYs) are evolutionarily conserved photoreceptors that mediate various light-induced responses in bacteria, plants, and animals. Plant cryptochromes govern a variety of critical growth and developmental processes including seed germination, flowering time and entrainment of the circadian clock. CRY's photocycle involves reduction of their flavin adenine dinucleotide (FAD)-bound chromophore, which is completely oxidized in the dark and semi to fully reduced in the light signaling-active state. Despite the progress in characterizing cryptochromes, important aspects of their photochemistry, regulation, and light-induced structural changes remain to be addressed. In this study, we determine the crystal structure of the photosensory domain of Arabidopsis CRY2 in a tetrameric active state. Systematic structure-based analyses of photo-activated and inactive plant CRYs elucidate distinct structural elements and critical residues that dynamically partake in photo-induced oligomerization. Our study offers an updated model of CRYs photoactivation mechanism as well as the mode of its regulation by interacting proteins.

Structural insights into the photoactivation of Arabidopsis CRY2

The blue-light receptor cryptochrome (CRY) in plants undergoes oligomerization to transduce blue-light signals after irradiation, but the corresponding molecular mechanism remains poorly understood. Here, we report the cryogenic electron microscopy structure of a blue-light-activated CRY2 tetramer at a resolution of 3.1 Å, which shows how the CRY2 tetramer assembles. Our study provides insights into blue-light-mediated activation of CRY2 and a theoretical basis for developing regulators of CRYs for optogenetic manipulation.

Regulatory Impact of the C-Terminal Tail on Charge Transfer Pathways in Drosophila Cryptochrome

Interconnected transcriptional and translational feedback loops are at the core of the molecular mechanism of the circadian clock. Such feedback loops are synchronized to external light entrainment by the blue light photoreceptor cryptochrome (CRY) that undergoes conformational changes upon light absorption by an unknown photoexcitation mechanism. Light-induced charge transfer (CT) reactions in Drosophila CRY (dCRY) are investigated by state-of-the-art simulations that reveal a complex, multi-redox site nature of CT dynamics on the microscopic level. The simulations consider redox-active chromophores of the tryptophan triad (Trp triad) and further account for pathways mediated by W314 and W422 residues proximate to the C-terminal tail (CTT), thus avoiding a pre-bias to specific W-mediated CT pathways. The conducted dissipative quantum dynamics simulations employ microscopically derived model Hamiltonians and display complex and ultrafast CT dynamics on the picosecond timescale, subtly balanced by the electrostatic environment of dCRY. In silicio point mutations provide a microscopic basis for rationalizing particular CT directionality and demonstrate the degree of electrostatic control realized by a discrete set of charged amino acid residues. The predicted participation of CT states in proximity to the CTT relates the directionality of CT reactions to the spatial vicinity of a linear interaction motif. The results stress the importance of CTT directional charge transfer in addition to charge transfer via the Trp triad and call for the use of full-length CRY models including the interactions of photolyase homology region (PHR) and CTT domains.

The Universally Conserved Residues Are Not Universally Required for Stable Protein Expression or Functions of Cryptochromes

Universally conserved residues (UCRs) are invariable amino acids evolutionarily conserved among members of a protein family across diverse kingdoms of life. UCRs are considered important for stability and/or function of protein families, but it has not been experimentally examined systematically. Cryptochromes are photoreceptors in plants or light-independent components of the circadian clocks in mammals. We experimentally analyzed 51 UCRs of Arabidopsis cryptochrome 2 (CRY2) that are universally conserved in eukaryotic cryptochromes from Arabidopsis to human. Surprisingly, we found that UCRs required for stable protein expression of CRY2 in plants are not similarly required for stable protein expression of human hCRY1 in human cells. Moreover, 74% of the stably expressed CRY2 proteins mutated in UCRs retained wild-type-like activities for at least one photoresponses analyzed. Our finding suggests that the evolutionary mechanisms underlying conservation of UCRs or that distinguish UCRs from non-UCRs determining the same functions of individual cryptochromes remain to be investigated.

A Case Study of Eukaryogenesis: The Evolution of Photoreception by Photolyase/Cryptochrome Proteins

Eukaryogenesis, the origin of the eukaryotes, is still poorly understood. Herein, we show how a detailed all-kingdom phylogenetic analysis overlaid with a map of key biochemical features can provide valuable clues. The photolyase/cryptochrome family of proteins are well known to repair DNA in response to potentially harmful effects of sunlight and to entrain circadian rhythms. Phylogenetic analysis of photolyase/cryptochrome protein sequences from a wide range of prokaryotes and eukaryotes points to a number of horizontal gene transfer events between ancestral bacteria and ancestral eukaryotes. Previous experimental research has characterised patterns of tryptophan residues in these proteins that are important for photoreception, specifically a tryptophan dyad, a canonical tryptophan triad, an alternative tryptophan triad, a tryptophan tetrad and an alternative tetrad. Our results suggest that the spread of the different triad and tetrad motifs across the kingdoms of life accompanied the putative horizontal gene transfers and is consistent with multiple bacterial contributions to eukaryogenesis.

Rapid Oxidation Following Photoreduction in the Avian Cryptochrome4 Photocycle

Avian magnetoreception is assumed to occur in the retina. Although its molecular mechanism is unclear, magnetic field-dependent formation and the stability of radical-containing photointermediate(s) are suggested to play key roles in a hypothesis called the radical pair mechanism. Chicken cryptochrome4 (cCRY4) has been identified as a candidate magnetoreceptive molecule due to its expression in the retina and its ability to form stable flavin neutral radicals (FADH^●) upon blue light absorption. Herein, we used millisecond flash photolysis to investigate the cCRY4 photocycle, in both the presence and absence of dithiothreitol (DTT); detecting the anion radical form of FAD (FAD^●-) under both conditions. Using spectral data obtained during flash photolysis and UV-visible photospectroscopy, we estimated the absolute absorbance spectra of the photointermediates, thus allowing us to decompose each spectrum into its individual components. Notably, in the absence of DTT, approximately 37% and 63% of FAD^●- was oxidized to FAD_OX and protonated to form FADH^●, respectively. Singular value decomposition analysis suggested the presence of two FAD^●- molecular species, each of which was destined to be oxidized to FAD_OX or protonated to FADH^●. A tyrosine neutral radical was also detected; however, it likely decayed concomitantly with the oxidation of FAD^●-. On the basis of these results, we considered the occurrence of bifurcation prior to FAD^●- generation, or during FAD^●- oxidization, and discussed the potential role played by the tyrosine radical in the radical pair mechanism.

Structural insights into BIC-mediated inactivation of Arabidopsis cryptochrome 2

Cryptochromes (CRYs) are blue-light receptors in plants that harbor FAD as a cofactor and regulate various physiological responses. Photoactivated CRYs undergo oligomerization, which increases the binding affinity to downstream signaling partners. Despite decades of research on the activation of CRYs, little is known about how they are inactivated. Binding of blue-light inhibitors of cryptochromes (BICs) to CRY2 suppresses its photoactivation, but the underlying mechanism remains unknown. Here, we report crystal structures of CRY2N (CRY2 PHR domain) and the BIC2-CRY2N complex with resolutions of 2.7 and 2.5 Å, respectively. In the BIC2-CRY2N complex, BIC2 exhibits an extremely extended structure that sinuously winds around CRY2N. In this way, BIC2 not only restrains the transfer of electrons and protons from CRY2 to FAD during photoreduction but also interacts with the CRY2 oligomer to return it to the monomer form. Uncovering the mechanism of CRY2 inactivation lays a solid foundation for the investigation of cryptochrome protein function.

The oligomeric structures of plant cryptochromes

Cryptochromes (CRYs) are a group of evolutionarily conserved flavoproteins found in many organisms. In plants, the well-studied CRY photoreceptor, activated by blue light, plays essential roles in plant growth and development. However, the mechanism of activation remains largely unknown. Here, we determined the oligomeric structures of the blue-light-perceiving PHR domain of Zea mays CRY1 and an Arabidopsis CRY2 constitutively active mutant. The structures form dimers and tetramers whose functional importance is examined in vitro and in vivo with Arabidopsis CRY2. Structure-based analysis suggests that blue light may be perceived by CRY to cause conformational changes, whose precise nature remains to be determined, leading to oligomerization that is essential for downstream signaling. This photoactivation mechanism may be widely used by plant CRYs. Our study reveals a molecular mechanism of plant CRY activation and also paves the way for design of CRY as a more efficient optical switch.

Mechanisms of Cryptochrome-Mediated Photoresponses in Plants

Cryptochromes are blue-light receptors that mediate photoresponses in plants. The genomes of most land plants encode two clades of cryptochromes, CRY1 and CRY2, which mediate distinct and overlapping photoresponses within the same species and between different plant species. Photoresponsive protein-protein interaction is the primary mode of signal transduction of cryptochromes. Cryptochromes exist as physiologically inactive monomers in the dark; the absorption of photons leads to conformational change and cryptochrome homooligomerization, which alters the affinity of cryptochromes interacting with cryptochrome-interacting proteins to form various cryptochrome complexes. These cryptochrome complexes, collectively referred to as the cryptochrome complexome, regulate transcription or stability of photoresponsive proteins to modulate plant growth and development. The activity of cryptochromes is regulated by photooligomerization; dark monomerization; cryptochrome regulatory proteins; and cryptochrome phosphorylation, ubiquitination, and degradation. Most of the more than 30 presently known cryptochrome-interacting proteins are either regulated by other photoreceptors or physically interactingwith the protein complexes of other photoreceptors. Some cryptochrome-interacting proteins are also hormonal signaling or regulatory proteins. These two mechanisms enable cryptochromes to integrate blue-light signals with other internal and external signals to optimize plant growth and development.

Adaptative transcriptional response of Dietzia cinnamea P4 strain to sunlight simulator

Responses to sunlight exposure of the oil-degrading Dietzia cinnamea P4 strain were evaluated by transcriptional levels of SOS genes, photoreactivation and genes involved in tolerance to high levels of reactive oxygen species. The P4 strain was exposed for 1 and 2 h and the magnitude of level changes in the mRNA was evaluated by qPCR. The results described the activation of the SOS system, with the decline of the repressor lexA gene levels and the concomitant increase of recA and uvrAD genes levels. The genes that participate in the photoreactivation process were also responsive to sunlight. The phrB gene encoding deoxyribodipyrimidine photo-lyase had its expression increased after 1-h exposure, while the phytAB genes showed a progressive increase over the studied period. The protective genes against reactive oxygen species, catalases, superoxides, peroxidases, and thioredoxins, had their expression rates detected under the conditions validated in this study. These results show a fast and coordinated response of genes from different DNA repair and tolerance mechanisms employed by strain P4, suggesting a complex concerted protective action against environmental stressors.

Cryptochrome mediated magnetic sensitivity in Arabidopsis occurs independently of light-induced electron transfer to the flavin

Arabidopsis cryptochrome-dependent magnetosensitivity occurs via a reaction that does not require light. This excludes radical pairs formed during light-triggered electron transfer to the flavin.

The Arabidopsis cryptochrome 2 I404F mutant is hypersensitive and shows flavin reduction even in the absence of light

The cryptochrome photoreceptor mutant cry2I404F exhibits hyperactivity in the dark, hypersensitivity in different light conditions, and in contrast to the wild-type protein, its flavin chromophore is reducible even in the absence of light. Plant cryptochromes (cry) are blue-light photoreceptors involved in multiple signaling pathways and various photomorphogenic responses. One biologically hyperactive mutant of a plant cryptochrome that was previously characterized is Arabidopsis cry1L407F (Exner et al. in Plant Physiol 154:1633-1645, 2010). Protein sequence alignments of different cryptochromes revealed that L407 in cry1 corresponds to I404 in cry2. Point mutation of Ile to Phe in cry2 in this position created a novel mutant. The present study provided a baseline data on the elucidation of the properties of cry2I404F. This mutant was still able to bind ATP-triggering conformational changes, as confirmed by partial tryptic digestion and thermo-FAD assays. Surprisingly, the FAD cofactor of cry2I404F was reduced by the addition of reductant even in the absence of light. In vivo, cry2I404F exhibited a cop phenotype in the dark and hypersensitivity to various light conditions compared to cry2 wild type. Overall, these data suggest that the hypersensitivity to red and blue light and hyperactivity of this novel mutant in the dark can be mostly accounted to structural alterations brought forth by the Ile to Phe mutation at position 404 that allows reduction of the flavin chromophore even in the absence of light.

MYC2 regulates ARR16, a component of cytokinin signaling pathways, in Arabidopsis seedling development

MYC2 is a basic helix-loop-helix transcription factor that acts as a repressor of blue light-mediated photomorphogenic growth; however, it promotes lateral root formation. MYC2 also regulates different phytohormone-signaling pathways in crucial manner. Arabidopsis response regulator 16 (ARR16) is a negative regulator of cytokinin signaling pathways. Here, we show that MYC2 directly binds to the E-box of ARR16 minimal promoter and negatively regulates its expression in a cytokinin-dependent manner. While ARR16 and MYC2 influence jasmonic acid and cytokinin signaling, the expression of ARR16 is regulated by cry1, GBF1, and HYH, the components of light signaling pathways. The transgenic studies show that the expression of ARR16 is regulated by MYC2 at various stages of development. The mutational studies reveal that ARR16 positively regulates the hypocotyl growth in blue light, and phenotypic analysis of atmyc2 arr16 double mutant further reveals that arr16 can suppress the short hypocotyl phenotype of atmyc2. Altogether, this work highlights MYC2-mediated transcriptional repression of ARR16 in Arabidopsis seedling development.

Dark, Light, and Temperature: Key Players in Plant Morphogenesis

Recent advances in plant thermomorphogenesis under different light conditions reveal the roles of plant photoreceptors in the control of thermomorphogenesis

Exploration of the effect of blue light on microRNAs involved in the accumulation of functional metabolites of longan embryonic calli through RNA-sequencing

BACKGROUND

The regulation of functional metabolites under light by structural genes and regulatory genes is understood but the roles of microRNAs in this pathway have rarely been reported and their regulation network is not yet clear.

RESULTS

Blue light was most conducive to promoting the synthesis of some functional metabolites in longan embryonic callus (ECs). In this study, we sequenced three small RNA libraries of constructed longan ECs under different light qualities (dark, blue, and white). A total of 29 and 22 miRNAs were differentially expressed in the dark versus blue (DB) and dark versus white (DW) combinations, respectively. According to Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis, most of the differentially expressed miRNA target genes were involved in plant hormone signal transduction, mitogen-activated protein kinase (MAPK) signaling, biosynthesis of unsaturated fatty acids, and so on. Cytoscape analysis of the target genes of miRNAs indicated that miR396b-5p and miR5139 had the most target genes in DB. Moreover, this study also found that miR171f_3 targeted DELLA, miR390e targeted BRI1, miR396b-5p targeted EBF1/2 and EIN3; these miRNAs participated in the blue light signaling network through their target genes and regulated the accumulation of longan functional metabolites.

CONCLUSIONS

The results of the study revealed that the expressions of phase-specific miRNAs vary with the change of functional metabolites in longan ECs. This study provides new insights into the molecular mechanisms that allow light to influence plant metabolism. © 2018 Society of Chemical Industry.

Enhanced intrinsic CYP3A4 activity in human hepatic C3A cells with optically controlled CRISPR/dCas9 activator complex

Human hepatic C3A cells have been applied in bioartificial liver development, although these cells display low intrinsic cytochrome P450 3A4 (CYP3A4) enzyme activity. We aimed to enhance CYP3A4 enzyme activity of C3A cells utilizing CRISPR gene editing technology. We designed two CYP3A4 expression enhanced systems applying clustered regularly interspaced short palindromic repeats (CRISPR) gene technology: a CRISPR-on activation system including dCas9-VP64-GFP and two U6-sgRNA-mCherry elements, and a light-controlled CRISPR-on activation system combining our CRISPR-on activation system with an optical control system to facilitate regulation of CYP3A4 expression for various applications. Results of enzymatic activity assays displayed increased CYP3A4 activity in C3A cells expressing the CRISPR-on activation system compared with C3A cells. In addition, CYP3A4 activity increased in C3A cells expressing the light-controlled CRISPR-on activation system under blue light radiation compared with C3A cells. Notably, there was no statistical difference in the increase of CYP3A4 protein amounts induced by these two methods. After expansion in culture, C3A cells with the light-controlled CRISPR-on activation system exhibited no statistical difference in CYP3A4 mRNA levels between generations. Our findings provide a method to stably enhance functional gene expression in bioartificial liver cells with the potential for large-scale cell expansion.

Beyond the photocycle-how cryptochromes regulate photoresponses in plants?

Cryptochromes (CRYs) are blue light receptors that mediate light regulation of plant growth and development. Land plants possess various numbers of cryptochromes, CRY1 and CRY2, which serve overlapping and partially redundant functions in different plant species. Cryptochromes exist as physiologically inactive monomers in darkness; photoexcited cryptochromes undergo homodimerization to increase their affinity to the CRY-signaling proteins, such as CIBs (CRY2-interacting bHLH), PIFs (Phytochrome-Interacting Factors), AUX/IAA (Auxin/INDOLE-3-ACETIC ACID), and the COP1-SPAs (Constitutive Photomorphogenesis 1-Suppressors of Phytochrome A) complexes. These light-dependent protein-protein interactions alter the activity of the CRY-signaling proteins to change gene expression and developmental programs in response to light. In the meantime, photoexcitation also changes the affinity of cryptochromes to the CRY-regulatory proteins, such as BICs (Blue-light Inhibitors of CRYs) and PPKs (Photoregulatory Protein Kinases), to modulate the activity, modification, or abundance of cryptochromes and photosensitivity of plants in response to the changing light environment.

ATP boosts lit state formation and activity of Arabidopsis cryptochrome 2

Cryptochrome (cry) blue light photoreceptors have important roles in the regulation of plant development. Their photocycle includes redox changes of their flavin adenine dinucleotide (FAD) chromophore, which is fully oxidised in the dark state and semi-reduced in the signalling-active lit state. The two Arabidopsis thaliana cryptochromes, cry1 and cry2, and the plant-type cryptochrome CPH1 from Chlamydomonas rheinhardtii bind ATP and other nucleotides. Binding of ATP affects the photocycle of these photoreceptors and causes structural alterations. However, the exact regions that undergo structural changes have not been defined, and most importantly it is not known whether ATP binding affects the biological activity of these photoreceptors in planta. Here we present studies on the effect of ATP on Arabidopsis cry2. Recombinant cry2 protein showed a high affinity for ATP (K_D of 1.09 ± 0.48 μm). Binding of ATP and other adenines promoted photoreduction of the FAD chromophore in vitro and caused structural changes, particularly in α-helix 21 which links the photosensory domain with the C-terminal extension. The constructed cry2Y399A mutant was unable to bind ATP and did not show enhancement of photoreduction by ATP. When this mutant gene was expressed in Arabidopsis null cry2 mutant plants it retained some biological activity, which was, however, lower than that of the wild type. Our results indicate that binding of ATP to cry2, and most likely to other plant-type cryptochromes, is not essential but boosts the formation of the signalling state and biological activity.

Blue Light Regulates Secondary Cell Wall Thickening via MYC2/MYC4 Activation of the NST1-Directed Transcriptional Network in Arabidopsis

Secondary cell walls (SCWs) are formed in some specific types of plant cells, providing plants with mechanical strength. During plant growth and development, formation of secondary cell walls is regulated by various developmental and environmental signals. The underlying molecular mechanisms are poorly understood. In this study, we analyzed the blue light receptor cryptochrome1 (cry1) mutant of Arabidopsis thaliana for its SCW phenotypes. During inflorescence stem growth, SCW thickening in the vasculature was significantly affected by blue light. cry1 plants displayed a decline of SCW thickening in fiber cells, while CRY1 overexpression led to enhanced SCW formation. Transcriptome analysis indicated that the reduced SCW thickening was associated with repression of the NST1-directed transcription regulatory networks. Further analyses revealed that the expression of MYC2/MYC4 that is induced by blue light activates the transcriptional network underlying SCW thickening. The activation is caused by direct binding of MYC2/MYC4 to the NST1 promoter. This study demonstrates that SCW thickening in fiber cells is regulated by a blue light signal that is mediated through MYC2/MYC4 activation of NST1-directed SCW formation in Arabidopsis.

CIB1 and CO interact to mediate CRY2-dependent regulation of flowering

Cryptochromes are photolyase-like photoreceptors. Arabidopsis CRY2 (cryptochrome 2) primarily mediates the photoperiodic regulation of floral initiation. CRY2 has been shown to promote FT (FLOWERING LOCUS T) mRNA expression in response to blue light by suppressing the degradation of the CO (CONSTANS) protein and activating CIB1 (CRY2-interacting bHLH1). Although CIB1 and CO are both transcriptional activators of FT, their relationship is unknown. Here, we show that CIB1 physically interacts with CO and promotes FT transcription in a CO-dependent manner. CRY2, CIB1, and CO form a protein complex in response to blue light to activate FT transcription, and the complex is regulated by the photoperiod and peaks at dusk along with higher FT expression. We also determined that CRY2 was recruited to the FT chromatin by CIB1 and CO and that all three proteins are bound to the same region within the FT promoter. Therefore, there is crosstalk between the CRY2-CO and CRY2-CIBs pathways, and CIB1 and CO act together to regulate FT transcription and flowering.

Photoreceptors Take Charge: Emerging Principles for Light Sensing

The first stage in biological signaling is based on changes in the functional state of a receptor protein triggered by interaction of the receptor with its ligand(s). The light-triggered nature of photoreceptors allows studies on the mechanism of such changes in receptor proteins using a wide range of biophysical methods and with superb time resolution. Here, we critically evaluate current understanding of proton and electron transfer in photosensory proteins and their involvement both in primary photochemistry and subsequent processes that lead to the formation of the signaling state. An insight emerging from multiple families of photoreceptors is that ultrafast primary photochemistry is followed by slower proton transfer steps that contribute to triggering large protein conformational changes during signaling state formation. We discuss themes and principles for light sensing shared by the six photoreceptor families: rhodopsins, phytochromes, photoactive yellow proteins, light-oxygen-voltage proteins, blue-light sensors using flavin, and cryptochromes.

Circadian clock activity of cryptochrome relies on tryptophan-mediated photoreduction

Cryptochromes (CRYs) entrain the circadian clocks of plants and animals to light. Irradiation of the Drosophila cryptochrome (dCRY) causes reduction of an oxidized flavin cofactor by a chain of conserved tryptophan (Trp) residues. However, it is unclear how redox chemistry within the Trp chain couples to dCRY-mediated signaling. Here, we show that substitutions of four key Trp residues to redox-active tyrosine and redox-inactive phenylalanine tune the light sensitivity of dCRY photoreduction, conformational activation, cellular stability, and targeted degradation of the clock protein timeless (TIM). An essential surface Trp gates electron flow into the flavin cofactor, but can be relocated for enhanced photoactivation. Differential effects of Trp-mediated flavin photoreduction on cellular turnover of TIM and dCRY indicate that these activities are separated in time and space. Overall, the dCRY Trp chain has evolutionary importance for light sensing, and its manipulation has implications for optogenetic applications of CRYs.

Nicotinamide Adenine Dinucleotides Arrest Photoreduction of Class II DNA Photolyases in FADH˙ State

All light-sensitive members of the photolyase/cryptochrome family rely on FAD as catalytic cofactor. Its activity is regulated by photoreduction, a light-triggered electron transfer process from a conserved tryptophan triad to the flavin. The stability of the reduced flavin depends on available external electron donors and oxygen. In this study, we show for the class II photolyase of Methanosarcina mazei, MmCPDII, that it utilizes physiologically relevant redox cofactors NADH and NADPH for the formation of the semiquinoid FAD in a light-dependent reaction. Using redox-inert variants MmCPDII/W388F and MmCPDII/W360F, we demonstrate that photoreduction by NADH and NADPH requires the class II-specific tryptophan cascade of MmCPDII. Finally, we confirmed that mutations in the tryptophan cascade can be introduced without any substantial structural disturbances by analyzing crystal structures of MmCPDII/W388F, MmCPDII/W360F and MmCPDII/Y345F.

Cryptochrome photoreceptors in green algae: Unexpected versatility of mechanisms and functions

Green algae have a highly complex and diverse set of cryptochrome photoreceptor candidates including members of the following subfamilies: plant, plant-like, animal-like, DASH and cryptochrome photolyase family 1 (CPF1). While some green algae encode most or all of them, others lack certain members. Here we present an overview about functional analyses of so far investigated cryptochrome photoreceptors from the green algae Chlamydomonas reinhardtii (plant and animal-like cryptochromes) and Ostreococcus tauri (CPF1) with regard to their biological significance and spectroscopic properties. Cryptochromes of both algae have been demonstrated recently to be involved to various extents in circadian clock regulation and in Chlamydomonas additionally in life cycle control. Moreover, CPF1 even performs light-driven DNA repair. The plant cryptochrome and CPF1 are UVA/blue light receptors, whereas the animal-like cryptochrome responds to almost the whole visible spectrum including red light. Accordingly, plant cryptochrome, animal-like cryptochrome and CPF1 differ fundamentally in their structural response to light as revealed by their visible and infrared spectroscopic signatures, and in the role of the flavin neutral radical acting as dark form or signaling state.

Hyperactivity of the Arabidopsis cryptochrome (cry1) L407F mutant is caused by a structural alteration close to the cry1 ATP-binding site

Plant cryptochromes (cry) act as UV-A/blue light receptors. The prototype, Arabidopsis thaliana cry1, regulates several light responses during the life cycle, including de-etiolation, and is also involved in regulating flowering time. The cry1 photocycle is initiated by light absorption by its FAD chromophore, which is most likely fully oxidized (FAD_ox) in the dark state and photoreduced to the neutral flavin semiquinone (FADH°) in its lit state. Cryptochromes lack the DNA-repair activity of the closely related DNA photolyases, but they retain the ability to bind nucleotides such as ATP. The previously characterized L407F mutant allele of Arabidopsis cry1 is biologically hyperactive and seems to mimic the ATP-bound state of cry1, but the reason for this phenotypic change is unclear. Here, we show that cry1L407F can still bind ATP, has less pronounced photoreduction and formation of FADH° than wild-type cry1, and has a dark reversion rate 1.7 times lower than that of the wild type. The hyperactivity of cry1L407F is not related to a higher FADH° occupancy of the photoreceptor but is caused by a structural alteration close to the ATP-binding site. Moreover, we show that ATP binds to cry1 in both the dark and the lit states. This binding was not affected by cry1's C-terminal extension, which is important for signal transduction. Finally, we show that a recently discovered chemical inhibitor of cry1, 3-bromo-7-nitroindazole, competes for ATP binding and thereby diminishes FADH° formation, which demonstrates that both processes are important for cry1 function.

Light-mediated Reversible Modulation of the Mitogen-activated Protein Kinase Pathway during Cell Differentiation and Xenopus Embryonic Development

Kinase activity is crucial for a plethora of cellular functions, including cell proliferation, differentiation, migration, and apoptosis. During early embryonic development, kinase activity is highly dynamic and widespread across the embryo. Pharmacological and genetic approaches are commonly used to probe kinase activities. Unfortunately, it is challenging to achieve superior spatial and temporal resolution using these strategies. Furthermore, it is not feasible to control the kinase activity in a reversible fashion in live cells and multicellular organisms. Such a limitation remains a bottleneck for achieving a quantitative understanding of kinase activity during development and differentiation. This work presents an optogenetic strategy that takes advantage of a bicistronic system containing photoactivatable proteins Arabidopsis thaliana cryptochrome 2 (CRY2) and the N-terminal domain of cryptochrome-interacting basic-helix-loop-helix (CIBN). Reversible activation of the mitogen-activated protein kinase (MAPK) signaling pathway is achieved through light-mediated protein translocation in live cells. This approach can be applied to mammalian cell cultures and live vertebrate embryos. This bicistronic system can be generalized to control the activity of other kinases with similar activation mechanisms and can be applied to other model systems.

Residues at a Single Site Differentiate Animal Cryptochromes from Cyclobutane Pyrimidine Dimer Photolyases by Affecting the Proteins' Preferences for Reduced FAD

Cryptochromes (CRYs) and photolyases belong to the cryptochrome/photolyase family (CPF). Reduced FAD is essential for photolyases to photorepair UV-induced cyclobutane pyrimidine dimers (CPDs) or 6-4 photoproducts in DNA. In Drosophila CRY (dCRY, a type I animal CRY), FAD is converted to the anionic radical but not to the reduced state upon illumination, which might induce a conformational change in the protein to relay the light signal downstream. To explore the foundation of these differences, multiple sequence alignment of 650 CPF protein sequences was performed. We identified a site facing FAD (Ala377 in Escherichia coli CPD photolyase and Val415 in dCRY), hereafter referred to as "site 377", that was distinctly conserved across these sequences: CPD photolyases often had Ala, Ser, or Asn at this site, whereas animal CRYs had Ile, Leu, or Val. The binding affinity for reduced FAD, but not the photorepair activity of E. coli photolyase, was dramatically impaired when replacing Ala377 with any of the three CRY residues. Conversely, in V415S and V415N mutants of dCRY, FAD was photoreduced to its fully reduced state after prolonged illumination, and light-dependent conformational changes of these mutants were severely inhibited. We speculate that the residues at site 377 play a key role in the different preferences of CPF proteins for reduced FAD, which differentiate animal CRYs from CPD photolyases.

Cryptochromes Orchestrate Transcription Regulation of Diverse Blue Light Responses in Plants

Blue light affects many aspects of plant growth and development throughout the plant lifecycle. Plant cryptochromes (CRYs) are UV-A/blue light photoreceptors that play pivotal roles in regulating blue light-mediated physiological responses via the regulated expression of more than one thousand genes. Photoactivated CRYs regulate transcription via two distinct mechanisms: indirect promotion of the activity of transcription factors by inactivation of the COP1/SPA E3 ligase complex or direct activation or inactivation of at least two sets of basic helix-loop-helix transcription factor families by physical interaction. Hence, CRYs govern intricate mechanisms that modulate activities of transcription factors to regulate multiple aspects of blue light-responsive photomorphogenesis. Here, we review recent progress in dissecting the pathways of CRY signaling and discuss accumulating evidence that shows how CRYs regulate broad physiological responses to blue light.

A Tightly Regulated Genetic Selection System with Signaling-Active Alleles of Phytochrome B

Selectable markers derived from plant genes circumvent the potential risk of antibiotic/herbicide-resistance gene transfer into neighboring plant species, endophytic bacteria, and mycorrhizal fungi. Toward this goal, we have engineered and validated signaling-active alleles of phytochrome B (eYHB) as plant-derived selection marker genes in the model plant Arabidopsis (Arabidopsis thaliana). By probing the relationship of construct size and induction conditions to optimal phenotypic selection, we show that eYHB-based alleles are robust substitutes for antibiotic/herbicide-dependent marker genes as well as surprisingly sensitive reporters of off-target transgene expression.

Rhodobacter sphaeroides CryB is a bacterial cryptochrome with (6-4) photolyase activity

Photolyases are efficient DNA repair enzymes that specifically repair either cyclobutane pyrimidine dimers or (6-4) photoproducts in a light-dependent cleavage reaction. The closely related classical cryptochrome blue light photoreceptors do not repair DNA lesions; instead they are involved in regulatory processes. CryB of Rhodobacter sphaeroides was until now described as a cryptochrome that affects light-dependent and singlet oxygen-dependent gene expression and is unusual in terms of its cofactor composition. Here we present evidence for a repair activity of (6-4) photoproducts by CryB and suggest a dual character combining the functions of cryptochromes and photolyases. We investigated the effects of crucial amino acids involved in cofactor or DNA lesion binding on the light-dependent recovery of cells after UV light exposure (in vivo photoreactivation). Remarkably, impairment of one of the two light absorbing cofactors, FAD or 6,7-dimethyl-8-ribityllumazine, only marginally affected the final survival rate but strongly decelerated photoreactivation kinetics. The impairment of both of them together through mutagenesis decreased CryB-dependent photoreactivation to the level of the ∆cryB knockout strain. The third cofactor, a [4Fe4S] iron-sulfur cluster, is indispensable for the structural integrity of the protein. The reduction of FAD via the conserved tryptophan W338, which is crucial for in vitro reduction and consequently DNA repair, is not required for in vivo photoreactivation, suggesting that this reduction pathway to FAD is dispensable in the cellular environment. This demonstrates that in vitro experiments give only limited information on in vivo photolyase activity.

An open-hardware platform for optogenetics and photobiology

In optogenetics, researchers use light and genetically encoded photoreceptors to control biological processes with unmatched precision. However, outside of neuroscience, the impact of optogenetics has been limited by a lack of user-friendly, flexible, accessible hardware. Here, we engineer the Light Plate Apparatus (LPA), a device that can deliver two independent 310 to 1550 nm light signals to each well of a 24-well plate with intensity control over three orders of magnitude and millisecond resolution. Signals are programmed using an intuitive web tool named Iris. All components can be purchased for under $400 and the device can be assembled and calibrated by a non-expert in one day. We use the LPA to precisely control gene expression from blue, green, and red light responsive optogenetic tools in bacteria, yeast, and mammalian cells and simplify the entrainment of cyanobacterial circadian rhythm. The LPA dramatically reduces the entry barrier to optogenetics and photobiology experiments.

The Blue Light-Dependent Polyubiquitination and Degradation of Arabidopsis Cryptochrome2 Requires Multiple E3 Ubiquitin Ligases

Cryptochromes are blue light receptors regulated by light-dependent ubiquitination and degradation in both plant and animal lineages. The Arabidopsis genome encodes two cryptochromes, CRY1 and CRY2, of which CRY2 undergoes blue light-dependent ubiquitination and 26S proteasome-dependent degradation. The molecular mechanism regulating blue light-dependent proteolysis of CRY2 is still not fully understood. We found that the F-box proteins ZEITLUPE (ZTL) and Lov Kelch Protein2 (LKP2), which mediate blue light suppression of degradation of the CRY2 signaling partner CIB1, are not required for the blue light-dependent CRY2 degradation. We further showed that the previously reported function of the COP1-SPA1 protein complex in blue light-dependent CRY2 degradation is more likely to be attributable to its cullin 4 (CUL4)-based E3 ubiquitin ligase activity than its activity as the cryptochrome signaling partner. However, the blue light-dependent CRY2 degradation is only partially impaired in the cul4 mutant, the cop1-5 null mutant and the spa1234 quadruple mutant, suggesting a possible involvement of additional E3 ubiquitin ligases in the regulation of CRY2. Consistent with this hypothesis, we demonstrated that the blue light-dependent CRY2 degradation is significantly impaired in the temperature-sensitive cul1 mutant allele (axr6-3), especially under the non-permissive temperature. Based on these and other results presented, we propose that photoexcited CRY2 undergoes Lys48-linked polyubiquitination catalyzed by the CUL4- and CUL1-based E3 ubiquitin ligases.