Curated Optogenetic Publication Database

Abstract: PixD is a blue light-using flavin (BLUF) photoreceptor that controls phototaxis in the cyanobacterium Synechocystis sp. PCC6803. PixD interacts with the response regulator-like protein PixE in a light-dependent manner, and this interaction is critical for light signal transduction in vivo. However, the structure of the PixD-PixE complex has not been determined. To improve our understanding of how PixD transmits its captured light signal to PixE, we used blue-native polyacrylamide gel electrophoresis to characterize the molecular mass of a recombinant PixD-PixE complex purified from Escherichia coli and found it to be 342 kDa, suggesting that the complex contains 10 PixD and 4 PixE monomers. The stoichiometry of the complex was confirmed by Western blotting. Specifically, three intermediate states, PixD(10)-PixE(1), PixD(10)-PixE(2), and PixD(10)-PixE(3), were detected. The apparent dissociation constant for PixE and PixD is ~5 μM. A docking simulation was performed using a modeled PixE structure and the PixD(10) crystal structure. The docking simulation showed how the molecules in the PixD(10)-PixE(4) structure interact. To verify the accuracy of the docked model, a site-directed mutagenesis study was performed in which Arg80 of PixE, which appears to be capable of interacting electrostatically with Asp135 of PixD in the predicted structure, was shown to be critical for complex formation as mutation of PixE Arg80 to Asp or Ala prevented PixD-PixE complex formation. This study provides a structural basis for future investigations of the light signal transduction mechanism involving PixD and PixE.

Abstract: We report an optogenetic method based on Arabidopsis thaliana cryptochrome 2 for rapid and reversible protein oligomerization in response to blue light. We demonstrated its utility by photoactivating the β-catenin pathway, achieving a transcriptional response higher than that obtained with the natural ligand Wnt3a. We also demonstrated the modularity of this approach by photoactivating RhoA with high spatiotemporal resolution, thereby suggesting a previously unknown mode of activation for this Rho GTPase.

Abstract: The photoreceptor phytochrome B (phyB) interconverts between the biologically active Pfr (λmax = 730 nm) and inactive Pr (λmax = 660 nm) forms in a red/far-red-dependent fashion and regulates, as molecular switch, many aspects of light-dependent development in Arabidopsis thaliana. phyB signaling is launched by the biologically active Pfr conformer and mediated by specific protein-protein interactions between phyB Pfr and its downstream regulatory partners, whereas conversion of Pfr to Pr terminates signaling. Here, we provide evidence that phyB is phosphorylated in planta at Ser-86 located in the N-terminal domain of the photoreceptor. Analysis of phyB-9 transgenic plants expressing phospho-mimic and nonphosphorylatable phyB-yellow fluorescent protein (YFP) fusions demonstrated that phosphorylation of Ser-86 negatively regulates all physiological responses tested. The Ser86Asp and Ser86Ala substitutions do not affect stability, photoconversion, and spectral properties of the photoreceptor, but light-independent relaxation of the phyB(Ser86Asp) Pfr into Pr, also termed dark reversion, is strongly enhanced both in vivo and in vitro. Faster dark reversion attenuates red light-induced nuclear import and interaction of phyB(Ser86Asp)-YFP Pfr with the negative regulator PHYTOCHROME INTERACTING FACTOR3 compared with phyB-green fluorescent protein. These data suggest that accelerated inactivation of the photoreceptor phyB via phosphorylation of Ser-86 represents a new paradigm for modulating phytochrome-controlled signaling.

Abstract: Near-infrared light is favourable for imaging in mammalian tissues due to low absorbance of hemoglobin, melanin, and water. Therefore, fluorescent proteins, biosensors and optogenetic constructs for optimal imaging, optical readout and light manipulation in mammals should have fluorescence and action spectra within the near-infrared window. Interestingly, natural Bacterial Phytochrome Photoreceptors (BphPs) utilize the low molecular weight biliverdin, found in most mammalian tissues, as a photoreactive chromophore. Due to their near-infrared absorbance BphPs are preferred templates for designing optical molecular tools for applications in mammals. Moreover, BphPs spectrally complement existing genetically-encoded probes. Several BphPs were already developed into the near-infrared fluorescent variants. Based on the analysis of the photochemistry and structure of BphPs we suggest a variety of possible BphP-based fluorescent proteins, biosensors, and optogenetic tools. Putative design strategies and experimental considerations for such probes are discussed.

Abstract: Growth and differentiation of multicellular systems is orchestrated by spatially restricted gene expression programs in specialized subpopulations. The targeted manipulation of such processes by synthetic tools with high-spatiotemporal resolution could, therefore, enable a deepened understanding of developmental processes and open new opportunities in tissue engineering. Here, we describe the first red/far-red light-triggered gene switch for mammalian cells for achieving gene expression control in time and space. We show that the system can reversibly be toggled between stable on- and off-states using short light pulses at 660 or 740 nm. Red light-induced gene expression was shown to correlate with the applied photon number and was compatible with different mammalian cell lines, including human primary cells. The light-induced expression kinetics were quantitatively analyzed by a mathematical model. We apply the system for the spatially controlled engineering of angiogenesis in chicken embryos. The system's performance combined with cell- and tissue-compatible regulating red light will enable unprecedented spatiotemporally controlled molecular interventions in mammalian cells, tissues and organisms.

Abstract: Light-oxygen-voltage (LOV) domains bind a flavin chromophore to serve as blue light sensors in a wide range of eukaryotic and prokaryotic proteins. LOV domains are associated with a variable effector domain or a separate protein signaling partner to execute a wide variety of functions that include regulation of kinases, generation of anti-sigma factor antagonists, and regulation of circadian clocks. Here we present the crystal structure, photocycle kinetics, association properties, and spectroscopic features of a full-length LOV domain protein from Rhodobacter sphaeroides (RsLOV). RsLOV exhibits N- and C-terminal helical extensions that form an unusual helical bundle at its dimer interface with some resemblance to the helical transducer of sensory rhodopsin II. The blue light-induced conformational changes of RsLOV revealed from a comparison of light- and dark-state crystal structures support a shared signaling mechanism of LOV domain proteins that originates with the light-induced formation of a flavin-cysteinyl photoadduct. Adduct formation disrupts hydrogen bonding in the active site and propagates structural changes through the LOV domain core to the N- and C-terminal extensions. Single-residue variants in the active site and dimer interface of RsLOV alter photoadduct lifetimes and induce structural changes that perturb the oligomeric state. Size exclusion chromatography, multiangle light scattering, small-angle X-ray scattering, and cross-linking studies indicate that RsLOV dimerizes in the dark but, upon light excitation, dissociates into monomers. This light-induced switch in oligomeric state may prove to be useful for engineering molecular associations in controlled cellular settings.

Abstract: Light-sensitive proteins are useful tools to control protein localization, activation and gene expression, but are currently limited to excitation with red or blue light. Here we report a novel optogenetic system based on the ultraviolet-B-dependent interaction of the Arabidopsis ultraviolet-B photoreceptor UVR8 with COP1 that can be performed in visible light background. We use this system to induce nuclear accumulation of cytoplasmic green fluorescent protein fused to UVR8 in cells expressing nuclear COP1, and to recruit a nucleoplasmic red fluorescent protein fused to COP1 to chromatin in cells expressing UVR8-H2B. We also show that ultraviolet-B-dependent interactions between DNA-binding and transcription activation domains result in a linear induction of gene expression. The UVR8-COP1 interactions in mammalian cells can be induced using subsecond pulses of ultraviolet-B light and last several hours. As UVR8 photoperception is based on intrinsic tryptophan residues, these interactions do not depend on the addition of an exogenous chromophore.

Abstract: Phytochromes are dimeric photoreceptors that regulate a range of responses in plants and microorganisms through interconversion of red light-absorbing (Pr) and far-red light-absorbing (Pfr) states. Photoconversion between these states is initiated by light-driven isomerization of a bilin cofactor, which triggers protein structural change. The extent of this change, and how light-driven structural changes in the N-terminal photosensory region are transmitted to the C-terminal regulatory domain to initiate the signalling cascade, is unknown. We have used pulsed electron-electron double resonance (PELDOR) spectroscopy to identify multiple structural transitions in a phytochrome from Synechocystis sp. PCC6803 (Cph1) by measuring distances between nitroxide labels introduced into the protein. We show that monomers in the Cph1 dimer are aligned in a parallel 'head-to-head' arrangement and that photoconversion between the Pr and Pfr forms involves conformational change in both the N- and C-terminal domains of the protein. Cryo-trapping and kinetic measurements were used to probe the extent and temporal properties of protein motions for individual steps during photoconversion of Cph1. Formation of the primary photoproduct Lumi-R is not affected by changes in solvent viscosity and dielectric constant. Lumi-R formation occurs at cryogenic temperatures, consistent with their being no major structural reorganization of Cph1 during primary photoproduct formation. All remaining steps in the formation of the Pfr state are affected by solvent viscosity and dielectric constant and occur only at elevated temperatures, implying involvement of a series of long-range solvent-coupled conformational changes in Cph1. We show that signalling is achieved through ultrafast photoisomerization where localized structural change in the GAF domain is transmitted and amplified to cause larger-scale and slower conformational change in the PHY and histidine kinase domains. This hierarchy of timescales and extent of structural change orientates the histidine kinase domain to elicit the desired light-activated biological response.

Abstract: Over the past decades, there has been growing recognition that light can provide a powerful stimulus for biological interrogation. Light-actuated tools allow manipulation of molecular events with ultra-fine spatial and fast temporal resolution, as light can be rapidly delivered and focused with sub-micrometre precision within cells. While light-actuated chemicals such as photolabile 'caged' compounds have been in existence for decades, the use of genetically encoded natural photoreceptors for optical control of biological processes has recently emerged as a powerful new approach with several advantages over traditional methods. Here, we review recent advances using light to control basic cellular functions and discuss the engineering challenges that lie ahead for improving and expanding the ever-growing optogenetic toolkit.

Abstract: The absorption and emission spectroscopic behavior of the photo-activated adenylate cyclase NgPAC2 from the amoeboflagellate Naegleria gruberi NEG-M strain was studied in the dark, during blue-light exposure and after blue-light exposure. The typical BLUF domain (BLUF = Blue Light sensor Using Flavin) flavin cofactor absorption and fluorescence photo-cycle dynamics was observed. For fresh samples a reversible concentration dependent protein oligomerization occurred showing up in free flavin binding and protein color center formation with increasing protein concentration. Thermal and temporal irreversible protein unfolding with loss of BLUF domain activity was investigated. Temperature dependent protein melting times and the apparent protein melting temperature were determined. The photodynamic behavior of the NgPAC2 is compared with the behavior of the previously investigated photo-activated cyclase NgPAC1 (nPAC) from the same N. gruberi NEG-M strain.

Abstract: BLUF (sensor of blue light using FAD) domain-containing proteins are one of three types of flavin-binding, blue-light-sensing proteins found in many bacteria and some algae. The other types of blue-light-sensing proteins are the cryptochromes and the light, oxygen, voltage (LOV) domain-containing proteins. BLUF proteins control a wide variety of light-dependent physiological activities including photosystem synthesis, biofilm formation and the photoavoidance response. The BLUF domain photochemical reaction is unique in that only small chromophore structural changes are involved in the light activation process, because the rigid flavin moiety is involved, rather than an isomerizable chromophore (e.g. phytochromobilin in phytochromes and retinal in rhodopsins). Recent spectroscopic, biochemical and structural studies have begun to elucidate how BLUF domains transmit the light-induced signal and identify related, subsequent changes in the domain structures. Herein, I review progress made to date concerning the physiological functions and the phototransduction mechanism of BLUF proteins.

Abstract: Light-oxygen-voltage (LOV) domains serve as the photosensory modules for a wide range of plant and bacterial proteins, conferring blue light-dependent regulation to effector activities as diverse as enzymes and DNA binding. LOV domains can also be engineered into a variety of exogenous targets, allowing similar regulation for new protein-based reagents. Common to these proteins is the ability for LOV domains to reversibly form a photochemical adduct between an internal flavin chromophore and the surrounding protein, using this to trigger conformational changes that affect output activity. Using the Erythrobacter litoralis protein EL222 model system that links LOV regulation to a helix-turn-helix (HTH) DNA binding domain, we demonstrated that the LOV domain binds and inhibits the HTH domain in the dark, releasing these interactions upon illumination [Nash, A. I., et al. (2011) Proc. Natl. Acad. Sci. U.S.A. 108, 9449-9454]. Here we combine genomic and in vitro selection approaches to identify optimal DNA binding sites for EL222. Within the bacterial host, we observe binding at several genomic sites using a 12 bp sequence consensus that is also found by in vitro selection methods. Sequence-specific alterations in the DNA consensus reduce EL222 binding affinity in a manner consistent with the expected binding mode, a protein dimer binding to two repeats. Finally, we demonstrate the light-dependent activation of transcription of two genes adjacent to an EL222 binding site. Taken together, these results shed light on the native function of EL222 and provide useful reagents for further basic and applications research of this versatile protein.

Abstract: Light inducible protein-protein interactions are powerful tools to manipulate biological processes. Genetically encoded light-gated proteins for controlling precise cellular behavior are a new and promising technology, called optogenetics. Here we exploited the blue light-induced transcription system in yeast and zebrafish, based on the blue light dependent interaction between two plant proteins, blue light photoreceptor Cryptochrome 2 (CRY2) and the bHLH transcription factor CIB1 (CRY-interacting bHLH 1). We demonstrate the utility of this system by inducing rapid transcription suppression and activation in zebrafish.

Abstract: Phytochromes are red/far-red photoreceptors using cysteine-linked linear tetrapyrrole (bilin) chromophores to regulate biological responses to light. Light absorption triggers photoisomerization of the bilin between the 15Z and 15E photostates. The related cyanobacteriochromes (CBCRs) extend the photosensory range of the phytochrome superfamily to shorter wavelengths of visible light. Several subfamilies of CBCRs have been described. Representatives of one such subfamily, including AnPixJ and NpR6012g4, exhibit red/green photocycles in which the 15Z photostate is red-absorbing like that of phytochrome but the 15E photoproduct is instead green-absorbing. Using recombinant expression of individual CBCR domains in Escherichia coli, we fully survey the red/green subfamily from the cyanobacterium Nostoc punctiforme. In addition to 14 new photoswitching CBCRs, one apparently photochemically inactive protein exhibiting intense red fluorescence was observed. We describe a novel orange/green photocycle in one of these CBCRs, NpF2164g7. Dark reversion varied in this panel of CBCRs; some examples were stable as the 15E photoproduct for days, while others reverted to the 15Z dark state in minutes or even seconds. In the case of NpF2164g7, dark reversion was so rapid that reverse photoconversion of the green-absorbing photoproduct was not significant in restoring the dark state, resulting in a broadband response to light. Our results demonstrate that red/green CBCRs can thus act as sensors for the color or intensity of the ambient light environment.

Abstract: Fluorescent proteins (FPs) are widely used as optical sensors, whereas other light-absorbing domains have been used for optical control of protein localization or activity. Here, we describe light-dependent dissociation and association in a mutant of the photochromic FP Dronpa, and we used it to control protein activities with light. We created a fluorescent light-inducible protein design in which Dronpa domains are fused to both termini of an enzyme domain. In the dark, the Dronpa domains associate and cage the protein, but light induces Dronpa dissociation and activates the protein. This method enabled optical control over guanine nucleotide exchange factor and protease domains without extensive screening. Our findings extend the applications of FPs from exclusively sensing functions to also encompass optogenetic control.

Abstract: Filamentous fungi are important model systems for understanding eukaryotic cellular processes, including the study of protein expression. A salient feature of fungi is the ability of the protein-processing machinery to perform all of the extensive posttranslational modifications needed in the complex world of eukaryotic organisms, making them great hosts for production of eukaryotic proteins. In the model organism Neurospora crassa, several regulatable promoters have been used for heterologous gene expression but all suffer from leaky expression absent stimuli or an inability to induce protein expression at levels greater than those seen in vivo. To increase and better control in vivo protein expression in Neurospora, we have harnessed the light-induced vvd promoter. vvd promoter-driven mRNA expression is dependent upon light, shows a graded response, and is rapidly shut off when returned to the dark. The vvd promoter is a highly tunable and regulatable system, which could be a useful instrument for those interested in efficient and controllable gene expression.

Abstract: Advanced gene regulatory systems are necessary for scientific research, synthetic biology, and gene-based medicine. An ideal system would allow facile spatiotemporal manipulation of gene expression within a cell population that is tunable, reversible, repeatable, and can be targeted to diverse DNA sequences. To meet these criteria, a gene regulation system was engineered that combines light-sensitive proteins and programmable zinc finger transcription factors. This system, light-inducible transcription using engineered zinc finger proteins (LITEZ), uses two light-inducible dimerizing proteins from Arabidopsis thaliana, GIGANTEA and the LOV domain of FKF1, to control synthetic zinc finger transcription factor activity in human cells. Activation of gene expression in human cells engineered with LITEZ was reversible and repeatable by modulating the duration of illumination. The level of gene expression could also be controlled by modulating light intensity. Finally, gene expression could be activated in a spatially defined pattern by illuminating the human cell culture through a photomask of arbitrary geometry. LITEZ enables new approaches for precisely regulating gene expression in biotechnology and medicine, as well as studying gene function, cell-cell interactions, and tissue morphogenesis.

Abstract: The survey focuses on recent aspects of photochemical reactions to cellular DNA that are implicated through the predominant formation of mostly bipyrimidine photoproducts in deleterious effects of human exposure to sunlight. Recent developments in analytical methods have allowed accurate and quantitative measurements of the main DNA photoproducts in cells and human skin. Highly mutagenic CC and CT bipyrimidine photoproducts, including cyclobutane pyrimidine dimers and pyrimidine (6-4) pyrimidone photoproducts (6-4PPs) are generated in low yields with respect to TT and TC photoproducts. Another striking finding deals with the formation of Dewar valence isomers, the third class of bipyrimidine photoproducts that is accounted for by UVA-mediated isomerization of initially UVB generated 6-4PPs. Cyclobutadithymine (T<>T) has been unambiguously shown to be involved in the genotoxicity of UVA radiation. Thus, T<>T is formed in UVA-irradiated cellular DNA according to a direct excitation mechanism with a higher efficiency than oxidatively generated DNA damage that arises mostly through the Type II photosensitization mechanism. C<>C and C<>T are repaired at rates intermediate between those of T<>T and 6-4TT. Evidence has been also provided for the occurrence of photosensitized reactions mediated by exogenous agents that act either in an independent way or through photodynamic effects.

Abstract: A variety of methods exist for inducible control of DNA transcription in yeast. These include the use of native yeast promoters or regulatory elements that are responsive to small molecules such as galactose, methionine, and copper, or engineered systems that allow regulation by orthogonal small molecules such as estrogen. While chemically regulated systems are easy to use and can yield high levels of protein expression, they often provide imprecise control over protein levels. Moreover, chemically regulated systems can affect many other proteins and pathways in yeast, activating signaling pathways or physiological responses. Here, we describe several methods for light mediated control of DNA transcription in vivo in yeast. We describe methodology for using a red light and phytochrome dependent system to induce transcription of genes under GAL1 promoter control, as well as blue light/cryptochrome dependent systems to control transcription of genes under GAL1 promoter or LexA operator control. Light is dose dependent, inexpensive to apply, easily delivered, and does not interfere with cellular pathways, and thus has significant advantages over chemical systems.

Abstract: Synthetic extracellular matrices provide a framework in which cells can be exposed to defined physical and biological cues. However no method exists to manipulate single cells within these matrices. It is desirable to develop such methods in order to understand fundamental principles of cell migration and define conditions that support or inhibit cell movement within these matrices. Here, we present a strategy for manipulating individual mammalian stem cells in defined synthetic hydrogels through selective optical activation of Rac, which is an intracellular signaling protein that plays a key role in cell migration. Photoactivated cell migration in synthetic hydrogels depended on mechanical and biological cues in the biomaterial. Real-time hydrogel photodegradation was employed to create geometrically defined channels and spaces in which cells could be photoactivated to migrate. Cell migration speed was significantly higher in the photo-etched channels and cells could easily change direction of movement compared to the bulk hydrogels.

Abstract: Phosphoinositides (PIs) are lipid components of cell membranes that regulate a wide variety of cellular functions. Here we exploited the blue light-induced dimerization between two plant proteins, cryptochrome 2 (CRY2) and the transcription factor CIBN, to control plasma membrane PI levels rapidly, locally, and reversibly. The inositol 5-phosphatase domain of OCRL (5-ptase(OCRL)), which acts on PI(4,5)P(2) and PI(3,4,5)P(3), was fused to the photolyase homology region domain of CRY2, and the CRY2-binding domain, CIBN, was fused to plasma membrane-targeting motifs. Blue-light illumination (458-488 nm) of mammalian cells expressing these constructs resulted in nearly instantaneous recruitment of 5-ptase(OCRL) to the plasma membrane, where it caused rapid (within seconds) and reversible (within minutes) dephosphorylation of its targets as revealed by diverse cellular assays: dissociation of PI(4,5)P(2) and PI(3,4,5)P(3) biosensors, disappearance of endocytic clathrin-coated pits, nearly complete inhibition of KCNQ2/3 channel currents, and loss of membrane ruffling. Focal illumination resulted in local and transient 5-ptase(OCRL) recruitment and PI(4,5)P(2) dephosphorylation, causing not only local collapse and retraction of the cell edge or process but also compensatory accumulation of the PI(4,5)P(2) biosensor and membrane ruffling at the opposite side of the cells. Using the same approach for the recruitment of PI3K, local PI(3,4,5)P(3) synthesis and membrane ruffling could be induced, with corresponding loss of ruffling distally to the illuminated region. This technique provides a powerful tool for dissecting with high spatial-temporal kinetics the cellular functions of various PIs and reversibly controlling the functions of downstream effectors of these signaling lipids.

Abstract: Phytochromes are photoreceptors in phototropic organisms that respond to light conditions by changing interactions between a response regulator and DNA. Bacterial phytochromes (BphPs) comprise an input photosensory core domain (PCD) and an output transducing domain (OTD). We report the structure of a BphP containing both PCD and the majority of its OTD, and demonstrate interaction with its cognate repressor. The OTD of RpBphP1, from Rhodopseudomonas palustris, is composed of a PAS/PAC domain and, to our knowledge, a hitherto unrecognized two-helix output sensor (HOS) domain. Unlike canonical BphPs, it does not transmit phosphorelay signals but forms a complex with the transcriptional repressor RpPpsR2 on photoconversion with far-red light. We show that HOS is essential for complex formation and that the anti-parallel dimer geometry is crucial in achieving HOS domain activation and protomer swapping under the control of light. These results provide insights into the steps taken by a two-component signaling system.

Abstract: Repeated cocaine administration increases the dendritic arborization of nucleus accumbens neurons, but the underlying signaling events remain unknown. Here we show that repeated exposure to cocaine negatively regulates the active form of Rac1, a small GTPase that controls actin remodeling in other systems. Further, we show, using viral-mediated gene transfer, that overexpression of a dominant negative mutant of Rac1 or local knockout of Rac1 is sufficient to increase the density of immature dendritic spines on nucleus accumbens neurons, whereas overexpression of a constitutively active Rac1 or light activation of a photoactivatable form of Rac1 blocks the ability of repeated cocaine exposure to produce this effect. Downregulation of Rac1 activity likewise promotes behavioral responses to cocaine exposure, with activation of Rac1 producing the opposite effect. These findings establish that Rac1 signaling mediates structural and behavioral plasticity in response to cocaine exposure.

Abstract: Highly complex synthetic gene circuits have been engineered in living organisms to develop systems with new biological properties. A precise trigger to activate or deactivate these complex systems is desired in order to tightly control different parts of a synthetic or natural network. Light represents an excellent tool to achieve this goal as it can be regulated in timing, location, intensity, and wavelength, which allows for precise spatiotemporal control over genetic circuits. Recently, light has been used as a trigger to control the biological function of small molecules, oligonucleotides, and proteins involved as parts in gene circuits. Light activation has enabled the construction of unique systems in living organisms such as band-pass filters and edge-detectors in bacterial cells. Additionally, light also allows for the regulation of intermediate steps of complex dynamic pathways in mammalian cells such as those involved in kinase networks. Herein we describe recent advancements in the area of light-controlled synthetic networks.

Abstract: A functional coupling of photosensory domains derived from photoreceptors to effector proteins is a promising strategy for engineering novel photoresponsive proteins in optogenetics. Here, we have fused the light-sensitive LOV2 domain from Avena sativa phototropin1 to the restriction enzyme PvuII to generate a genetically encoded, light-controllable endonuclease. By analyzing several LOV-PvuII fusion enzymes, variants were obtained that show a 3-fold difference in DNA cleavage activity, when illuminated with blue light or kept in the dark. The effect is fully reversible over multiple photocycles. Depending on the particular fusion interface, the LOV-PvuII variants obtained had a bidirectional polarity in photoactivation; i.e., increased DNA cleavage activity was observed either in the dark state, with a compact folded LOV domain, or in the blue light photoexcitation state, when the LOV domain is partially unfolded.