Showing 501 - 525 of 647 results
A light-inducible organelle-targeting system for dynamically activating and inactivating signaling in budding yeast.
Protein localization plays a central role in cell biology. Although powerful tools exist to assay the spatial and temporal dynamics of proteins in living cells, our ability to control these dynamics has been much more limited. We previously used the phytochrome B- phytochrome-interacting factor light-gated dimerization system to recruit proteins to the plasma membrane, enabling us to control the activation of intracellular signals in mammalian cells. Here we extend this approach to achieve rapid, reversible, and titratable control of protein localization for eight different organelles/positions in budding yeast. By tagging genes at the endogenous locus, we can recruit proteins to or away from their normal sites of action. This system provides a general strategy for dynamically activating or inactivating proteins of interest by controlling their localization and therefore their availability to binding partners and substrates, as we demonstrate for galactose signaling. More importantly, the temporal and spatial precision of the system make it possible to identify when and where a given protein's activity is necessary for function, as we demonstrate for the mitotic cyclin Clb2 in nuclear fission and spindle stabilization. Our light-inducible organelle-targeting system represents a powerful approach for achieving a better understanding of complex biological systems.
The UVR8 UV-B Photoreceptor: Perception, Signaling and Response.
Ultraviolet-B radiation (UV-B) is an intrinsic part of sunlight that is accompanied by significant biological effects. Plants are able to perceive UV-B using the UV-B photoreceptor UVR8 which is linked to a specific molecular signaling pathway and leads to UV-B acclimation. Herein we review the biological process in plants from initial UV-B perception and signal transduction through to the known UV-B responses that promote survival in sunlight. The UVR8 UV-B photoreceptor exists as a homodimer that instantly monomerises upon UV-B absorption via specific intrinsic tryptophans which act as UV-B chromophores. The UVR8 monomer interacts with COP1, an E3 ubiquitin ligase, initiating a molecular signaling pathway that leads to gene expression changes. This signaling output leads to UVR8-dependent responses including UV-B-induced photomorphogenesis and the accumulation of UV-B-absorbing flavonols. Negative feedback regulation of the pathway is provided by the WD40-repeat proteins RUP1 and RUP2, which facilitate UVR8 redimerization, disrupting the UVR8-COP1 interaction. Despite rapid advancements in the field of recent years, further components of UVR8 UV-B signaling are constantly emerging, and the precise interplay of these and the established players UVR8, COP1, RUP1, RUP2 and HY5 needs to be defined. UVR8 UV-B signaling represents our further understanding of how plants are able to sense their light environment and adjust their growth accordingly.
An optogenetic tool for the activation of endogenous diaphanous-related formins induces thickening of stress fibers without an increase in contractility.
We have developed an optogenetic technique for the activation of diaphanous-related formins. Our approach is based on fusion of the light-oxygen-voltage 2 domain of Avena sativa Phototrophin1 to an isolated Diaphanous Autoregulatory Domain from mDia1. This "caged" diaphanous auto-regulatory domain was inactive in the dark but in the presence of blue light rapidly activated endogenous diaphanous-related formins. Using an F-actin reporter, we observed filopodia and lamellipodia formation as well as a steady increase in F-actin along existing stress fibers, starting within minutes of photo-activation. Interestingly, we did not observe the formation of new stress fibers. Remarkably, a 1.9-fold increase in F-actin was not paralleled by an increase in myosin II along stress fibers and the amount of tension generated by the fibers, as judged by focal adhesion size, appeared unchanged. Our results suggest a decoupling between F-actin accumulation and contractility in stress fibers and demonstrate the utility of photoactivatable diaphanous autoregulatory domain for the study of diaphanous-related formin function in cells.
Phytochrome-interacting factors have both shared and distinct biological roles.
Phytochromes are plant photoreceptors that perceive red and far-red light. Upon the perception of light in Arabidopsis, light-activated phytochromes enter the nucleus and act on a set of interacting proteins, modulating their activities and thereby altering the expression levels of ∼10% of the organism's entire gene complement. Phytochromeinteracting factors (PIFs) belonging to Arabidopsis basic helix-loop-helix (bHLH) subgroup 15 are key interacting proteins that play negative roles in light responses. Their activities are post-translationally countered by light-activated phytochromes, which promote the degradation of PIFs and directly or indirectly inhibit their binding to DNA. The PIFs share a high degree of similarity, but examinations of pif single and multiple mutants have indicated that they have shared and distinct functions in various developmental and physiological processes. These are believed to stem from differences in both intrinsic protein properties and their gene expression patterns. In an effort to clarify the basis of these shared and distinct functions, we compared recently published genome-wide ChIP data, developmental gene expression maps, and responses to various stimuli for the various PIFs. Based on our observations, we propose that the biological roles of PIFs stem from their shared and distinct DNA binding targets and specific gene expression patterns.
A light-triggered protein secretion system.
Optical control of protein interactions has emerged as a powerful experimental paradigm for manipulating and studying various cellular processes. Tools are now available for controlling a number of cellular functions, but some fundamental processes, such as protein secretion, have been difficult to engineer using current optical tools. Here we use UVR8, a plant photoreceptor protein that forms photolabile homodimers, to engineer the first light-triggered protein secretion system. UVR8 fusion proteins were conditionally sequestered in the endoplasmic reticulum, and a brief pulse of light triggered robust forward trafficking through the secretory pathway to the plasma membrane. UVR8 was not responsive to excitation light used to image cyan, green, or red fluorescent protein variants, allowing multicolor visualization of cellular markers and secreted protein cargo as it traverses the cellular secretory pathway. We implemented this novel tool in neurons to demonstrate restricted, local trafficking of secretory cargo near dendritic branch points.
Optogenetic elevation of endogenous glucocorticoid level in larval zebrafish.
The stress response is a suite of physiological and behavioral processes that help to maintain or reestablish homeostasis. Central to the stress response is the hypothalamic-pituitary-adrenal (HPA) axis, as it releases crucial hormones in response to stress. Glucocorticoids (GCs) are the final effector hormones of the HPA axis, and exert a variety of actions under both basal and stress conditions. Despite their far-reaching importance for health, specific GC effects have been difficult to pin-down due to a lack of methods for selectively manipulating endogenous GC levels. Hence, in order to study stress-induced GC effects, we developed a novel optogenetic approach to selectively manipulate the rise of GCs triggered by stress. Using this approach, we could induce both transient hypercortisolic states and persistent forms of hypercortisolaemia in freely behaving larval zebrafish. Our results also established that transient hypercortisolism leads to enhanced locomotion shortly after stressor exposure. Altogether, we present a highly specific method for manipulating the gain of the stress axis with high temporal accuracy, altering endocrine and behavioral responses to stress as well as basal GC levels. Our study offers a powerful tool for the analysis of rapid (non-genomic) and delayed (genomic) GC effects on brain function and behavior, feedbacks within the stress axis and developmental programming by GCs.
A circularly permuted photoactive yellow protein as a scaffold for photoswitch design.
Upon blue light irradiation, photoactive yellow protein (PYP) undergoes a conformational change that involves large movements at the N-terminus of the protein. We reasoned that this conformational change might be used to control other protein or peptide sequences if these were introduced as linkers connecting the N- and C-termini of PYP in a circular permutant. For such a design strategy to succeed, the circularly permuted PYP (cPYP) would have to fold normally and undergo a photocycle similar to that of the wild-type protein. We created a test cPYP by connecting the N- and C-termini of wild-type PYP (wtPYP) with a GGSGGSGG linker polypeptide and introducing new N- and C-termini at G115 and S114, respectively. Biophysical analysis indicated that this cPYP adopts a dark-state conformation much like wtPYP and undergoes wtPYP-like photoisomerization driven by blue light. However, thermal recovery of dark-state cPYP is ∼10-fold faster than that of wtPYP, so that very bright light is required to significantly populate the light state. Targeted mutations at M121E (M100 in wtPYP numbering) were found to enhance the light sensitivity substantially by lengthening the lifetime of the light state to ∼10 min. Nuclear magnetic resonance (NMR), circular dichroism, and UV-vis analysis indicated that the M121E-cPYP mutant also adopts a dark-state structure like that of wtPYP, although protonated and deprotonated forms of the chromophore coexist, giving rise to a shoulder near 380 nm in the UV-vis absorption spectrum. Fluorine NMR studies with fluorotryptophan-labeled M121E-cPYP show that blue light drives large changes in conformational dynamics and leads to solvent exposure of Trp7 (Trp119 in wtPYP numbering), consistent with substantial rearrangement of the N-terminal cap structure. M121E-cPYP thus provides a scaffold that may allow a wider range of photoswitchable protein designs via replacement of the linker polypeptide with a target protein or peptide sequence.
Biomedically relevant circuit-design strategies in mammalian synthetic biology.
The development and progress in synthetic biology has been remarkable. Although still in its infancy, synthetic biology has achieved much during the past decade. Improvements in genetic circuit design have increased the potential for clinical applicability of synthetic biology research. What began as simple transcriptional gene switches has rapidly developed into a variety of complex regulatory circuits based on the transcriptional, translational and post-translational regulation. Instead of compounds with potential pharmacologic side effects, the inducer molecules now used are metabolites of the human body and even members of native cell signaling pathways. In this review, we address recent progress in mammalian synthetic biology circuit design and focus on how novel designs push synthetic biology toward clinical implementation. Groundbreaking research on the implementation of optogenetics and intercellular communications is addressed, as particularly optogenetics provides unprecedented opportunities for clinical application. Along with an increase in synthetic network complexity, multicellular systems are now being used to provide a platform for next-generation circuit design.
Multi-chromatic control of mammalian gene expression and signaling.
The emergence and future of mammalian synthetic biology depends on technologies for orchestrating and custom tailoring complementary gene expression and signaling processes in a predictable manner. Here, we demonstrate for the first time multi-chromatic expression control in mammalian cells by differentially inducing up to three genes in a single cell culture in response to light of different wavelengths. To this end, we developed an ultraviolet B (UVB)-inducible expression system by designing a UVB-responsive split transcription factor based on the Arabidopsis thaliana UVB receptor UVR8 and the WD40 domain of COP1. The system allowed high (up to 800-fold) UVB-induced gene expression in human, monkey, hamster and mouse cells. Based on a quantitative model, we determined critical system parameters. By combining this UVB-responsive system with blue and red light-inducible gene control technology, we demonstrate multi-chromatic multi-gene control by differentially expressing three genes in a single cell culture in mammalian cells, and we apply this system for the multi-chromatic control of angiogenic signaling processes. This portfolio of optogenetic tools enables the design and implementation of synthetic biological networks showing unmatched spatiotemporal precision for future research and biomedical applications.
Photoswitchable protein degradation: a generalizable control module for cellular function?
In this issue of Chemistry & Biology, Renicke et al. report a photosensitive degron (psd) consisting of the LOV2 domain fused to a protein degradation sequence. This design enabled light-dependent protein degradation in yeast. When psd was fused to cell-cycle-dependent proteins, it controlled cell cycle by light with spatiotemporal precision.
A LOV2 domain-based optogenetic tool to control protein degradation and cellular function.
Light perception is indispensable for plants to respond adequately to external cues and is linked to proteolysis of key transcriptional regulators. To provide synthetic light control of protein stability, we developed a generic photosensitive degron (psd) module combining the light-reactive LOV2 domain of Arabidopsis thaliana phot1 with the murine ornithine decarboxylase-like degradation sequence cODC1. Functionality of the psd module was demonstrated in the model organism Saccharomyces cerevisiae. Generation of conditional mutants, light regulation of cyclin-dependent kinase activity, light-based patterning of cell growth, and yeast photography exemplified its versatility. In silico modeling of psd module behavior increased understanding of its characteristics. This engineered degron module transfers the principle of light-regulated degradation to nonplant organisms. It will be highly beneficial to control protein levels in biotechnological or biomedical applications and offers the potential to render a plethora of biological processes light-switchable.
Optogenetic tools for mammalian systems.
Light is fundamental to life on earth. Therefore, nature has evolved a multitude of photoreceptors that sense light across all kingdoms. This natural resource provides synthetic biology with a vast pool of light-sensing components with distinct spectral properties that can be harnessed to engineer novel optogenetic tools. These devices enable control over gene expression, cell morphology and signaling pathways with superior spatiotemporal resolution and are maturing towards elaborate applications in basic research, in the production of biopharmaceuticals and in biomedicine. This article provides a summary of the recent advances in optogenetics that use light for the precise control of biological functions in mammalian cells.
Nuclear actin network assembly by formins regulates the SRF coactivator MAL.
Formins are potent activators of actin filament assembly in the cytoplasm. In turn, cytoplasmic actin polymerization can promote release of actin from megakaryocytic acute leukemia (MAL) protein for serum response factor (SRF) transcriptional activity. We found that formins polymerized actin inside the mammalian nucleus to drive serum-dependent MAL-SRF activity. Serum stimulated rapid assembly of actin filaments within the nucleus in a formin-dependent manner. The endogenous formin mDia was regulated with an optogenetic tool, which allowed for photoreactive release of nuclear formin autoinhibition. Activated mDia promoted rapid and reversible nuclear actin network assembly, subsequent MAL nuclear accumulation, and SRF activity. Thus, a dynamic polymeric actin structure within the nucleus is part of the serum response.
Live imaging of multicolor-labeled cells in Drosophila.
We describe LOLLIbow, a Brainbow-based live imaging system with applications in developmental biology and neurobiology. The development of an animal, including the environmentally sensitive adaptation of its brain, is thought to proceed through continual orchestration among diverse cell types as they divide, migrate, transform and interact with one another within the body. To facilitate direct visualization of such dynamic morphogenesis by individual cells in vivo, we have modified the original Brainbow for Drosophila in which live imaging is practical during much of its development. Our system offers permanent fluorescent labels that reveal fine morphological details of individual cells without requiring dissection or fixation of the samples. It also features a non-invasive means to control the timing of stochastic tricolor cell labeling with a light pulse. We demonstrate applicability of the new system in a variety of settings that could benefit from direct imaging of the developing multicellular organism with single-cell resolution.
Guiding lights: recent developments in optogenetic control of biochemical signals.
Optogenetics arises from the innovative application of microbial opsins in mammalian neurons and has since been a powerful technology that fuels the advance of our knowledge in neuroscience. In recent years, there has been growing interest in designing optogenetic tools extendable to broader cell types and biochemical signals. To date, a variety of photoactivatable proteins (refers to induction of protein activity in contrast to fluorescence) have been developed based on the understanding of plant and microbial photoreceptors including phototropins, blue light sensors using flavin adenine dinucleotide proteins, cryptochromes, and phytochromes. Such tools offered researchers reversible, quantitative, and precise spatiotemporal control of enzymatic activity, protein-protein interaction, protein translocation, as well as gene transcription in cells and in whole animals. In this review, we will briefly introduce these photosensory proteins, describe recent developments in optogenetics, and compare and contrast different methods based on their advantages and limitations.
A predicted structure for the PixD-PixE complex determined by homology modeling, docking simulations, and a mutagenesis study.
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.
Optogenetic protein clustering and signaling activation in mammalian cells.
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.
Phosphorylation of phytochrome B inhibits light-induced signaling via accelerated dark reversion in Arabidopsis.
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.
Engineering of bacterial phytochromes for near-infrared imaging, sensing, and light-control in mammals.
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.
A red/far-red light-responsive bi-stable toggle switch to control gene expression in mammalian cells.
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.
Ultraviolet-B-mediated induction of protein-protein interactions in mammalian cells.
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.
Ultrafast red light activation of Synechocystis phytochrome Cph1 triggers major structural change to form the Pfr signalling-competent state.
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.
Optogenetic control of cell function using engineered photoreceptors.
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.
Photo-dynamics and thermal behavior of the BLUF domain containing adenylate cyclase NgPAC2 from the amoeboflagellate Naegleria gruberi NEG-M strain.
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.
Light detection and signal transduction in the BLUF photoreceptors.
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.