On the in vivo redox state of flavin-containing photosensory receptor proteins

Reduction midpoint potential of a paradigm light-oxygen-voltage receptor and its modulation by methionine residues

Light-dependent adaptations of organismal physiology, development, and behavior abound in nature and depend on sensory photoreceptors. As one class, light-oxygen-voltage (LOV) photoreceptors harness flavin-nucleotide chromophores to sense blue light. Photon absorption drives the LOV receptor to its signaling state, characterized by a metastable thioadduct between the flavin and a conserved cysteine residue. With this cysteine absent, LOV receptors instead undergo photoreduction to the flavin semiquinone which however can still elicit downstream physiological responses. Irrespective of the cysteine presence, the LOV photochemical response thus entails a formal reduction of the flavin. Against this backdrop, we here investigate the reduction midpoint potential E 0 in the paradigmatic LOV2 domain from Avena sativa phototropin 1 (AsLOV2), and how it can be deliberately varied. Replacements of residues at different sites near the flavin by methionine consistently increase E 0 from its value of around -280 mV by up to 40 mV. Moreover, methionine introduction invariably impairs photoactivation efficiency and thus renders the resultant AsLOV2 variants less light-sensitive. Although individual methionine substitutions also affect the stability of the signaling state and downstream allosteric responses, no clear-cut correlation with the redox properties emerges. With a reduction midpoint potential near -280 mV, AsLOV2 and, by inference, other LOV receptors may be partially reduced inside cells which directly affects their light responsiveness. The targeted modification of the chromophore environment, as presently demonstrated, may mitigate this effect and enables the design of LOV receptors with stratified redox sensitivities.

Light-Mediated Decreases in Cyclic di-GMP Levels Inhibit Structure Formation in Pseudomonas aeruginosa Biofilms

Light is known to trigger regulatory responses in diverse organisms, including slime molds, animals, plants, and phototrophic bacteria. However, light-dependent processes in nonphototrophic bacteria, and those of pathogens in particular, have received comparatively little research attention. In this study, we examined the impact of light on multicellular development in Pseudomonas aeruginosa, a leading cause of biofilm-based bacterial infections. We grew P. aeruginosa strain PA14 in a colony morphology assay and found that growth under prolonged exposure to low-intensity blue light inhibited biofilm matrix production and thereby the formation of vertical biofilm structures (i.e., "wrinkles"). Light-dependent inhibition of biofilm wrinkling was correlated with low levels of cyclic di-GMP (c-di-GMP), consistent with the role of this signal in stimulating matrix production. A screen of enzymes with the potential to catalyze c-di-GMP synthesis or degradation identified c-di-GMP phosphodiesterases that contribute to light-dependent inhibition of biofilm wrinkling. One of these, RmcA, was previously characterized by our group for its role in mediating the effect of redox-active P. aeruginosa metabolites called phenazines on biofilm wrinkle formation. Our results suggest that an RmcA sensory domain that is predicted to bind a flavin cofactor is involved in light-dependent inhibition of wrinkling. Together, these findings indicate that P. aeruginosa integrates information about light exposure and redox state in its regulation of biofilm development.IMPORTANCE Light exposure tunes circadian rhythms, which modulate the immune response and affect susceptibility to infection in plants and animals. Though molecular responses to light are defined for model plant and animal hosts, analogous pathways that function in bacterial pathogens are understudied. We examined the response to light exposure in biofilms (matrix-encased multicellular assemblages) of the nonphotosynthetic bacterium Pseudomonas aeruginosa We found that light at intensities that are not harmful to human cells inhibited biofilm maturation via effects on cellular signals. Because biofilm formation is a critical factor in many types of P. aeruginosa infections, including burn wound infections that may be exposed to light, these effects could be relevant for pathogenicity.

Signal transduction in photoreceptor histidine kinases

Two-component systems (TCS) constitute the predominant means by which prokaryotes read out and adapt to their environment. Canonical TCSs comprise a sensor histidine kinase (SHK), usually a transmembrane receptor, and a response regulator (RR). In signal-dependent manner, the SHK autophosphorylates and in turn transfers the phosphoryl group to the RR which then elicits downstream responses, often in form of altered gene expression. SHKs also catalyze the hydrolysis of the phospho-RR, hence, tightly adjusting the overall degree of RR phosphorylation. Photoreceptor histidine kinases are a subset of mostly soluble, cytosolic SHKs that sense light in the near-ultraviolet to near-infrared spectral range. Owing to their experimental tractability, photoreceptor histidine kinases serve as paradigms and provide unusually detailed molecular insight into signal detection, decoding, and regulation of SHK activity. The synthesis of recent results on receptors with light-oxygen-voltage, bacteriophytochrome and microbial rhodopsin sensor units identifies recurring, joint signaling strategies. Light signals are initially absorbed by the sensor module and converted into subtle rearrangements of α helices, mostly through pivoting and rotation. These conformational transitions propagate through parallel coiled-coil linkers to the effector unit as changes in left-handed superhelical winding. Within the effector, subtle conformations are triggered that modulate the solvent accessibility of residues engaged in the kinase and phosphatase activities. Taken together, a consistent view of the entire trajectory from signal detection to regulation of output emerges. The underlying allosteric mechanisms could widely apply to TCS signaling in general.

Blue-Light Receptors for Optogenetics

Sensory photoreceptors underpin light-dependent adaptations of organismal physiology, development, and behavior in nature. Adapted for optogenetics, sensory photoreceptors become genetically encoded actuators and reporters to enable the noninvasive, spatiotemporally accurate and reversible control by light of cellular processes. Rooted in a mechanistic understanding of natural photoreceptors, artificial photoreceptors with customized light-gated function have been engineered that greatly expand the scope of optogenetics beyond the original application of light-controlled ion flow. As we survey presently, UV/blue-light-sensitive photoreceptors have particularly allowed optogenetics to transcend its initial neuroscience applications by unlocking numerous additional cellular processes and parameters for optogenetic intervention, including gene expression, DNA recombination, subcellular localization, cytoskeleton dynamics, intracellular protein stability, signal transduction cascades, apoptosis, and enzyme activity. The engineering of novel photoreceptors benefits from powerful and reusable design strategies, most importantly light-dependent protein association and (un)folding reactions. Additionally, modified versions of these same sensory photoreceptors serve as fluorescent proteins and generators of singlet oxygen, thereby further enriching the optogenetic toolkit. The available and upcoming UV/blue-light-sensitive actuators and reporters enable the detailed and quantitative interrogation of cellular signal networks and processes in increasingly more precise and illuminating manners.

The Brucella abortus virulence regulator, LovhK, is a sensor kinase in the general stress response signalling pathway

In the intracellular pathogen Brucella abortus, the general stress response (GSR) signalling system determines survival under acute stress conditions in vitro, and is required for long-term residence in a mammalian host. To date, the identity of the Brucella sensor kinase(s) that function to perceive stress and directly activate GSR signalling have remained undefined. We demonstrate that the flavin-binding sensor histidine kinase, LovhK (bab2_0652), functions as a primary B. abortus GSR sensor. LovhK rapidly and specifically phosphorylates the central GSR regulator, PhyR, and activates transcription of a set of genes that closely overlaps the known B. abortus GSR regulon. Deletion of lovhK severely compromises cell survival under defined oxidative and acid stress conditions. We further show that lovhK is required for cell survival during the early phase of mammalian cell infection and for establishment of long-term residence in a mouse infection model. Finally, we present evidence that particular regions of primary structure within the two N-terminal PAS domains of LovhK have distinct sensory roles under specific environmental conditions. This study elucidates new molecular components of a conserved signalling pathway that regulates B. abortus stress physiology and infection biology.