Engineering of Phycourobilin Synthase: PubS to a Two-Electron Reductase.

Phycourobilin:ferredoxin oxidoreductase (PubS) belongs to the ferredoxin-dependent bilin reductase (FDBR) family and catalyzes the reduction of the C15=C16 double bond, followed by the C4=C5 double bond of biliverdin IXα to produce phycourobilin. Among the diverse FDBR enzymes that catalyze site-specific reduction reactions of bilins, PubS lineage is the only one that reduces the C4=C5 double bond. This family can be broadly divided into four-electron reduction enzymes, which catalyze two successive two-electron reductions, such as PubS, and two-electron reduction enzymes, which catalyze a single two-electron reduction. The crystal structures of diverse FDBRs, excluding PubS, have unraveled that there are two distinct binding modes in the substrate-binding pocket. In this study, we focused on the arginine (Arg) residues that is considered crucial for substrate-binding mode in two-electron reduction enzymes. Through sequence alignment and comparison with the predicted structure of PubS, we identified a residue in PubS that corresponds to the Arg residue in the two-electron reduction enzymes. We further introduced mutations to avoid the steric hindrance associated with changes in the binding mode. Biochemical characterization of these variants showed that we successfully modified PubS from a four-electron reduction enzyme to a two-electron reduction enzyme with the accumulation of radicals. Our results provide insight into the molecular mechanisms of the chromophore binding mode and proton donation from acidic residues.

Intracellular biliverdin dynamics during ferroptosis.

Ferroptosis is a cell death mechanism mediated by iron-dependent lipid peroxidation. Although ferroptosis has garnered attention as a cancer-suppressing mechanism, there are still limited markers available for identifying ferroptotic cells or assessing their sensitivity to ferroptosis. The study focused on biliverdin, an endogenous reducing substance in cells, and examined the dynamics of intracellular biliverdin during ferroptosis using a biliverdin-binding cyanobacteriochrome. It was found that intracellular biliverdin decreases during ferroptosis and that this decrease is specific to ferroptosis among different forms of cell death. Furthermore, the feasibility of predicting sensitivity to ferroptosis by measuring intracellular biliverdin was demonstrated using a ferroptosis model induced by the re-expression of the transcription factor BACH1. These findings provide further insight into ferroptosis research and are expected to contribute to the development of cancer therapies that exploit ferroptosis.

Phytochromobilin Binding and Specific Amino Acid Residues Near The Chromophore Contribute To Orange Light Perception By The Dualchrome Phytochrome Region.

A novel photoreceptor dualchrome 1 (DUC1), containing a fused structure of cryptochrome and phytochrome, was discovered in the marine green alga Pycnococcus provasolli. The DUC1 phytochrome region (PpDUC1-N) binds to the bilin (linear tetrapyrrole) chromophores, phytochromobilin (PΦB) or phycocyanobilin (PCB), and reversibly photoconverts between the orange-absorbing dark-adapted state and the far-red-absorbing photoproduct state. This contrasts with typical phytochromes, which photoconvert between the red-absorbing dark-adapted and far-red-absorbing photoproduct states. In this study, we examined the molecular mechanism of PpDUC1-N to sense orange light by identifying the chromophore species synthesized by P. provasolli and the amino acid residues within the PpDUC1-N responsible for sensing orange light in the dark-adapted state. We focused on the PcyA homolog of P. provasolli (PpPcyA). Coexpression with the photoreceptors followed by an enzymatic assay revealed that PpPcyA synthesized PCB. Next, we focused on the PpDUC1-N GAF domain responsible for chromophore binding and light sensing. Ten amino acid residues were selected as the mutagenesis target near the chromophore. Replacement of these residues with those conserved in typical phytochromes revealed that three mutations (F290Y/M304S/L353M) resulted in a 23-nm red-shift in the dark-adapted state. Finally, we combined these constructs to obtain the PΦB-binding F290Y/M304S/L353M mutant and a 38-nm red-shift was observed compared with the PCB-binding wild-type PpDUC1. The binding chromophore species and the key residues near the chromophore contribute to blue-shifted orange light sensing in the dark-adapted state of the PpDUC1-N.

Red/green cyanobacteriochromes acquire isomerization from phycocyanobilin to phycoviolobilin.

Cyanobacteriochromes (CBCRs) are unique cyanobacteria-specific photoreceptors that share a distant relation with phytochromes. Most CBCRs contain conserved cysteine residues known as canonical Cys, while some CBCRs have additional cysteine residues called second Cys within the DXCF motif, leading to their classification as DXCF CBCRs. They typically undergo a process where they incorporate phycocyanobilin (PCB) and subsequently isomerize it to phycoviolobilin (PVB). Conversely, CBCRs with conserved Trp residues and without the second Cys are called extended red/green (XRG) CBCRs. Typical XRG CBCRs bind PCB without undergoing PCB-to-PVB isomerization, displaying red/green reversible photoconversion, and there are also atypical CBCRs that exhibit diverse photoconversions. We discovered novel XRG CBCRs with Cys residue instead of the conserved Trp residue. These novel XRG CBCRs exhibited the ability to isomerize PCB to PVB, displaying green/teal reversible photoconversion. Through sequence- and structure-based comparisons coupled with mutagenesis experiments, we identified three amino acid residues, including the Cys residue, crucial for facilitating PCB-to-PVB isomerization. This research expands our understanding of the diversity of XRG CBCRs, highlighting the remarkable molecular plasticity of CBCRs.

Functional Modification of Cyanobacterial Phycobiliprotein and Phycobilisomes through Bilin Metabolism Control.

Phycobilisomes (PBSs) are light-harvesting antenna complexes in cyanobacteria that adapt to diverse light environments through the use of phycobiliproteins within the PBS structures. Freshwater cyanobacteria, such as Synechococcus elongatus PCC 7942, thrive under red light because of the presence of phycocyanin (PC) and its chromophore, phycocyanobilin (PCB), in the PBS. Cyanobacteria in shorter-wavelength light environments such as green light, employ phycoerythrin paired with phycoerythrobilin (PEB) along with PC in the PBS. Synthetic biology studies have shown that PEB production can be achieved by expression of the heterologous PEB synthases 15,16-dihydrobiliverdin:ferredoxin oxidoreductase (PebA) and PEB:ferredoxin oxidoreductase (PebB), leading to PEB accumulation and cellular browning. This approach is genetically unstable, and the properties of the resulting PEB-bound PBS complexes remain uncharacterized. In this study, we engineered a novel strain of Synechococcus 7942 PEB1 with finely tuned control of PEB biosynthesis. PEB1 exhibited a reversible change in the color of the culture from green to brown and pink based on PebA and PebB induction levels. High induction led to complete PCB-to-PEB substitution, causing the disassembly of the PBS rod complex. In contrast, low induction levels of PebA and PebB resulted in the formation of a stable chimeric PBS complex with partial PCB-to-PEB substitution. This acclimation enabled efficient light harvesting in the green spectrum and energy transfer to the photosynthetic reaction center. These findings, which improve our understanding of PBS and highlight the structural importance of the bilin composition, provide a foundation for future studies on PBS adaptation in bioengineering, synthetic biology, and renewable energy.

Crucial Residue for Tuning Thermal Relaxation Kinetics in the Biliverdin-binding Cyanobacteriochrome Photoreceptor Revealed by Site-saturation Mutagenesis.

Cyanobacteriochromes (CBCRs) are cyanobacterial photoreceptors distantly related to the phytochromes sensing red and far-red light reversibly. Only the cGMP phosphodiesterase/Adenylate cyclase/FhlA (GAF) domain is needed for chromophore incorporation and proper photoconversion. The CBCR GAF domains covalently ligate linear tetrapyrrole chromophores and show reversible photoconversion between two light-absorbing states. In most cases, the two light-absorbing states are stable under dark conditions, but in some cases, the photoproduct state undergoes thermal relaxation back to the dark-adapted state during thermal relaxation. In this study, we examined the engineered CBCR GAF domain, AnPixJg2_BV4. AnPixJg2_BV4 covalently binds biliverdin IX-alpha (BV) and shows reversible photoconversion between a far-red-absorbing Pfr dark-adapted state and an orange-absorbing Po photoproduct state. Because the BV is an intrinsic chromophore of mammalian cells and absorbs far-red light penetrating into deep tissues, BV-binding CBCR molecules are useful for the development of optogenetic and bioimaging tools used in mammals. To obtain a better developmental platform molecule, we performed site-saturation random mutagenesis on the Phe319 position. We succeeded in obtaining variant molecules with higher chromophore-binding efficiency and higher molar extinction coefficient. Furthermore, we observed a wide variation in thermal relaxation kinetics, with an 81-fold difference between the slowest and fastest rates. Both molecules with relatively slow and fast thermal relaxation would be advantageous for optogenetic control.

Conformational change in an engineered biliverdin-binding cyanobacteriochrome during the photoconversion process.

Cyanobacteriochromes (CBCRs) derived from cyanobacteria are linear-tetrapyrrole-binding photoreceptors related to the canonical red/far-red reversible phytochrome photoreceptors. CBCRs contain chromophore-binding cGMP-specific phosphodiesterase/adenylate cyclase/FhlA (GAF) domains that are highly diverse in their primary sequences and are categorized into many subfamilies. Among this repertoire, the biliverdin (BV)-binding CBCR GAF domains receive considerable attention for their in vivo optogenetic and bioimaging applications because BV is a mammalian intrinsic chromophore and can absorb far-red light that penetrates deep into the mammalian body. The typical BV-binding CBCR GAF domain exhibits reversible photoconversion between far-red-absorbing dark-adapted and orange-absorbing photoproduct states. Herein, we applied various biochemical and spectral studies to identify the details of the conformational change during this photoconversion process. No oligomeric state change was observed, whereas the surface charge would change with a modification of the α-helix structures during the photoconversion process. Combinatorial analysis using partial protease digestion and mass spectrometry identified the region where the conformational change occurred. These results provide clues for the future development of optogenetic tools.

Introduction of reversible cysteine ligation ability to the biliverdin-binding cyanobacteriochrome photoreceptor.

Cyanobacteriochrome (CBCR) photoreceptors are distantly related to the canonical red/far-red reversible phytochrome photoreceptors. In the case of the CBCRs, only the GAF domain is required for chromophore incorporation and photoconversion. The GAF domains of CBCR are highly diversified into many lineages to sense various colors of light. These CBCR GAF domains are divided into two types: those possessing only the canonical Cys residue and those with both canonical and second Cys residues. The canonical Cys residue stably ligates to the chromophore in both cases. The second Cys residue mostly shows reversible adduct formation with the chromophore during photoconversion for spectral tuning. In this study, we focused on the CBCR GAF domain AnPixJg2_BV4, which possesses only the canonical Cys residue. AnPixJg2_BV4 covalently ligates to the biliverdin (BV) chromophore and shows far-red/orange reversible photoconversion. Because BV is a mammalian intrinsic chromophore, BV-binding molecules are advantageous for in vivo optogenetic and bioimaging tool development. To obtain a better developmental platform molecule, we performed site-saturation random mutagenesis and serendipitously obtained a unique variant molecule that showed far-red/blue reversible photoconversion, in which the Cys residue was introduced near the chromophore. This introduced Cys residue functioned as the second Cys residue that reversibly ligated with the chromophore. Because the position of the introduced Cys residue is distinct from the known second Cys residues, the variant molecule obtained in this study would expand our knowledge about the spectral tuning mechanism of CBCRs and contribute to tool development.

Role of hypoxanthine-guanine phosphoribosyltransferase in the metabolism of fairy chemicals in rice.

Fairy chemicals (FCs), 2-azahypoxanthine (AHX), imidazole-4-carboxamide (ICA), and 2-aza-8-oxohypoxanthine (AOH), are molecules with many diverse functions in plants. The defined biosynthetic pathway for FCs is a novel purine metabolism in which they are biosynthesized from 5-aminoimidazole-4-carboxamide. Here, we show that one of the purine salvage enzymes, hypoxanthine-guanine phosphoribosyltransferase (HGPRT), recognizes AHX and AOH as substrates. Two novel compounds, AOH ribonucleotide and its ribonucleoside which are the derivatives of AOH, were enzymatically synthesized. The structures were determined by mass spectrometry, 1D and 2D NMR spectroscopy, and X-ray single-crystal diffraction analysis. This report demonstrates the function of HGPRT and the existence of novel purine metabolism associated with the biosynthesis of FCs in rice.

Evidence for an early green/red photocycle that precedes the diversification of GAF domain photoreceptor cyanobacteriochromes.

Phytochromes are linear tetrapyrrole-binding photoreceptors in eukaryotes and bacteria, primarily responding to red and far-red light signals reversibly. Among the GAF domain-based phytochrome superfamily, cyanobacteria-specific cyanobacteriochromes show various optical properties covering the entire visible region. It is unknown what physiological demands drove the evolution of cyanobacteriochromes in cyanobacteria. Here, we utilize ancestral sequence reconstruction and biochemical verification to show that the resurrected ancestral cyanobacteriochrome proteins reversibly respond to green and red light signals. pH titration analyses indicate that the deprotonation of the bound phycocyanobilin chromophore is crucial to perceive green light. The ancestral cyanobacteriochromes show only modest thermal reversion to the green light-absorbing form, suggesting that they evolved to sense the incident green/red light ratio. Many cyanobacteria can utilize green light for photosynthesis using phycobilisome light-harvesting complexes. The green/red sensing cyanobacteriochromes may have allowed better acclimation to changing light environments by rearranging the absorption capacity of the phycobilisome through chromatic acclimation.

Novel cyanobacteriochrome photoreceptor with the second Cys residue showing atypical orange/blue reversible photoconversion.

Cyanobacteriochromes (CBCRs) are cyanobacterial linear tetrapyrrole-binding photoreceptors distantly related to phytochromes. Only the GAF domain is needed for chromophore incorporation and proper photoconversion of the CBCRs. Most CBCR GAF domains possess the canonical Cys residue stably ligating to the chromophore. DXCF-type CBCR GAF domains also possess a second Cys residue within the DXCF motif. This second Cys residue reversibly ligates to the C10 of the chromophore. The Cys adduct formation is mostly observed for the dark-adapted state but not for the photoproduct state. In this study, we discovered novel CBCR GAF domains with a DXCI motif instead of the DXCF motif. Since these CBCR GAF domains are categorized into two subfamilies (DXCI-1 and DXCI-2), the GAF domains from each subfamily were analyzed. Although the CBCR GAF domain belonging to the DXCI-2 subfamily showed orange/green reversible photoconversion without transient Cys ligation, the CBCR GAF domain belonging to the DXCI-1 subfamily showed reversible photoconversion between an orange-absorbing dark-adapted state and a blue-absorbing photoproduct state. This indicates that the second Cys residue is covalently bound to the C10 of the chromophore in the photoproduct state but not in the dark-adapted state. Since the covalent bond formation in the photoproduct state is atypical, site-directed mutagenesis was conducted to understand the molecular mechanism of this GAF domain. The Ile residue within the DXCI motif may be key for covalent bond formation in the photoproduct state.

A red light-responsive photoswitch for deep tissue optogenetics.

Red light penetrates deep into mammalian tissues and has low phototoxicity, but few optogenetic tools that use red light have been developed. Here we present MagRed, a red light-activatable photoswitch that consists of a red light-absorbing bacterial phytochrome incorporating a mammalian endogenous chromophore, biliverdin and a photo-state-specific binder that we developed using Affibody library selection. Red light illumination triggers the binding of the two components of MagRed and the assembly of split-proteins fused to them. Using MagRed, we developed a red light-activatable Cre recombinase, which enables light-activatable DNA recombination deep in mammalian tissues. We also created red light-inducible transcriptional regulators based on CRISPR-Cas9 that enable an up to 378-fold activation (average, 135-fold induction) of multiple endogenous target genes. MagRed will facilitate optogenetic applications deep in mammalian organisms in a variety of biological research areas.

Identification of significant residues for intermediate accumulation in phycocyanobilin synthesis.

Phycocyanobilin, the primary pigment of both light perception and light-harvesting in cyanobacteria, is synthesized from biliverdin IXα (BV) through intermediate 181, 182-dihydrobiliverdin (181, 182-DHBV) by a phycocyanobilin:ferredoxin oxidoreductase (PcyA). In our previous study, we discovered two PcyA homologs (AmPcyAc and AmPcyAp) derived from Acaryochloris marina MBIC 11017 (A. marina) that exceptionally uses chlorophyll d as the primary photosynthetic pigment, absorbing longer wavelength far-red light than chlorophyll a, the photosynthetic pigment found in most cyanobacteria. Biochemical characterization of the two PcyA homologs identified functional diversification of these two enzymes: AmPcyAc provides 181, 182-DHBV, and PCB to the cyanobacteriochrome (CBCR) photoreceptors, whereas, AmPcyAp specifically provides PCB to the light-harvesting phycobilisome subunit. In this study, we focused on the residues necessary for 181, 182-DHBV supply to the CBCR photoreceptors by AmPcyAc. Based on the SyPcyA structure, we concentrated on the 30 residues that constitute the substrate-binding pocket. Among them, we discovered that Leu151 and Val225 in AmPcyAc were both substituted with isoleucine. During the enzymatic reaction, the SyPcyA variant molecule, possessing V225I and L151I replacements, accumulates the 181, 182-DHBV and supplies it to a CBCR molecule derived from A. marina. It is worth noting that the substitution of Val225 with isoleucine was specifically conserved among the Acaryochloris genus. Collectively, we propose that the specific evolution of PcyA among the Acaryochloris genus may correlate with the acquisition of Chl. d synthetic ability and growth in long-wavelength far-red light environments.

Identification of a dual orange/far-red and blue light photoreceptor from an oceanic green picoplankton.

Photoreceptors are conserved in green algae to land plants and regulate various developmental stages. In the ocean, blue light penetrates deeper than red light, and blue-light sensing is key to adapting to marine environments. Here, a search for blue-light photoreceptors in the marine metagenome uncover a chimeric gene composed of a phytochrome and a cryptochrome (Dualchrome1, DUC1) in a prasinophyte, Pycnococcus provasolii. DUC1 detects light within the orange/far-red and blue spectra, and acts as a dual photoreceptor. Analyses of its genome reveal the possible mechanisms of light adaptation. Genes for the light-harvesting complex (LHC) are duplicated and transcriptionally regulated under monochromatic orange/blue light, suggesting P. provasolii has acquired environmental adaptability to a wide range of light spectra and intensities.

An Engineered Biliverdin-Compatible Cyanobacteriochrome Enables a Unique Ultrafast Reversible Photoswitching Pathway.

Cyanobacteriochromes (CBCRs) are promising optogenetic tools for their diverse absorption properties with a single compact cofactor-binding domain. We previously uncovered the ultrafast reversible photoswitching dynamics of a red/green photoreceptor AnPixJg2, which binds phycocyanobilin (PCB) that is unavailable in mammalian cells. Biliverdin (BV) is a mammalian cofactor with a similar structure to PCB but exhibits redder absorption. To improve the AnPixJg2 feasibility in mammalian applications, AnPixJg2_BV4 with only four mutations has been engineered to incorporate BV. Herein, we implemented femtosecond transient absorption (fs-TA) and ground state femtosecond stimulated Raman spectroscopy (GS-FSRS) to uncover transient electronic dynamics on molecular time scales and key structural motions responsible for the photoconversion of AnPixJg2_BV4 with PCB (Bpcb) and BV (Bbv) cofactors in comparison with the parent AnPixJg2 (Apcb). Bpcb adopts the same photoconversion scheme as Apcb, while BV4 mutations create a less bulky environment around the cofactor D ring that promotes a faster twist. The engineered Bbv employs a reversible clockwise/counterclockwise photoswitching that requires a two-step twist on ~5 and 35 picosecond (ps) time scales. The primary forward Pfr → Po transition displays equal amplitude weights between the two processes before reaching a conical intersection. In contrast, the primary reverse Po → Pfr transition shows a 2:1 weight ratio of the ~35 ps over 5 ps component, implying notable changes to the D-ring-twisting pathway. Moreover, we performed pre-resonance GS-FSRS and quantum calculations to identify the Bbv vibrational marker bands at ~659,797, and 1225 cm-1. These modes reveal a stronger H-bonding network around the BV cofactor A ring with BV4 mutations, corroborating the D-ring-dominant reversible photoswitching pathway in the excited state. Implementation of BV4 mutations in other PCB-binding GAF domains like AnPixJg4, AM1_1870g3, and NpF2164g5 could promote similar efficient reversible photoswitching for more directional bioimaging and optogenetic applications, and inspire other bioengineering advances.

Unusual ring D fixation by three crucial residues promotes phycoviolobilin formation in the DXCF-type cyanobacteriochrome without the second Cys.

Cyanobacteriochromes are linear tetrapyrrole-binding photoreceptors produced by cyanobacteria. Their chromophore-binding GAF domains are categorized into many lineages. Among them, dual Cys-type cyanobacteriochrome GAF domains possessing not only a highly conserved 'first Cys' but also a 'second Cys' are found from multiple lineages. The first Cys stably attaches to C31 of the A-ring, while the second Cys mostly shows reversible ligation to the C10 of the chromophore. Notably, the position of the second Cys in the primary sequence is diversified, and the most abundant dual Cys-type GAF domains have a 'second Cys' within the DXCF motif, which are called DXCF GAF domains. It has been long known that the second Cys in the DXCF GAF domains not only shows the reversible ligation but also is involved in isomerization activity (reduction in C4=C5 double bond) from the initially incorporated phycocyanobilin to phycoviolobilin. However, comprehensive site-directed mutagenesis on the DXCF GAF domains, AM1_6305g1 and AM1_1499g1, revealed that the second Cys is dispensable for isomerization activity, in which three residues participate by fixing the C- and D-rings. Fixation of the chromophore on both sides of the C5 bridge is necessary, even though one side of the fixation site is far from this bridge, with the other side at C31 fixed by the first Cys.

Comparison of the Forward and Reverse Photocycle Dynamics of Two Highly Similar Canonical Red/Green Cyanobacteriochromes Reveals Unexpected Differences.

Cyanobacteriochromes (CBCRs) are cyanobacterial photoreceptors that exhibit photochromism between two states: a thermally stable dark-adapted state and a metastable light-adapted state with bound linear tetrapyrrole (bilin) chromophores possessing 15Z and 15E configurations, respectively. The photodynamics of canonical red/green CBCRs have been extensively studied; however, the time scales of their excited-state lifetimes and subsequent ground-state evolution rates widely differ and, at present, remain difficult to predict. Here, we compare the photodynamics of two closely related red/green CBCRs that have substantial sequence identity (∼68%) and similar chromophore environments: AnPixJg2 from Anabaena sp. PCC 7120 and NpR6012g4 from Nostoc punctiforme. Using broadband transient absorption spectroscopy on the primary (125 fs to 7 ns) and secondary (7 ns to 10 ms) time scales together with global analysis modeling, our studies revealed that AnPixJg2 and NpR6012g4 have comparable quantum yields for initiating the forward (15ZPr → 15EPg) and reverse (15EPg → 15ZPr) reactions, which proceed through monotonic and nonmonotonic mechanisms, respectively. In addition to small discrepancies in the kinetics, the secondary reverse dynamics resolved unique features for each domain: intermediate shunts in NpR6012g4 and a Meta-Gf intermediate red-shifted from the 15ZPr photoproduct in AnPixJg2. Overall, this study supports the conclusion that sequence similarity is a useful criterion for predicting pathways of the light-induced evolution and quantum yield of generating primary intermediate Φp within subfamilies of CBCRs, but more studies are still needed to develop a comprehensive molecular level understanding of these processes.

Transient electronic and vibrational signatures during reversible photoswitching of a cyanobacteriochrome photoreceptor.

Cyanobacteriochromes (CBCRs) are an emerging class of photoreceptors that are distant relatives of the phytochromes family. Unlike phytochromes, CBCRs have gained popularity in optogenetics due to their highly diverse spectral properties spanning the UV to near-IR region and only needing a single compact binding domain. AnPixJg2 is a CBCR that can reversibly photoswitch between its red-absorbing (15ZPr) and green-absorbing (15EPg) forms of the phycocyanobilin (PCB) cofactor. To reveal primary events of photoconversion, we implemented femtosecond transient absorption spectroscopy with a homemade LED box and a miniature peristaltic pump flow cell to track transient electronic responses of the photoexcited AnPixJg2 on molecular time scales. The 525 nm laser-induced Pg-to-Pr reverse conversion exhibits a ~3 ps excited-state lifetime before reaching the conical intersection (CI) and undergoing further relaxation on the 30 ps time scale to generate a long-lived Lumi-G ground state intermediate en route to Pr. The 650 nm laser-induced Pr-to-Pg forward conversion is less efficient than reverse conversion, showing a longer-lived excited state which requires two steps with ~13 and 217 ps time constants to enter the CI region. Furthermore, using a tunable ps Raman pump with broadband Raman probe on both the Stokes and anti-Stokes sides, we collected the pre-resonance ground-state femtosecond stimulated Raman spectroscopy (GS-FSRS) data with mode assignments aided by quantum calculations. Key vibrational marker bands at ~850, 1050, 1615, and 1649 cm-1 of the Pr conformer exhibit a notable blueshift to those of the Pg conformer inside AnPixJg2, reflecting the PCB chromophore terminal D (major) and A (minor) ring twist along the primary photoswitching reaction coordinate. This integrated ultrafast spectroscopy and computational platform has the potential to elucidate photochemistry and photophysics of more CBCRs and photoactive proteins in general, providing the highly desirable mechanistic insights to facilitate the rational design of functional molecular sensors and devices.

Phytochromes and Cyanobacteriochromes: Photoreceptor Molecules Incorporating a Linear Tetrapyrrole Chromophore.

In this chapter, we summarize the molecular mechanisms of the linear tetrapyrrole-binding photoreceptors, phytochromes, and cyanobacteriochromes. We especially focus on the color-tuning mechanisms and conformational changes during the photoconversion process. Furthermore, we introduce current status of development of the optogenetic tools based on these molecules. Huge repertoire of these photoreceptors with diverse spectral properties would contribute to development of multiplex optogenetic regulation. Among them, the photoreceptors incorporating the biliverdin IXα chromophore is advantageous for in vivo optogenetics because this is intrinsic in the mammalian cells, and absorbs far-red light penetrating into deep mammalian tissues.