Phytochromes are red/far red light photochromic photoreceptors that direct many photosensory behaviors in the bacterial, fungal, and plant kingdoms. They consist of an N-terminal domain that covalently binds a bilin chromophore and a C-terminal region that transmits the light signal, often through a histidine kinase relay. Using x-ray crystallography, we recently solved the first three-dimensional structure of a phytochrome, using the chromophore-binding domain of Deinococcus radiodurans bacterial phytochrome assembled with its chromophore, biliverdin IX␣. Now, by engineering the crystallization interface, we have achieved a significantly higher resolution model. This 1.45 Å resolution structure helps identify an extensive buried surface between crystal symmetry mates that may promote dimerization in vivo. It also reveals that upon ligation of the C3 2 carbon of biliverdin to Cys 24 , the chromophore A-ring assumes a chiral center at C2, thus becoming 2(R),3(E)-phytochromobilin, a chemistry more similar to that proposed for the attached chromophores of cyanobacterial and plant phytochromes than previously appreciated. The evolution of bacterial phytochromes to those found in cyanobacteria and higher plants must have involved greater fitness using more reduced bilins, such as phycocyanobilin, combined with a switch of the attachment site from a cysteine near the N terminus to one conserved within the cGMP phosphodiesterase/ adenyl cyclase/FhlA domain. From analysis of site-directed mutants in the D. radiodurans phytochrome, we show that this bilin preference was partially driven by the change in binding site, which ultimately may have helped photosynthetic organisms optimize shade detection. Collectively, these three-dimensional structural results better clarify bilin/protein interactions and help explain how higher plant phytochromes evolved from prokaryotic progenitors. Phytochromes (Phys)3 comprise a ubiquitous superfamily of photoreceptors present in the plant, fungal, and bacterial kingdoms. They play critical roles in various light-regulated processes, ranging from phototaxis and pigmentation in bacteria to seed germination, chloroplast development, shade avoidance, and flowering in higher plants (1, 2). Phys are homodimeric complexes with each polypeptide containing an N-terminal chromophore-binding domain (CBD) that autocatalytically attaches via a thioether linkage a single linear tetrapyrrole (or bilin) chromophore, a PHY domain that is important for spectral integrity, and a C-terminal domain that promotes dimerization and often signal transmission (3). Through unique interactions among the bilin and the CBD and PHY domains, Phys can exist as two metastable conformers, a red light (R)-absorbing Pr form and a far red light (FR)-absorbing Pfr form. By photoconverting between Pr and Pfr, Phys act as unique light-regulated switches in various signal transduction cascades. In bacteria and fungi, a canonical histidine kinase (HK) domain is typically present downstream of the PHY domain, thus allowing these P...

The ability of phytochromes (Phy) to act as photointerconvertible light switches in plants and microorganisms depends on key interactions between the bilin chromophore and the apoprotein that promote bilin attachment and photointerconversion between the spectrally distinct red light-absorbing Pr conformer and far red light-absorbing Pfr conformer. Using structurally guided site-directed mutagenesis combined with several spectroscopic methods, we examined the roles of conserved amino acids within the bilin-binding domain of Deinococcus radiodurans bacteriophytochrome with respect to chromophore ligation and Pr/Pfr photoconversion. Incorporation of biliverdin IX␣ (BV), its structure in the Pr state, and its ability to photoisomerize to the first photocycle intermediate are insensitive to most single mutations, implying that these properties are robust with respect to small structural/electrostatic alterations in the binding pocket. In contrast, photoconversion to Pfr is highly sensitive to the chromophore environment. Many of the variants form spectrally bleached Meta-type intermediates in red light that do not relax to Pfr. Particularly important are Asp-207 and His-260, which are invariant within the Phy superfamily and participate in a unique hydrogen bond matrix involving the A, B, and C pyrrole ring nitrogens of BV and their associated pyrrole water. Resonance Raman spectroscopy demonstrates that substitutions of these residues disrupt the Pr to Pfr protonation cycle of BV with the chromophore locked in a deprotonated Meta-R c -like photoconversion intermediate after red light irradiation. Collectively, the data show that a number of contacts contribute to the unique photochromicity of Phy-type photoreceptors. These include residues that fix the bilin in the pocket, coordinate the pyrrole water, and possibly promote the proton exchange cycle during photoconversion.The phytochrome (Phy) 5 superfamily encompasses a large and diverse set of photoreceptors present in the plant, fungal, and bacterial kingdoms where they play critical roles in various light-regulated processes (1-3). These processes range from the control of phototaxis, pigmentation, and photosynthetic potential in proteobacteria and cyanobacteria to seed germination, chloroplast development, shade avoidance, and flowering time in higher plants. Phys are unique among photoreceptors in being able to assume two stable, photointerconvertible conformers, designated Pr and Pfr based on their respective absorption maxima in the red and far-red spectral regions. By cycling between Pr and Pfr, Phys act as light-regulated switches in various photosensory cascades.Phys are homodimeric complexes with each polypeptide containing a single bilin (or linear tetrapyrrole) chromophore, which binds autocatalytically via a thioether linkage to a positionally conserved cysteine (1-3). The photosensing portion typically contains Per/Arndt/Sim (PAS) and cGMP phosphodiesterase/adenyl cyclase/FhlA (GAF) domains, which are essential for bilin binding and Pr assembly, and t...

Phytochromes are photochromic photoreceptors responsible for a myriad of red/far-red light-dependent processes in plants and microorganisms. Interconversion is initially driven by photoreversible isomerization of bilin, but how this alteration directs the photostate-dependent changes within the protein to actuate signaling is poorly understood. Here, we describe the structure of the Deinococcus phytochrome photosensory module in its near complete far-red light-absorbing Pfr state. In addition to confirming the 180° rotation of the D-pyrrole ring, the dimeric structure clearly identifies downstream rearrangements that trigger large-scale conformational differences between the dark-adapted and photoactivated states. Mutational analyses verified the importance of residues surrounding the bilin in Pfr stabilization, and protease sensitivity assays corroborated photostate alterations that propagate along the dimeric interface. Collectively, these data support a cooperative "toggle" model for phytochrome photoconversion and advance our understanding of the allosteric connection between the photosensory and output modules.

The extremely acidic environment of the mammalian stomach, with a pH range usually between 1 and 3, represents a stressful challenge for enteric pathogenic bacteria such as Escherichia coli before they enter into the intestine. The hdeA gene of E. coli was found to be acid inducible and was revealed by genetic studies to be important for the acid survival of the strain. This study was performed in an attempt to characterize the mechanism of the activity of the HdeA protein. Our data provided in this report strongly suggest that HdeA employs a novel strategy to modulate its chaperone activity: it possesses an ordered conformation that is unable to bind denatured substrate proteins under normal physiological conditions (i.e. at neutral pH) and transforms into a globally disordered conformation that is able to bind substrate proteins under stress conditions (i.e. at a pH below 3). Furthermore, our data indicate that HdeA exposes hydrophobic surfaces that appear to be involved in the binding of denatured substrate proteins at extremely low pH values. In light of our observations, models are proposed to explain the action of HdeA in both a physiological and a molecular context.

Phytochromes are a collection of bilin-containing photoreceptors that regulate numerous photoresponses in plants and microorganisms through their ability to photointerconvert between a red light-absorbing, ground state Pr and a far-red light-absorbing, photoactivated state Pfr1,2. While the structures of several phytochromes as Pr have been determined3-7, little is known about the structure of Pfr and how it initiates signaling. Here, we describe the three-dimensional solution structure of the bilin-binding domain as Pfr using the cyanobacterial phytochrome from Synechococcus OSB’. Contrary to predictions, light-induced rotation of the A but not the D pyrrole ring is the primary motion of the chromophore during photoconversion. Subsequent rearrangements within the protein then affect intra- and interdomain contact sites within the phytochrome dimer. From our models, we propose that phytochromes act by propagating reversible light-driven conformational changes in the bilin to altered contacts between the adjacent output domains, which in most phytochromes direct differential phosphotransfer.

Phytochromes are a collection of bilin-containing photoreceptors that regulate a diverse array of processes in microorganisms and plants through photoconversion between two stable states, a red light-absorbing Pr form, and a far red light-absorbing Pfr form. Recently, a novel set of phytochrome-like chromoproteins was discovered in cyanobacteria, designated here as cyanochromes, that instead photoconvert between stable blue and green light-absorbing forms Pb and Pg, respectively. Here, we show that the distinctive absorption properties of cyanochromes are facilitated through the binding of phycocyanobilin via two stable cysteine-based thioether linkages within the cGMP phosphodiesterase/adenyl cyclase/FhlA domain. Absorption, resonance Raman and infrared spectroscopy, and molecular modeling of the Te-PixJ GAF (cGMP phosphodiesterase/adenyl cyclase/FhlA) domain assembled with phycocyanobilin are consistent with attachments to the C3 1 carbon of the ethylidene side chain and the C4 or C5 carbons in the A-B methine bridge to generate a double thioether-linked phycoviolobilin-type chromophore. These spectroscopic methods combined with NMR data show that the bilin is fully protonated in the Pb and Pg states and that numerous conformation changes occur during Pb 3 Pg photoconversion. Also identified were a number of photochromically inactive mutants with strong yellow or red fluorescence that may be useful for fluorescence-based cell biological assays. Phylogenetic analyses detected cyanochromes capable of different signaling outputs in a wide range of cyanobacterial species. One unusual case is the Synechocystis cyanochrome Etr1 that also binds ethylene, suggesting that it works as a hybrid receptor to simultaneously integrate light and hormone signals. Phytochromes (Phys)3 comprise a large and diverse superfamily of photoreceptors that regulate a wide range of physiological responses in plants, fungi, bacteria, and cyanobacteria (1-3). They are unique among photoreceptors by being able to photoconvert between two stable states, a red light-absorbing Pr form that is typically the dark-adapted and biologically inactive conformer and a far-red light-absorbing Pfr form that requires light for its production and is typically the biologically active conformer. By interconverting between Pr and Pfr, Phys act as light-regulated switches in controlling processes ranging from phototaxis and pigmentation in bacteria to seed germination, photomorphogenesis, and flowering time in higher plants.Light absorption by Phys is directed by a bilin (or linear tetrapyrrole) chromophore produced by the oxidative cleavage of heme. Although bacterial and fungal Phys use the immediate cleavage product biliverdin (BV), cyanobacterial and higher plant Phys use phycocyanobilin (PCB) and phytochromobilin, respectively, produced by enzymatic reduction of BV (1, 2). The bilin is then covalently bound autocatalytically to the photosensory unit of the apoprotein, which typically contains a sequence of Per/Arndt/Sim (PAS), cGMP phosphodiesterase/aden...

Purpose This paper aims to investigate the question concerning whether gender diversity in the boardroom matters to lenders or not? Design/methodology/approach To answer this question, the authors use the data from 2009 to 2015 of all A-share listed companies on the Shanghai and Shenzhen stock exchanges. The authors use ordinary least squares regression and firm fixed effect regression to draw our inferences. To check and control the issue of endogeneity the authors use one-year lagged gender diversity regression, two-stage least squares regression, propensity score matching method and Heckman two-stage regression. Findings The results suggest that the presence of female directors on the board reduces managerial opportunistic behavior and information asymmetry and, consequently, creditors’ perceptions about the probability of loan default and the cost of debt. The authors find that lenders charge 4 per cent less from borrowers that have at least one female board member than they do from borrowers with no female board members. The authors also find that the board structure (i.e. gender diversity) of government-owned firms also matters to lenders, as government-owned firms that have gender-diverse boards have a lower cost of debt (i.e. 5 per cent lower interest rate). Practical Implications The findings have implications for individual borrowers and for regulators. For example, borrowers can get debt financing at lower rates by altering their boards’ composition (i.e. through gender diversity). From the regulatory perspective, the results support recent legislative initiatives around the world regarding female directors’ representation on boards. Originality Value This paper makes several contributions. First, beyond the recent studies on boardroom gender, the authors investigate the relationship between gender diversity in the boardroom and the cost of debt. Second, the authors extend the literature on the association between government ownership and cost of debt by first time providing evidence that the board composition (e.g. gender diversity) of government-owned firms also matters to the lenders. The other contributions are discussed in the introduction section.

Phytochromes are a collection of dimeric photoreceptors that direct a diverse array of responses in plants and microorganisms through photoconversion between a red light-absorbing ground state Pr, and a far-red light-absorbing photoactivated state Pfr. Photoconversion from Pr to Pfr is initiated by a light-driven rotation within the covalently attached bilin, which then triggers a series of protein conformational changes in the binding pocket. These movements ultimately affect an appended output module, which often has reversible protein kinase activity. Propagation of the light signal from the bilin to the output module likely depends on the dimerization interface but its architecture and response to phototransformation remain unclear. Here, we used single particle cryoelectron microscopy to determine the quaternary arrangement of the phytochrome dimer as Pr, using the bacteriophytochrome (BphP) from Deinococcus radiodurans . Contrary to the long-standing view that the two monomers are held together solely via their C-terminal region, we provide unambiguous evidence that the N-terminal bilin-binding region of BphP also provides a dimerization interface with the C-terminal kinase domain appearing as a more flexible appendage. The BphP monomers dimerize in parallel with the polypeptides intimately twisting around each other in a right-handed fashion. Based on this electron microscopic picture, we propose that the light-driven conformational changes transmitted from the chromophore to the output module along the spine of this extensive dimer interface is the central feature underpinning phytochrome signaling.