Pr Form Research Articles

Chromophore-apoprotein interactions in Synechocystis sp. PCC6803 phytochrome Cph1.

The secondary, tertiary, and quaternary structures of the Synechocystis Cph1 phytochrome were investigated by absorption and circular dichroism spectroscopy, size exclusion chromatography, and limited proteolysis. The Cph1 protein was coexpressed with a bacterial thioredoxin in Escherichia coli, reconstituted in vitro with tetrapyrrole chromophores, and purified by chitin affinity chromatography. The resultant Cph1 holoproteins were essentially pure and had the specific absorbance ratio (SAR) of 0.8-0.9. Circular dichroism spectroscopy and limited proteolysis showed that the chromophore binding induced marked conformational changes in the Cph1 protein. The alpha-helical content increased to 42-44% in the holoproteins from 37% in the apoprotein. However, no significant difference in the secondary structure was detected between the Pr and Pfr forms. The tertiary structure of the Cph1 apoprotein appeared to be relatively flexible but became more compact and resistant to tryptic digestion upon chromophore binding. Interestingly, a small chromopeptide of about 30 kDa was still predominant even after longer tryptic digestion. The N-terminal location of this chromopeptide was confirmed by expression in E. coli and in vitro reconstitution with chromophores of the 32.5 kDa N-terminal fragment of the Cph1 protein. This chromopeptide was fully photoreversible with the spectral characteristic similar to that of the full-size Cph1 protein. The Cph1 protein forms dimers through the C-terminal region. These results suggest that the prokaryotic Cph1 phytochrome shares the structural and conformational characteristics of plant phytochromes, such as the two-domain structure consisting of the relatively compact N-terminal and the relatively flexible C-terminal regions, in addition to the chromophore-induced conformational changes.

Nuclear and cytosolic events of light-induced, phytochrome-regulated signaling in higher plants.

Plants as sessile organisms cannot walk away but have to adapt to changes in their natural environments. Light is obviously the most important external factor, being the energy source. To regulate their fitness, the photosynthetic capacity of plants must be adapted to changes in the ambient light environment. Therefore, plants have evolved a large set of photoreceptors to monitor light quality and quantity, ranging from the UV to the infrared part of the spectrum. At least three different photoreceptor classes have been identified and analyzed: (i) UV‐B receptors, primarily characterized by action spectroscopy (Wellmann, 1983); (ii) the blue UV‐A photoreceptors, cry1 and cry2 and phototropine (Ahmad and Cashmore, 1993; Christie et al ., 1998; Lin et al ., 1998); and (iii) the red/far‐red reversible phytochromes. These photoreceptors control different aspects of plant growth and development, and in this review we focus on the physiological and molecular events regulated by the most characterized plant photoreceptors, the phytochromes. In higher plants, phytochromes are encoded by a small multigene family; five genes ( PHYA–PHYE , Sharrock and Quail, 1989; Clack et al ., 1994) have been identified in Arabidopsis . Phytochromes exist as dimers composed of two 125 kDa polypeptides, each carrying a covalently linked tetrapyrrol chromophore in the N‐terminal domain and dimerization domains in the C‐terminal domain. The photosensory function of the molecule is based on its capacity for reversible interconversion between the red light‐absorbing Pr form and the far‐red light absorbing Pfr form following sequential absorption of red and far‐red light. Photosignal perception by the receptor is followed by conformational changes, and activates, through an as yet poorly understood mechanism, signaling pathways leading to changes in the expression of genes that underlie developmental responses to light (Quail et al ., 1995). In order to finely tune their …

Sequences within both the N- and C-terminal domains of phytochrome A are required for PFR ubiquitination and degradation.

Photoconversion of the plant photoreceptor phytochrome A (phyA) from its inactive Pr form to its biologically active Pfr from initiates its rapid proteolysis. Previous kinetic and biochemical studies implicated a role for the ubiquitin/26S proteasome pathway in this breakdown and suggested that multiple domains within the chromoprotein are involved. To further resolve the essential residues, we constructed a series of mutant PHY genes in vitro and analyzed the Pfr-specific degradation of the resulting photoreceptors expressed in transgenic tobacco. One important site is within the C-terminal half of the polypeptide as its removal stabilizes oat phyA as Pfr. Within this half is a set of conserved lysines that are potentially required for ubiquitin attachment. Substitution of these lysines did not prevent ubiquitination or breakdown of Pfr, suggesting either that they are not the attachment sites or that other lysines can be used in their absence. A small domain just proximal to the C-terminus is essential for the form-dependent breakdown of the holoprotein. Removal of just six amino acids in this domain generated a chromoprotein that was not rapidly degraded as Pfr. Using chimeric photoreceptors generated from potato PHYA and PHYB, we found that the N-terminal half of phyA is also required for Pfr-specific breakdown. Only those chimeras containing the N-terminal sequences from phyA were ubiquitinated and rapidly degraded as Pfr. Taken together, our data demonstrate that, whereas an intact C-terminal domain is essential for phyA degradation, the N-terminal domain is responsible for the selective recognition and ubiquitination of Pfr.

Higher-plant phytochrome: "I used to date histidine, but now I prefer serine".

The primary photoreceptors of higher plants are the phytochromes and cryptochromes—quite distinct from the rhodopsins that animals use for vision. Phytochrome mediates responses to red/far-red light through a tetrapyrolle chromophore (1, 2), whereas the cryptochromes respond to blue/UV-A light through a flavin (3, 4). An important and distinguishing feature of the phytochrome light-sensing system is that it measures, not the absolute amount of red/far-red light, but the ratio of the amount of light (the number of photons) corresponding to these two regions of the spectrum. Phytochrome acts as a light-regulated molecular switch, with the capacity to undergo repeated interconversion between the red light-absorbing phytochrome (Pr) form and the far-red light-absorbing phytochrome (Pfr).

The Complexity of the Pr to Pfr Phototransformation Kinetics Is an Intrinsic Property of Native Phytochrome*

AbstractSeveral possible origins of the complex phytochrome red to far‐red light‐absorbing phytochrome (Pr Pfr) phototransformation kinetics in the nanosecond‐to‐second time range have been examined. Heterogeneity based on protein sequence is ruled out as an origin of the multi‐component kinetics because recombinant 124 kDa oat phytochrome A apoprotein reconstituted with phytochro‐mobilin and the native protein are very similar in this regard throughout this time range. The Pr forms of native 124 kDa oat phytochrome A and of a homogeneous recombinant 65 kDa chromoprotein fragment exhibit thermochromic properties interpreted as arising in each case from the presence of two Pr species in thermal equilibrium. They exhibit identical photochemical properties. The complex kinetics therefore cannot result from Pr heterogeneity either. Thus, the presence of two Pr forms in equilibrium (Pr,675 and Pr,655) and the complex multiex‐ponential PrPfr phototransformation kinetics observed in all time ranges are intrinsic properties of the homogeneous holoprotein of oat phytochrome A.

Interactions ofSrF2andPrF3with TiC(111) and Si(111) surfaces studied by low-energyD+scattering spectroscopy

On the basis of neutralization of scattered low-energy ${\mathrm{D}}^{+}$ ions, the nature of the bonding of ionic molecules adsorbed on metal and semiconductor surfaces has been investigated. It is found that ${\mathrm{SrF}}_{2}$ reacts with the active dangling-bond states of the TiC(111) surface and the ionic bonding between ${\mathrm{Sr}}^{2+}$ and ${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ ions is strongly weakened. However, the ionicity of the adsorbates recovers after oxygenation or hydrogenation of the TiC(111) surface since H or O passivates the dangling-bond states at the interface. On the other hand, the dangling bond of Si(111) has relatively little effect on the ionic Sr-F bond formation and rather dissociation of ${\mathrm{SrF}}_{2}$ is promoted at an elevated temperature due to preferential reaction of F with Si. In terms of ${\mathrm{PrF}}_{3},$ ionicity is strongly reduced on both Si(111) and TiC(111) surfaces and oxygenation of the surface has very little effect, suggesting that ${\mathrm{PrF}}_{3}$ is dissociatively adsorbed and Pr forms covalent or metallic bonds with the substrate.

Modes of action of phytochromes

through largely unknown pathways into molecular and physiological changes that modulate growth and developPhytochrome is a family of photoreceptors whose ment. Phytochromes posses two photo-interconvertible apoproteins are encoded by divergent genes. Three forms, the R-absorbing form (Pr) and the FR-absorbing modes of action of phytochromes, very-low-fluence form (Pfr) and are synthesised in the Pr form. In recent responses (VLFR), low-fluence responses (LFR) and years it has been established that phytochromes are a high-irradiance responses (HIR), have been considered small family of photoreceptors whose apoproteins are in the literature to define the quantitative relationship encoded by diVerent genes (Clack et al., 1994; Hauser between response and predicted levels of the far-red et al., 1995). Traditionally, three ‘modes of action’ of light absorbing form of phytochrome. The results of phytochromes have been described: very-low-fluence recent experiments with phytochrome mutant and responses ( VLFR); low-fluence responses (LFR) and transgenic seedlings are consistent with the notion high-irradiance responses (HIR). Assigning particular that phytochrome B, and in some cases other stable responses to particular molecular species of phytochromes phytochromes, mediate LFR whereas phytochrome A is one of the important areas of research in the field mediates both VLFR and HIR. The mechanisms of photomorphogenesis. This review seeks to bring involved in phytochrome A inductive responses (VLFR) together data on the modes of action of phytochrome versus HIR appear to be different in some ways. with information on diVerent phytochrome molecular However, VLFR, LFR and HIR can converge to control species. the transcriptional activity of a single gene promoter. The analysis of phytochrome transduction chains is The implications of these findings for the under- currently concentrating the eVorts of many laboratories. standing of phytochrome phototransduction are Experimental approaches include the characterization of discussed. the photoreceptor molecule, in particular the biological significance of diVerent domains (Cherry et al., 1993;

Phytochrome degradation

ABSTRACTPlants actively modulate the levels of the various phyto‐chrome isoforms during their life cycle to optimize light absorption and perception. For phytochrome A (phyA), one of the most influential methods of control is selective turnover of the photoreceptor upon photoconversion from the red‐absorbing form (Pr) to the far‐red‐absorbing form (Pfr). Whereas the Pr form has a half‐life of approximately 1 week, the Pfr form is rapidly degraded with a half‐life of 1–2 h. The ubiquitin/26S proteasome pathway has been implicated in phyA breakdown. In this proteolytic pathway, multiple ubiquitins are covalently attached to proteins committed for degradation; these ubiquitin‐protein conjugates then serve as intermediates in the breakdown of the target protein by the 26S proteasome, a multi‐subunit proteolytic complex. In several plant species, ubiquitin‐phyA conjugates have been detected in vivo following Pfr formation that show accumulation and decay kinetics expected for Pfr degradation intermediates. Analyses of phyA mutants and phyA/phyB chimeras expressed in transgenic plants have been particularly useful in mapping domains within the chromoprotein that are necessary for Pfr degradation. Several domains have been identified within both the N‐ and C‐terminal portions of the photoreceptor that presumably serve as recognition and/or acceptor sites for ubiquitination

Chromophore: apoprotein interactions in phytochrome A*

ABSTRACTPhytochromes are molecular light switches by virtue of their photochromic red/far‐red reversibility. The His‐324 residue next to the chromophore‐linked Cys‐323 plays a critical role in conferring photochromism to the tetrapyrrole chromophore in native phytochrome A. The chromophore appears to be enclosed between the amphiphilic α‐helical chains in a hydrophobic pocket. The absorbance maxima of both the Pr and the Pfr forms of pea phytochrome A are blue‐shifted by 10 and 20 nm, respectively, upon C‐terminal truncation. We speculate that the quaternary structure of the phytochrome A molecule involves some interactions of the C‐terminal half with the chromophore domain. The Pfr conformation of phytochrome includes an amphiphilic α‐helix of the amino terminal chain, which occurs in 113 ms after picosecond photoisomerization of the Pr form. Compared to α‐helical folding, unfolding of the α‐helix occurs faster in about 310 μs upon phototransformation of the Pfr form of phytochrome A. The photochromic transformation of phytochrome A modulates protein kinase‐catalysed phosphorylation sites in vivo and in vitro, but only a subtle local change in conformation is detectable in the phosphorylated phytochromes. This suggests that the post‐translational modification serves as a surface label, rather than a transducer‐activating trigger, for the recognition of a putative phytochrome receptor.

Resonance raman analysis of chromophore structure in the lumi-R photoproduct of phytochrome.

Resonance Raman vibrational spectra of the Pr, lumi-R, and Pfr forms of phytochrome have been obtained using low-temperature trapping and room temperature flow techniques in conjunction with shifted-excitation Raman difference spectroscopy (SERDS). The Pr to lumi-R photoconversion exhibits a thermal barrier and is completely blocked at 30 K, indicating that thermally assisted protein relaxation is necessary for the primary photochemistry. When Pr is converted to lumi-R, new bands appear in the C = C and C = N stretching regions at 1651, 1636, 1590, and 1569 cm-1, indicating that a significant structural change of the chromophore has occurred. The photoconversion also results in an 18 cm-1 decrease in the N-H rocking band in lumi-R. Normal mode calculations correlate this frequency drop with a change in the geometry of the C15 methine bridge of the phytochromobilin chromophore. Additionally, a C = N stretching mode marker band shifts from 1576 cm-1 in Pr to 1569 cm-1 in lumi-R and to 1552 cm-1 in Pfr. Normal mode calculations show that the frequency drop of this band in the lumi-R-->Pfr interconversion is an indication of a C14-C15 syn-->anti conformational change. Moderately intense hydrogen out-of-plane modes that occur at 805 cm-1 in Pr shift to 829 and 847 cm-1 upon photoconversion to lumi-R and are replaced by a very intense mode at 814 cm-1 in Pfr. These observations indicate that the C and D rings of the chromophore in Pr and lumi-R are moderately planar but that they become highly distorted in Pfr. This information suggests that the primary photochemistry in phytochrome is a Z-->E isomerization of the C15 = C16 bond of Pr giving lumi-R. This is followed by a thermal syn-->anti C14-C15 conformational relaxation to form Pfr. A four-state model is presented to explain the chromophore structural changes in Pr, lumi-R, and Pfr that uses hydrogen bonding to the surrounding protein to stabilize the high-energy Pfr C15 = C16, C14-C15, E,anti chromophore structure. This implicates an anchor and release mechanism between the chromophore and protein that might lead to altered biological signaling in the plant.

Mechanism of native oat phytochrome photoreversion: a time-resolved absorption investigation.

The regulation of plant photomorphogenesis is mediated by the thermal reactions that follow light absorption by the phytochrome photoreceptor. Phytochromes are tetrapyrrolic chromoproteins that exist in two photochromically interconvertible forms, a red light absorbing species, Pr, and a far-red light absorbing form, Pfr. Upon irradiation with 670 nm light, the inactive, red light sensing Pr form is converted to the active Pfr form. Although the forward phototransformation has been studied extensively by several groups using various techniques, the Pfr-->Pr photoreversion reaction that occurs upon irradiation with 730 nm light is not as thoroughly characterized. In this study, time-resolved absorption (TROD) spectroscopy is used to examine the intermediate species involved in the phytochrome photoreversion mechanism at 10 degrees C. Analysis of the TROD data identifies three species with lifetimes of 320 ns, 265 microseconds, and 5.5 ms. TROD results are described in terms of the simplest parallel and sequential kinetic models. Comparison of intermediate spectra from these mechanisms with those of previously reported species from flash photoreversion and low-temperature studies indicates that Pfr photoreversion follows a sequential pathway that does not share any intermediates with the Pr phototransformation pathway.

Fluorescence and photochemical characterization of phytochrome in de-etiolated pea mutant lip

The red-light-absorbing form of phytochrome, Pr, and its phototransformation into the first stable photoproduct, lumi-R, under red light ( λ r = 667 nm) and back under far-red light ( λ fr = 696 nm) at 85 K were investigated with the use of low temperature fluorescence spectroscopy in the stems and roots of lip mutant pea seedlings, which have many of the characteristics of light-grown plants even when grown in the dark (S. Frances et al., Plant Cell, 4 (1992) 1519–1530). The spectroscopic and photochemical parameters of Pr were found to be similar in wild-type and lip mutant etiolated seedlings. The two Pr forms with high and low extents of phototransformation into lumi-R, i.e. Pr′ and Pr″ respectively, detected at low temperatures (V. Sineshchekov, Photochem. Photobiol., 59 (1994) 77–86) were shown to be roughly equivalent in the wild-type and the lip mutant pea. Fluence time-response curves for the phototransformation of Pr′ into lumi-R and back in lip stems revealed three subpopulations of Pr similar to those obtained in the wild-type pea. The total phytochrome content [P tot] determined from the Pr fluorescence intensity in the stems of the mutant was approximately 30% of the wild type. However, mutant and wild-type roots contained similar amounts of Pr. Thus any contribution to the mutant phenotype made by phytochrome may be due to changes in the total concentration of phytochrome rather than variations in the relative content or modification of the various Pr species.

Genetic Evidence That the Red-Absorbing Form of Phytochrome B Modulates Gravitropism in Arabidopsis thaliana.

Hypocotyls of dark-grown Arabidopsis seedlings exhibit strong negative gravitropism, whereas in red light, gravitropism is strongly reduced. Red/far-red light-pulse experiments and analysis of specific phytochrome-deficient mutants indicate that the red-absorbing (Pr) form of phytochrome B regulates normal hypocotyl gravitropism in darkness, and depletion of Pr by photoconversion to the far-red-absorbing form attenuates hypocotyl gravitropism. These studies provide genetic evidence that the Pr form of phytochrome has an active function in plant development.

In vitro formation of a photoreversible adduct of phycocyanobilin and tobacco apophytochrome B.

The light-stable tobacco phytochrome apoprotein (PHYB) expressed in yeast can be assembled with phycocyanobilin to give a photoreversible adduct. The spectral properties of the reconstituted PHYB-phycocyanobilin species were determined by absorbacen and difference absorbance spectroscopies. The holoprotein exhibits absorbance maxima at 408 nm and 712 nm for the far-red-light-absorbing (Pfr) form and 356 nm and 658 nm for the red-light-absorbing (Pr) form. The ligation of the chromophores to the dimeric PHYB apoprotein resulted in a PHYB-phycocyanobilin adduct with the spectral properties of the Pr form. Kinetic analyses of the in vitro reconstitution for PHYB apoprotein under saturating concentrations of phycocyanobilin revealed a pseudo first-order rate constant of 2.8 x 10(-2)s.-1. The similarity with the reported rate constant for the reconstitution of light-labile phytochrome (PHYA) from oat [Li, L. & Lagarias, J.C. (1992) Phytochrome assembly, J. Biol. Chem. 267, 19,204-19,210] suggests that the mechanisms of chromophore attachment are probably very similar for PHYA and PHYB.

ULTRAVIOLET RESONANCE RAMAN SPECTRA OF PHYTOCHROME: A COMPARISON OF THE ENVIRONMENTS OF TRYPTOPHAN SIDE CHAINS BETWEEN RED LIGHT‐ABSORBING AND FAR‐RED LIGHT‐ABSORBING FORMS

AbstractUltraviolet resonance Raman spectra of phytochrome in the red light‐absorbing form (Pr) and the far‐red light‐absorbing form (Pfr) are reported. The spectra excited at 240‐nm provide structural information about the protein part of phytochrome. The protein contains only a very small amount of β‐sheet structure and most of the tyrosine side chains are located in hydrophobic environments. Indole rings of tryptophan (Trp) interact with neighboring groups in the Pr form and these interactions become weaker with the conversion from Pr to Pfr. Some Trp side chains of Pfr are surrounded by aliphatic groups but such is not the case in Pr. These changes in the environment occur at the same time as changes in orientation of Trp side chains. Our observations suggest that interactions between Trp residues and the tetrapyrrolic chromophore occur in the Pr form and that the strength of these interactions diminishes in the Pfr form.

Expression of Oat-phyA-cDNA in a Suspension Cell Culture of Transgenic Tobacco: A Single-Cell System for the Study of Phytochrome Function

A culture of callus cells has been developed from a transgenic line of tobacco which contains an introduced phyA-cDNA encoding phytochrome A. Suspension cultures of the cells were shown to accumulate a significant immunodetectable level of the heterologous phytochrome, but not of the native phyA-gene product. The red-irradiated form (Pfr) of the heterologous phytochrome was specifically degraded in vivo, and the red-irradiated (Pfr) and far-red-irradiated (Pr) forms demonstrated different patterns of in vitro proteolytic cleavage. These results strongly suggested that the phytochrome apoprotein was associated with a chromophore moiety which mediated red/far-red sensitive conformational changes of the molecule. Exogenous application of 4-amino-5-hexynoic acid (AHA) to the transgenic suspension cultures resulted in the accumulation of a population of phytochrome which was stable under red light and gave identical patterns of in vitro digestion in the red and far-red irradiated forms, i.e. the spectral activity of phytochrome was inhibited. Application of exogenous 5-aminolevulinic acid (ALA) or biliverdin overcame the inhibitory effects of AHA to restore spectral sensitivity of the phytochrome pool. These results are consistent with the proposed pathway of phytochrome chromophore biosynthesis in intact plant systems. Thus, the transgenic suspension cultures provided a single-cell system in which spectrally-active phytochrome, apparently indistinguishable from the native phytochrome synthesized in etiolated seedlings, was accumulated. Photoregulation of expression of the genes encoding the small subunit of ribulosc-1,5-tephosphate carboxylase and chlorophyll aib binding proteins demonstrated that the heterologous phytochrome population mediated rapid changes in gene expression in the de-differentiated cells. It is therefore proposed that such a suspension culture of transgenic cells offers a model system for the study of phytochrome function.

Identification of Surface-Exposed Parts of Red-Light- and Far-Red-Light- Absorbing Forms of Native Pea Phytochrome by Limited Proteolysis

Peptide fragments were obtained by limited proteolysis with trypsin and Staphylococcus aureus V 8 protease from either the PR or the PFR form of 121-kDa phytochrome purified from etiolated pea (Pisum sativum L.) shoots. Patterns of bands after polyacrylamide gel electrophoresis in the presence of SDS of the digests were different, with some bands appearing preferentially when the digestions were carried out with the PR or the PFR form. Amino-terminal sequences of the fragments were analyzed to determine the exact locations of the amino-termini of the fragments within the amino acid sequence of the apoprotein of pea phytochrome. The amino acid compositions of some of the sequenced fragments were determined in order to confirm the carboxy-terminal amino acids. Three cleavage regions were identified as kinetically favored sites of cleavage of PFR (Arg-746 to Lys-752, around Glu-877 and around Arg-1010), whereas only one was identified for PR (Glu-38 to Arg-62). Regions of Glu-255, Arg-383, Arg-583 to Glu-620 and Lys-1093 to Glu-1115 were also identified as potential sites of proteolytic cleavage in both forms of the phytochrome. Other cleavage sites, the specificities of which have not yet been determined, are Glu-404, Glu-695 and Lys-1045. Surface-exposed parts of phytochrome in the PR and PFR forms are discussed.

Fourier transform resonance Raman spectroscopy of phytochrome.

The Pr and Pfr forms of phytochrome in H2O and D2O have been studied by Fourier transform resonance Raman spectroscopy with near-infrared excitation (1064 nm). It is demonstrated that this technique is a powerful method for analyzing the chromophore structures of photosensitive pigments. The high spectral quality allows discussion of vibrational assignments based on an empirical approach using previously published data obtained from model compounds. The reduction in intensity of a high-frequency band assigned to the ring-C/D methine bridge vibration is an indication for the non-coplanarity of the ring D in Pfr. The high intensity of a C-H out-of-plane vibration also supports this hypothesis. In Pr, a broad peak at approximately 1100 cm-1 is assigned to an out-of-plane vibration of a strongly hydrogen-bonded pyrrole C=NH+ group. It is missing in Pfr, suggesting deprotonation of the corresponding ring during the transformation from Pr to Pfr.

Phytochrome assembly. Defining chromophore structural requirements for covalent attachment and photoreversibility.

Assembly of holophytochrome in the plant cell requires covalent attachment of the linear tetrapyrrole chromophore precursor, phytochromobilin, to a unique cysteine in the nascent apoprotein. In this investigation we compare chromophore analogs with the natural chromophore precursor for their ability to attach covalently to recombinant oat apophytochrome and to form photoactive holoproteins. Ethylidene-containing analogs readily form covalent adducts with apophytochrome, whereas chromophores lacking this double bond are poor substrates for attachment. Kinetic measurements establish that although the chromophore binding site on apophytochrome is best tailored to phytochromobilin, apophytochrome will accommodate the two analogs with modified D-rings, phycocyanobilin and phycoerythrobilin. The phycocyanobilin-apophytochrome adduct is photoactive and undergoes a light-induced protein conformational change similar to the native holoprotein. By contrast, the phycoerythrobilin adduct is locked into a photochemically inactive protein conformation that is similar to the red light-absorbing Pr form of phytochrome. These results support the hypothesis that the photoconversion from Pr to Pfr, the far red light- absorbing form of phytochrome, involves the photoisomerization of the C15 double bond. Knowledge gained from these studies provides impetus for rational design of chromophore analogs whose insertion into apophytochrome should elicit profound changes in light-mediated plant growth and development.

Magnetic-susceptibility and heat-capacity measurements on Pr-rich PrBa2-xPrxCu3O7-y compounds.

The compound ${\mathrm{PrBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{y}}$, though isostructural to the 90-K superconducting compounds, R${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{y}}$ (R=rare earth and Y), is semiconducting. Moreover, it orders magnetically with ${\mathit{T}}_{\mathit{N}}$\ensuremath{\sim}16 K and also exhibits a large electronic specific-heat coefficient \ensuremath{\gamma}. We have prepared Pr-rich ${\mathrm{PrBa}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Pr}}_{\mathit{x}}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{y}}$ (0\ensuremath{\le}x\ensuremath{\le}1) compounds to study the effect of excess Pr at the Ba site on the structure, magnetic ordering, and electronic specific-heat coefficient of this system. It is observed that single-phase compounds in the above series form up to x\ensuremath{\sim}0.8. The stoichiometric ${\mathrm{PrBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{y}}$ is orthorhombic but the compounds with excess Pr form in the closely related tetragonal structure. Magnetic-susceptibility and heat-capacity measurements reveal that both ${\mathit{T}}_{\mathit{N}}$ and \ensuremath{\gamma} decrease with increasing Pr concentration suggesting a common origin underlying the two.