Action Spectra for Conversions of Phycochrome c from Nostoc muscorum

The reversibly photochromic pigment, phycochrome c, was extracted from the blue-green alga Nostoc muscorum strain A. Action spectra were determined for in vitro conversions of the pigment from the short wavelength to the long wavelength form and vice versa. The action peak for the absorbance decrease at 650 nm is at 630 nm. During this decrease there is only a slight increase of the absorbance in the green region. Green and yellow light (maximum efficiency at 580 nm) completely restores absorbance at 650 nm. The observations are explained by the existence of three spectrally different forms of phycochrome c: P^c₆₃₀ and P^c₆₅₀ which equilibrate in darkness and P^c₅₈₀ which is reversibly photoconvertible to P^c₆₃₀. We have also measured the absorbance changes brought about by saturating irradiations with light of various wavelengths (“photostationary state spectrum”). Extreme photostationary states were obtained with about 650 nm and 500 nm light.     

Phycochrome d, a New Photochromic Pigment from the Blue-Green Alga, Tolypothrix distorta

A new reversibly photochromic pigment, phycochrome d, has been found in extracts of the blue-green alga Tolypothrix distorta. This phycochrome exhibits an absorbance increase in the red region (maximum at about 650 nm) when irradiated with 650 nm light, and a corresponding absorbance decrease when irradiated with 610 nm light. The absorbance difference spectrum and action spectra for in vitro conversions were determined.     

Photochromic pigments in akinetes and pigment characteristics of akinetes in comparison with vegetative cells of Anabaena variabilis

Since akinete germination is triggered by light and the action spectrum for this process has features in common with the spectra of the two photochromic pigments, phycochromes b and d, a search was made for the presence of these phycochromes in akinetes of the blue-green alga. Anabaena variabilis Kützing. Allophycocyanin-B was also looked for, since the action spectrum for akinete germination points to a possible participation of this pigment too. Isoelectric focusing was used for purification of the pigments. The different fractions were investigated for phycochromes b and d by measuring the absorbance difference spectra: for phycochrome b. 500 nm irradiated minus 570 nm irradiated, and for phycochrome d, 650 nm irradiated minus 610 nm irradiated. For determination of allophycocyanin-B. fourth derivative analysis of absorption spectra was made for some of the fractions from the isoelectric focusing column. Phycochrome b was also assayed for by measuring in vivo absorption difference spectra. The assays were positive for all three pigments. The complete photosynthetic pigment systems were also studied by in vivo fluorescence measurements on both akinetes and vegetative cells of Anabaena variabilis. Fluorescence emission and excitation spectra at selected emission wavelengths were measured at room temperature and liquid nitrogen temperature. The energy transfer from phycoerythrocyanin to phycocyanin is very efficient under all conditions, as is the energy transfer from phycocyanin to allophycocyanin at room temperature. At low temperature, however, phycocyanin is partly decoupled from allophycocyanin, particularly in the akinetes; the energy transfer from allophycocyanin to chlorophyll a is less efficient at low temperature in both types of cells, but especially in akinetes. Delayed light emission was measured for both types of cells and found to be very weak in akinetes compared to vegetative cells. From this study it would seem that akinetes lack an active photosystem II, although the 691 nm peak in the 570 nm excited low temperature fluorescence emission spectrum proves the presence of photosystem II chlorophyll, and also its energetic connection to the phycobilisomes.     

Biosynthesis in isolatedAcetabularia chloroplasts II. Plastid pigments

Summary The plastid pigments — chlorophylls and carotenoids — of the alga,Acetabularia, have been chromatographically separated and identified. These pigments were found to become radio-active during incubations of an isolated chloroplast fraction with¹⁴CO₂. Specific activity calculations indicate that appreciable amounts of synthesis were occurringin vitro. The phytol and porphyrin moieties of chlorophyll a were both radioactive; thus the pigments were being formed completely from recent photosynthetic products. A comparison of the incorporation of¹⁴CO₂ into plastid pigmentsin vivo andin vitro suggests that the isolated chloroplasts form the pigments at their normalin vivo rates.     

Photochromic Pigments from Blue-Green Algae: Phycochromes a, b, and c

Aqueous extracts of blue-green algae were fractionated by electrofocusing. In all algae investigated, fractions with iso-electric points at or near 4.6 showed photochromic behaviour analogous to that of phytochrome, although they were sensitive to light of shorter wavelength. Three main types of photochromic pigments were found: Phycochrome a (in Tolypothrix distorta, Phormidium luridum, Nostoc muscorum 1453/12, and Anacystis nidulans) has one form absorbing maximally at about 590 nm (formed under red light) and one absorbing maximally at about 630 nm (formed under green light). Phycochrome b (in Tolypothrix distorta) has one form absorbing maximally near 510 nm and one form absorbing maximally at 570 nm (formed in yellow-green and blue-green light, respectively). Phycochrome c (in Nostoc muscorum A and probably in Tolypothrix tenuis) has one form absorbing maximally at 650 nm (formed under green light) and one absorbing very weakly in the green region (formed under red light). The conversion of Phormidium phycochrome a from its red-absorbing form to its green-absorbing form causes the same spectral change as if an f-chromophore of phycocyanin were transformed into an s-chromophore. The quantum yield for this conversion is estimated to be 0.1, while the quantum yield for the reversion is estimated to be 0.4 on the assumption that the absorption coefficients are those of f- and s-chromophores. Phycochrome c is less light-sensitive than phycochromes a and b.     

Relationships between nitrate level,nitrate reductase activity and anaerobic nitrite production inPisum sativum leaf tissue

Anaerobic nitrite production (thein vivo NO₃-R activity) in an incubation medium lacking exogenous nitrate but containing 0.5%n-propanol and 0.1% Triton X-100 showed higher correlation (y - ax ^b) with the level of endogenous nitrate inPisum sativum L. leaves than thein vitro nitrate reductase activity. Thein vivo NO₃-R activity correlated well with thein vitro activity up to the 50 ppm NO₃-N level of endogenous nitrate. The ratioin vivo: in vitro activity slightly decreased with increasing level of endogenous nitrate in leaf tissue.     

Phototaxis in a dinoflagellate: Action spectra as evidence for a two-pigment system

Summary Action spectra were determined in the UV region of the spectrum for the first phase of the phototactic response (stop response) and for the phytochrome pigment associated with this response in the dinoflagellate Gyrodinium dorsum Kofoid. Differences between these action spectra indicate the participation of two pigments in phototaxis. Following R (620 nm) irradiation of the phytochrome, the stop response maxima occur at 470 and 280-nm; after FR irradiation they shift to 490 and 300–310 nm. These maxima suggest that the photoreceptor pigment for phototaxis is a carotenoprotein. The action spectrum shift following the different phytochrome conversions may represent a trans to cis isomer change by the carotenoid. The absorption maximum of P_R in the UV appears to be at 320 nm, which is consistent with the shift of the R absorption maximum to shorter wavelengths (620 nm) as compared to higher plants. The P_FR absorption maximum appears as a broad band between 360 and 390 nm. Comparison of P_R to P_FR conversions by different intensities of 620-nm and 320-nm light indicates that at lower intensities the logarithm of the threshold for the stop response is inversely proportional to the logarithm of the intensity of the sensitizing light. The ratio of response activation by R and UV light is about 4:1.Abbreviations FR far-red - R red - P_FR far-red-absorbing form of phytochrome - P_R red-absorbing form of phytochrome - UV ultraviolet     

The in vivo and in vitro Reconstitution of Pigment-protein Complexes,and its Implication in Acquiring a New System

Reconstitution is one of the most fundamental and powerful tools to investigate pigment—protein complexes, for example, light-harvesting complexes and reaction center complexes. Two reconstitution methods, in vitro and in vivo, have been applied to complexes. In vitro reconstitution methods were first developed using isolated proteins and pigments, and recently using over-expressed proteins. This method enables analysis of pigment binding, pigment stoichiometry, and protein flexibility when accepting extrinsic pigments, however, it has not yet been successfully applied to the core antenna system. In vivo reconstitution, which was developed using genetic modifications, is applicable even on core systems. In this Review, the in vivo reconstitution is mainly considered on the basis of the in vitro reconstitution, because the former was a recent development and will be expanded to many systems. When genes for a new pigment are acquired and expressed, the new pigment is incorporated into a pre-exiting complex(es) and becomes functional when it is accepted by this complex(es), or abandoned if it is not. This process is postulated to occur during the evolutionary process(es) of antenna and reaction centers, and it is now possible to reproduce this evolutionary developmental pathway(s). Several examples of in vivo reconstitution are given and considered from the viewpoint of evolutionary implication with regards to the antenna and reaction centers.This revised version was published online in October 2005 with corrections to the Cover Date.     

Chloroplast ultrastructure,photosynthetic apparatus activities and production of steviol glycosides in <Emphasis Type="Italic">Stevia rebaudiana in vivo</Emphasis> and <Emphasis Type="Italic">in vitro</Emphasis>

The accumulation of steviol glycosides (SGs) in cells of Stevia rebaudiana Bertoni both in vivo and in vitro was related to the extent of the development of the membrane system of chloroplasts and the content of photosynthetic pigments. Chloroplasts of the in vitro plants, unlike those of the intact plants, had poorly developed membrane system. The callus cells grown in the light contained proplastids of almost round shape and their thylakoid system was represented by short thylakoids sometimes forming a little number of grana consisting of 2–3 thylakoids. In cells of the etiolated in vitro regenerants and the callus culture grown in the dark, only proplastids practically lacking the membrane system were observed. All the chloroplasts having developed thylakoids and forming at least a little number of grana were equipped with photochemically active reaction centers of photosystems 1 and 2. Leaves of in vivo plants accumulated greater amount of the pigments than leaves of the in vitro plants. In both the callus culture grown in the light and the etiolated in vitro regenerants, the content of the pigments was one order of magnitude lower than that in leaves of the intact plants. The callus tissue grown in the dark contained merely trace amounts of the pigments. Leaves of the intact and the in vitro plants did not exhibit any significant differences in photosynthetic O₂ evolution rate. However, photosynthetic O₂ evolution rate in the callus cells was much lower than that in the differentiated plant cells. The in vitro cell cultures containing merely proplastids did not practically produce SGs. However, after transferring these cultures in the light, both the formation of chloroplasts and the production of SGs in them were detected.     

Adenylate cyclase toxin translocates across target cell membrane without forming a pore

The adenylate cyclase toxin‐haemolysin of Bordetella (CyaA) targets CD11b⁺ myeloid phagocytes and translocates across their cytoplasmic membrane an adenylate cyclase (AC) enzyme that catalyses conversion of cytosolic ATP into cAMP. In parallel, CyaA acts as a cytolysin forming cation‐selective pores, which permeabilize cell membrane and eventually provoke cell lysis. Using cytolytic activity, potassium efflux and patch‐clamp assays, we show that a combination of substitutions within the pore‐forming (E570Q) and acylation‐bearing domain (K860R) ablates selectively the cell‐permeabilizing activity of CyaA. At the same time, however, the capacity of such mutant CyaA to translocate the AC domain across cytoplasmic membrane into cytosol of macrophage cells and to elevate cellular cAMP concentrations remained intact. Moreover, the combination of E570Q+K860R substitutions suppressed the residual cytolytic activity of the enzymatically inactive CyaA/OVA/AC^‐ toxoid on CD11b‐expressing monocytes, while leaving unaffected the capacity of the mutant toxoid to deliver in vitro a reporter CD8⁺ T cell epitope from ovalbumin (OVA) to the cytosolic pathway of dendritic cells for MHC class I‐restricted presentation and induce in vivo an OVA‐specific cytotoxic T cell response. CyaA, hence, employs a mechanism of AC enzyme domain translocation across cellular membrane that avoids passage across the cytolytic pore formed by toxin oligomers.     

The photosensitive retinal pigments of fishes from relatively turbid coastal waters

Digitonin extracts have been prepared from the retinae of a dozen species of marine and euryhaline teleost fishes from turbid water habitats. Spectrophotometric analysis of the extracts shows that the photosensitive retinal pigments of these species have maximum absorption above 500 mµ. In nine species there are retinene₁ pigments with λ_max between 504 and 512 mµ. In the marine but euryhaline mullet, Mugil cephalus, there is a porphyropsin with λ_max 520 mµ. A mixture of rhodopsin and porphyropsin in an extract of a marine puffer, Sphoeroides annulatus, was disclosed by partial bleaching with colored light. In addition, one other species has a 508 mµ pigment, of which the nature of the chromophore was not determined. The habitats in which these fishes live are relatively turbid, with the water greenish or yellowish in color. The spectral transmission of such waters is probably maximal between 520 and 570 mµ. It is suggested that the fishes have become adapted to these conditions by small but significant shifts in spectral absorption of their retinal pigments. These pigments are decidedly more effective than rhodopsin in absorption of wavelengths above 500 mµ. This offers a possible interpretation of the confusing array of retinal pigments described from marine and euryhaline fishes.     

Responses of Tobacco Plantlets to Change of Irradiance During Transfer from in vitro to ex vitro Conditions

Chlorophyll a fluorescence kinetics, net photosynthetic rate (P _N), water relations, and photosynthetic pigment contents were studied during acclimation of in vitro grown tobacco to higher irradiance (HL; 700 mol m^–2 s^–1). Plantlets were grown on medium containing sucrose in glass vessels (G-plants) or in Magenta boxes (M-plants) with better CO₂ supply in the latter ones. The effect of HL was studied either (1) in plantlets grown under original in vitro conditions (closed vessels), (2) in in vitro plantlets exposed to ambient CO₂ concentration (covers removed), or (3) in plantlets transplanted to ex vitro into pots with sand and nutrient solution. Higher P _N, and fraction of closed photosystem 2 (PS2) centres (1 – q_P), and lower content of xanthophyll cycle pigments were found in M-plants compared to G-plants. HL treatment caused photoinhibition particularly in plants kept in closed vessels. This was indicated by the decrease in the ratio of F_v/F_m and by the increase in non-photochemical quenching, 1 – qp, and content of xanthophyll cycle pigments. Better CO₂ supply ensured by the removal of closure lead to the moderate reduction of symptoms of photoinhibition, although stomatal conductance (g _s), transpiration rate (E), and P _N were negatively affected. The main reason was the decrease in relative air humidity, which caused similar reduction of P _N, E, and g _s after the transfer of plantlets to ex vitro. Nevertheless, plant response to HL seemed not to be affected by any possible root injury caused by transfer to ex vitro. The differences in contents of xanthophyll cycle pigments, degree of de-epoxidation, P _N, and quenching parameters between M- and G-plantlets were still significant 7 d after ex vitro transfer and HL acclimation.     

Influence of divalent cations on the conformation of phosphorothioate oligodeoxynucleotides: a circular dichroism study

Phosphorothioate oligodeoxynucleotides (ODNs) have been extensively investigated in vivo and in vitro for antisense control of gene expression. It has been shown that cellular uptake of phosphorothioate ODNs in some in vitro cell systems increases in the presence of divalent cations. In this work, we analyze the conformation of phosphorothioate ODNs and specific changes induced in it by various divalent cations using circular dichroism (CD) spectroscopy. CD data were obtained with several phosphorothioate ODNs in the absence and presence of the divalent cations Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺ and Mn²⁺. All CD spectra indicated stable conformations of the ODNs in solution. The spectra were strongly dependent on ODN sequence and composition. Some ODNs such as T₂₃ and another with ‘random’ distribution of bases showed CD spectra characteristic of B-form DNA. Other ODNs which had at least three consecutive guanines in their sequences exhibited spectra characteristic of parallel G-tetraplexes. CD spectra of antisense ODNs exhibited specific responses to divalent cations. Changes in the conformation were not simply due to ionic strength effects. Mn²⁺ diminished secondary structure in some ODNs. Group II divalent ions stabilized the parallel G-tetraplexes, and Mg²⁺ generally had the weakest stabilizing efficiency. Each sequence/ion combination had a specific response so these effects cannot be generalized. These sequence-dependent, divalent ion-sensitive, and structurally unique solution conformations may be related to ion-mediated ODN uptake.     

Effect of a single dose of inducers and inhibitors on the rate of synthesis of cytochromes and reductases in liver organelles

Summary Measurement of the effect of drugs on the in vivo rates of synthesis of rabbit liver organelle bound proteins were measured following individual treatments with the inducers phenobarbital, 3-methylcholanthrene and PCB (a mixture of polychlorinated biphenyls) and the inhibitors, cycloheximide, aflatoxin 13, chloramphenicol and actinomycin D. Following their isolation from a homogenate containing the combined livers of ¹⁴C-leucine injected experimental animals and 3H-leucine injected control animals, purified fractions of the following proteins were prepared: microsomal cytochrome b₅, cytochrome P-450, NADH-cytochrome b₅ reductase, NADPH-cytochrome P-450 reductase and proteolipids, outer mitochrondrial membrane cytochrome b5, NADH-cytochrome b₅ reductase and proteolipids, inner mitochrondrial membrane cytochrome c, NADH dehydrogenase and proteolipids, intermitochrondrial membrane cytochrome b₅ and circulating serum albumin.This research was supported by grants from the PSC-BHE Research Award Program of The City University of New York, US Public Health Service Grant SO 7 PRO7132-07 and the Alma Toorock Fund for Cancer Research.     

BIO-OPTICAL CHARACTERISTICS AND PHOTOADAPTIVE RESPONSES IN THE TOXIC AND BLOOM-FORMING DINOFLAGELLATES GYRODINIUM AUREOLUM,GYMNODINIUM GALATHEANUM,AND TWO STRAINS OF PROROCENTRUM MINIMUM1

Photoadaptive responses in the toxic and bloom-forming dinoflagellates Gyrodinium aureolum Hulbert, Gymnodinium galatheanum Braarud, and two strains of Prorocentrum minimum (Pavillard)Schiller were evaluated with respect to pigment composition, light-harvesting characteristics, carbon and nitrogen contents, and growth rates in shade- and light-adapted cells. The two former species were grown at scalar irradiances of 30 and 170 μmol · m ^?2 at a 12-h daylength at 20° C. The two strains of P. minimum were grown at 35 and 500 μmol. m^?2· s^?1 at a 2-h daylength at 20° C. For the first time, chlorophyll (chl) c₃, characteristic of several bloom-forming prymnesiophytes, was detected in G. aureolum and G. galatheanum. Photoadaptional status affected the pigment composition strongly, and the interpretation of the variation depended on whether the pigment composition was normalized per cell, carbon, or chl a. Species-specific and photoadaptional differences in chl a-specific absorption (°a_c, 400–700 nm) and chl a-normalized fluorescence excitation spectra of photosystem II fluorescence with or without addition of DCMU (°F and °F_DCMU 400–700 nm) were evident. Gyrodinium aureolum and G. galatheanum exhibited in vivo spectral characteristics similar to chl c₃-containing prymnesiophytes in accordance with their similar pigmentation. Prorocentrum minimum had in vivo absorption and fluorescence characteristics typical for peridinin-containing dinoflagellates. Species-specific differences in in vivo absorption were also observed as a function of package effect vs. growth irradiance. This effect could be explained by differences in intracellular pigment content, cell size/shape, and chloroplast morphology/numbers. Light- and shade-adapted cells of P. minimum contained 43 and 17% of photoprotective carotenoids (diadino + diatoxanthin) relative to chl a, respectively. The photoprotective function of these carotenoids was clearly observed as a reduction in °F and °F ^DCMU at 400–540 nm compared to °ac in light-adapted cells of P. minimum. Spectrally weighted light absorption (normalized to chl a and carbon, 400–700 nm) varied with species and growth conditions. The use of quantum-corrected and normalized fluorescence excitation spectra with or without DCMU-treated cells to estimate photosynthetically usable light is discussed. The usefulness of in vitro absorption and fluorescence excitation spectra for estimation of the degradation status of chl a and the ratio of chl a to total pigments is also discussed.     

1,4-Dihydroquinazolin-3(2H)-yl benzamide derivatives as anti-inflammatory and analgesic agents with an improved gastric profile: Design,synthesis, COX-1/2 inhibitory activity and molecular docking study

The design and synthesis of a new series of 1,4-dihydroquinazolin-3(2H)-yl benzamide derivatives (4a–o) as anti-inflammatory and analgesic agents and COX-1/2 inhibitors are reported. The target compounds (4a–o) were synthesized using a two-step scheme, and their chemical structures were confirmed with ¹H NMR, ¹³C NMR, and mass spectra and elemental analysis. Compounds 4b, 4d, 4h, 4l, 4n and 4o showed the best in vitro COX-2 inhibitory activity (IC₅₀ 0.04–0.07 μM), which was nearly the same as that of the reference drug celecoxib (IC₅₀ 0.049 μM), but had a lower selectivity index, as dictated in our target design. In the in vivo anti-inflammatory inhibition assay, compounds 4b, 4c, 4e, 4f, 4m and 4o showed better oedema inhibition percentages, ranging from 38.1% to 54.1%, than did diclofenac sodium (37.8%). An in vivo analgesic assay revealed that compounds 4b and 4n had a potential analgesic effect 4- to 21-fold more potent than that of indomethacin and diclofenac sodium. All the tested compounds showed an improved ulcerogenic index when compared to indomethacin. In the synthesized series, compound 4b showed the best biological activity in all the experiments. The docking study results agreed with the in vitro COX inhibition assay results. Moreover, the predicted in silico studies of all the compounds support their potential as drug candidates.     

Novel tetrazole and cyanamide derivatives as inhibitors of cyclooxygenase-2 enzyme: design,synthesis, anti-inflammatory evaluation,ulcerogenic liability and docking study

Nineteen new compounds containing tetrazole and/or cyanamide moiety have been designed and synthesised. Their structures were confirmed using spectroscopic methods and elemental analyses. Anti-inflammatory activity for all the synthesised compounds was evaluated in vivo. The most active compounds 4c, 5a, 5d–f, 8a and b and 9a and b were further investigated for their ulcerogenic liability and analgesic activity. Pyrazoline derivatives 9b and 8b bearing trimethoxyphenyl part and SO₂NH₂ or SO₂Me pharmacophore showed equal or nearly the same ulcerogenic liability (UI: 0.5, 0.75, respectively), to celecoxib (UI: 0.50). Most of tested compounds showed potent central and/or peripheral analgesic activities. Histopathological investigations were done to evaluate test compounds effect on rat's gastric tissue. The obtained results were in consistent with the in vitro data on COX evaluation. Docking study was also done for all the target compounds inside COX-2-active site.     

Hemopoietic progenitor cells of W anemic mice studied in vivo and in vitro

The proliferation and differentiation of hemopoietic cells from genetically anemic W^v/W^x,W/W^v, and W^v/W^v mice, and from nonanemic carrier W/+, W^b/+, and W^v/+ mice have been evaluated in vivo by transplantation techniques and in vitro by the agar gel culture method. Marrow from anemic and carrier mice contained progenitor cells which were decreased in number and formed small, often rudimentary, colonies in the spleens of irradiated recipient mice. Proliferation and differentiation of both erythropoietic and leukopoietic progenitor cells were delayed and reduced, but erythropoiesis was more severely affected than leukopoiesis. The severity of the hemopoietic impairment was gene-dose dependent. The W gene effect on leukopoietic progenitor cells was not secondary to anemia or to abnormal erythropoiesis. The marrow cells of anemic and carrier mice which form colonies of granulocytic and mononuclear cells in vitro were neither decreased in number nor impaired in proliferation and differentiation. Hypertransfusion of red blood cells increased the frequency of in vitro colony-forming cells, but not that of in vivo progenitor cells. The data demonstrate that colony-forming cells which proliferate in the agar gel cultures in vitro are distinct from the in vivo colony-forming cells and suggest that the former are primitive members of the granulocytic cell line. Perhaps in vitro CFU are in an intermediate stage of differentiation between in vivo CFU and myeloblasts, analogous to that which has been suggested for the erythropoietin-sensitive cell in the red cell series. W mutant alleles appear to act, therefore, at or very near the beginning of hemopoietic differentiation.     

The synthesis of 6,9-deepoxy-6,9-N-phenylimino-Δ-prostaglandin I1 a novel inhibitor of leukotriene C and D synthesis

The pyrrole analog of prostacyclin, 6,9-deepoxy-6,9-N-phenylimino-Δ^6,8-prostaglandin I₁ was synthetized from PGF₂α methyl ester. This pyrroloprostacyclin (U-60, 257) and its methyl ester (U-56, 467) have been shown to inhibit leukotriene C/D biosynthesis and antagonized leukotriene C/D contractions in vitro. Antigen induced bronchopulmonary changes in monkeys and guinea pigs are inhibited by U-60, 257 in vivo.     

Interconversions of Chlorophylls a and b Synthesized from Exogenous Chlorophyllides a and b in Etiolated and Post-Etiolated Rye Seedlings

A comparative study of reciprocal conversions of chlorophylls a and b (Chl aand Chl b) in etiolated and post-etiolated rye seedlings (Secale cereale L.) was performed. The production of these pigments was initiated by infiltration of exogenous chlorophyllides a and b (Chlide a and b). It was shown that Chlide b, when infiltrated into etiolated rye seedlings, was esterified, producing Chl b. A major portion of Chl b (more than 80%) was transformed into Chl aduring long-term seedling dark exposure. The high rate of Chl b conversion into Chl a in the pool of pigments of exogenous origin was also observed during the lag-phase when there was no chlorophyll formation from endogenous precursors. The infiltration of Chlide a resulted in Chl a formation. The efficiency of its conversion into Chl b was low (about 1%) in the etiolated seedlings but increased during their greening. In the post-etiolated seedlings infiltrated with Chlide b, which were preliminary illuminated for 6–12 h, the Chl /Chl a ratio was almost similar in the pools of pigments synthesized from both exogenous and endogenous precursors. The rates of direct and reverse reactions responsible for the interconversion of Chl aand Chl b depended on the stage of the formation of the photosynthetic apparatus during greening of etiolated seedlings, when the particular structural components are formed in a definite sequence.