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Fluorometry is an effective research tool in biology and medicine; it is widely used in the study of the photosynthetic pigment apparatus in vivo. This method can be applied to the key plant photoreceptor phytochrome (phy). The fluorescence of phytochrome in plants was recorded for the first time in the group of the author, and a spectrofluorometric technique for its in vivo study was developed. The photophysical and photochemical properties of the pigment were described, and the photoreceptor was shown to be present in plants as two phenomenological types-active (at cryogenic temperatures) and water-soluble (Pr') and inactive and amphiphilic (Pr″). The scheme of the photoreaction explaining their photochemical distinctions was proposed. Phytochrome A was shown to comprise both types (phyA' and phyA″), whereas phytochrome B was only the second type. For phyA', distinct conformers have been detected. phyA' and phyA″ differ by the N-terminus of the molecule, possibly by serine phosphorylation. They mediate, respectively, the very low fluence and high irradiance photoresponses. Light, internal factors (kinase/phosphatase balance, pH), and hormones (jasmonate) were shown to affect the content and functions of the two phyA pools. All this points to the effectiveness of the developed method for invivo investigations of the phytochrome system. The data obtained can be applied in practical terms in agrobiology and light culture, as well as in the use of phytochrome as a new nanotool and a fluorescent probe., Competing Interests: Declaration of competing interest The author declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Masson SAS. All rights reserved.)

Phytochrome (phy) system in plants comprising a small number of phytochromes with phyA and phyB as major ones is responsible for acquiring light information in the red-far-red region of the solar spectrum. It provides optimal strategy for plant development under changing light conditions throughout all its life cycle beginning from seed germination and seedling establishment to fruiting and plant senescence. The phyA was shown to participate in the regulation of this cycle which is especially evident at its early stages. It mediates three modes of reactions-the very low and low fluence responses (VLFR and LFR) and the high irradiance responses (HIR). The phyA is the sole light receptor in the far-red spectral region responsible for plant's survival under a dense plant canopy where light is enriched with the far-red component. Its appearance is believed to be one of the main factors of plants' successful evolution. So far, it is widely accepted that one molecular phyA species is responsible for its complex functional manifestations. In this review, the evidence of the existence of two distinct phyA types-major, light-labile and soluble phyA' and minor, relatively light-stable and amphiphilic phyA″-is presented as what may account for the diverse modes of phyA action.

The evolution of oxygenic photosynthesis, respiration and photoperception are connected with the appearance of cyanobacteria. The key compounds, which are involved in these processes, are tetrapyrroles: open chain - bilins and cyclic - chlorophylls and heme. The latter are characterized by their covalent bond with the apoprotein resulting in the formation of biliproteins. This type of photoreceptors is unique in that it can perform important and opposite functions-light-harvesting in photosynthesis with the participation of phycobiliproteins and photoperception mediated by phycochromes and phytochromes. In this review, cyanobacterial phycobiliproteins and phytochrome Cph1 are considered from a comparative point of view. Structural features of these pigments, which provide their contrasting photophysical and photochemical characteristics, are analyzed. The determining factor in the case of energy migration with the participation of phycobiliproteins is blocking the torsional relaxations of the chromophore, its D-ring, in the excited state and their freedom, in the case of phytochrome photoisomerization. From the energetics point of view, this distinction is preconditioned by the height of the activation barrier for the photoreaction and relaxation in the excited state, which depends on the degree of the chromophore fixation by its protein surroundings., (© 2019 American Society for Photobiology.)

The photosynthetic apparatus accomplishes two major functions in plants - solar energy conversion and protection of the plant from photodestruction. Its highly orchestrated formation includes coordinated biosynthesis of chlorophyll (Chl) and of its binding to matrix proteins. Light plays here the central role driving both metabolic and regulatory processes. The regulation is achieved via operation of sophisticated photoreceptor machinery with the phytochrome system as its main component. This review concentrates on Chl a biosynthesis and the role of phytochrome A (phyA) in this process. The mechanism of action of phyA and the specificity of its state in the plant has been described, in particular, the existence of two native types with different modes of action. This review touches upon the dependence of the effects of phyA on tissues and organs of the plant and its species, genetic modifications, and hormonal status.

Phytochrome A (phyA) mediates different photoresponses what may be connected with the existence of its two types, phyA' and phyA'', differing by spectroscopic, photochemical and functional properties. We investigated a role of phyA phosphorylation in their formation turning to transgenic Arabidopsis thaliana (L. Heynh.) phyA or phyAphyB mutants overexpressing rice wild-type phyA (phyA WT) or mutant phyA (phyA SA) with the first 10 serines substituted by alanines. This prevents phyA phosphorylation at these sites and modifies photoresponses. Etiolated seedlings were employed and phyA parameters were evaluated with the use of low temperature fluorescence spectroscopy and photochemistry. Germination of seeds was induced by white light (WL) pre-treatment for 15min or 3h. Emission spectra of rice phyA WT and phyA SA were similar and their total content was comparable. However, the phyA'/phyA'' proportion in phyA WT was high and varied with the duration of the WL pre-treatment, whereas in phyA SA it was substantially shifted towards phyA'' and did not depend on the pre-illumination. This suggests that phyA SA comprises primarily or exclusively the phyA'' pool and supports the notion that the two phyA types differ by the state of serine phosphorylation. phyA'' was also found to be much more effective in the germination induction than phyA'.

Cyanobacterial phytochromes are a diverse family of light receptors controlling various biological functions including phototaxis. In addition to canonical bona fide phytochromes of the well characterized Cph1/plant-like clade, cyanobacteria also harbor phytochromes that absorb green, violet or blue light. The Synechocystis PCC 6803 Cph2 photoreceptor, a phototaxis inhibitor, is unconventional in bearing two distinct chromophore-binding GAF domains. Whereas the C-terminal GAF domain is most likely involved in blue-light perception, the first two domains correspond to a Cph1-like photosensory module lacking the PAS domain. Biochemical and spectroscopic studies show that this region switches between red (P(r) ) and far-red (P(fr) ) absorbing states. Unlike Cph1, the P(fr) state of Cph2 decays rapidly in darkness. Mutations close to the PCB chromophore further destabilize the P(fr) state without drastically affecting the spectroscopic features such as the quantum efficiency of P(r) →P(fr) conversion, fluorescence, or the Resonance-Raman signature of the chromophore. Overall, the PAS-less photosensory module of Cph2 resembles Cph1 including its mode of isomerisation, but the P(fr) state is unstable., (© 2010 The Authors. Photochemistry and Photobiology © 2010 The American Society of Photobiology.)

Phytochrome A (phyA) is a versatile plant photoreceptor that mediates responses to brief light exposures (very low fluence responses, VLFR) as well as to prolonged irradiation (high irradiance responses, HIR). We identified the phyA-303 mutant allele of Arabidopsis thaliana bearing an R384K substitution in the GAF subdomain of the N-terminal half of phyA. phyA-303 showed reduced phyA spectral activity, almost normal VLFR, and severely impaired HIR. Recombinant N-terminal half oat of PHYA bearing the phyA-303 mutation showed poor incorporation of chromophore in vitro, despite the predicted relatively long distance (>13 A) between the mutation and the closest ring of the chromophore. Fusion proteins bearing the N-terminal domain of oat phyA, beta-glucuronidase, green fluorescent protein, and a nuclear localization signal showed physiological activity in darkness and mediated VLFR but not HIR. At equal protein levels, the phyA-303 mutation caused slightly less activity than the fusions containing the wild-type sequence. Taken together, these studies highlight the role of the N-terminal domain of phyA in signaling and of distant residues of the GAF subdomain in the regulation of phytochrome bilin-lyase activity.

Effect of dehydration of plant tissues on the two native phenomenological phytochrome A (phyA) pools - major, variable and soluble phyA' and minor, relatively conserved and presumably membrane(protein)-associated phyA'' - was investigated on etiolated seedlings of barley and maize. With the use of in situ low-temperature fluorescence spectroscopy and photochemistry, it was found that even a considerable loss of water (up to 75-85% of the initial fresh weight) by coleoptiles does not bring about noticeable alterations of the spectroscopic and photochemical parameters of phytochrome pointing to a relative stability of the phyA'/phyA'' system in this regard. However, extreme dehydration (loss of weight 90%) of plant tissues including freeze-drying caused dramatic changes of the phytochrome properties - blue shift of the emission maximum and its widening and reduction in the extent of the Pr photoconversion into lumi-R at 85 K and into Pfr at 273 K. Rehydration of the dried tissues did not reverse the spectroscopic changes and did not recover the Pr-->lumi-R phototransformation at 85 K but restored the ability of Pr to photoconvert into Pfr at ambient temperatures. At the same time, the total phytochrome content was not affected by these treatments. These effects were interpreted as an irreversible transformation of phyA' into phyA'' upon extreme loss of water by plant tissues suggesting that water may play a role in stabilizing the conformation of the major and soluble phyA' species. The data also imply that phyA in dry and imbibing seeds is likely represented primarily by its phyA'' isoform.

Phytochrome (phy) A in its two native isoforms (phyA' and phyA") and the active (Pchlide(655)) and inactive (Pchlide(633)) protochlorophyllides were investigated by low-temperature fluorescence spectroscopy in the tips of rice (Oryza sativa L. Japonica cv Nihonmasari) coleoptiles from wild type (WT) and the jasmonate-deficient mutant hebiba. The seedlings were either grown in the dark or under pulsed (FRp) or continuous (FRc) far-red light (lambda(a) >/= 720 nm) of equal fluences. In the dark, the mutant had a long mesocotyl and a short coleoptile, whereas the situation was reversed under FR: short mesocotyl and long coleoptile, suggesting that the effect is mediated by phyA. Under these conditions the WT displayed a short coleoptile and emergence of the first leaf. In the dark, the spectroscopic and photochemical properties of phyA, its content and the proportion of its two pools, phyA' and phyA", were virtually identical between WT and hebiba. However, the total content of protochlorophyllides was higher in the mutant. Upon illumination with FRc, [phyA] declined in the WT and the ratio between phyA' and phyA" shifted towards phyA". In hebiba, the light-induced decline of [phyA] was less pronounced and the ratio between phyA' and phyA" did not shift. Moreover, in the WT, FRp stimulated the biosynthesis of Pchlide(655), whereas FRc was inhibiting. In contrast, in the mutant, both FRp and FRc stimulated the synthesis of Pchlide(655). This means that FRc caused the opposite effect in hebiba. This difference correlates with a slower photodestruction of primarily the light-labile phyA' pool in hebiba.

Different modes of the phytochrome function are connected with its polymorphism, the major isoforms being phytochromes A and B (phyA and phyB). In its turn, phyA comprises two native species, phyA' and phyA'', whose precise nature and functions remain obscure. With the use of in situ fluorescence spectroscopy, we investigated their properties in a mutant of pea, phyA-3D, characterized by exaggerated photoresponses and impaired photodestruction of phyA. The mutation is a substitution of alanine by valine at the position 194 in phyA. The phyA-3DphyB and phyB mutants were also investigated. In dark-grown plants, all the lines had the content and properties of the two phyA species very similar to the wild type. However, a considerably more intense reduction in [phyA] without changes in the phyA'/phyA'' equilibrium was found in far-red grown mutant plants suggesting a hypersensitivity of phyA-3D with regard to its autoregulation. On the contrary, under red illumination, a higher stability of phyA-3D was observed confirming our earlier findings. This allows a conclusion that the A194V substitution in phyA-3D not only impairs its destruction but also enhances its signaling ability, suggesting a role of this locus in modulation of its activity.

Phytochrome A (phyA) is an important photoreceptor controlling many processes throughout the plant life cycle. It is unique within the phytochrome family for its ability to mediate photomorphogenic responses to continuous far-red light and for the strong photocontrol of its transcript level and protein stability. Here we describe a dominant mutant of garden pea (Pisum sativum) that displays dramatically enhanced responses to light, early photoperiod-independent flowering, and impaired photodestruction of phyA. The mutant carries a single base substitution in the PHYA gene that is genetically inseparable from the mutant phenotype. This substitution is predicted to direct the replacement of a conserved Ala in an N-terminal region of PHYA that is highly divergent between phyA and other phytochromes. This result identifies a region of the phyA photoreceptor molecule that may play an important role in its fate after photoconversion.

Phytochrome (phy), a 124 kDa biliprotein, mediates plants' perception of environmental light conditions including quantity, quality and duration of light. The complex phenomenology of phy function is connected with its polymorphism, the major phys being phyA and phyB. PhyA mediates irreversible photoresponses in the very low and high fluence ranges (VLFR and HIR) primarily in the far-red (FR) spectral region, whereas phyB mediates the 'classical' R/FR reversible responses in the low fluence range (LFR). This phyA specificity is determined at the level of (i) intramolecular events, (ii) turnover, phyA being light-labile, and (iii) nuclear-cytoplasmic partitioning and interaction with partner proteins. A unique feature of phyA is that two native isoforms, phyA' and phyA'', comprise it, distinguished by spectroscopic and photochemical properties, localization and abundance in plant tissues, light stability, and other properties. They differ by the post-translational modification at the 6 kDa N-terminus, possibly phosphorylation, phyA' being phosphorylated and phyA'' dephosphorylated. Both species participate in the light-induced nuclear-cytoplasmic partitioning. The light-labile phyA' is responsible for de-etiolation (VLFR and HIR modes), whereas the relatively more light-stable phyA'' could be active throughout the whole life cycle. PhyA'' interferes with the action of phyA' and this interaction may be part of the fine tuning mechanism of the phyA function. Finally, within the phyA' pool there are different conformers in thermal equilibrium, that differ by the activation and kinetic parameters of the Pr-->lumi-R photoreaction. This heterogeneity of phyA may account, at least partially, for the complex dynamics of its photoprocesses and the phenomenology of photoresponses.

Phytochrome A signaling shows two photobiologically discrete outputs: so-called very-low-fluence responses (VLFR) and high-irradiance responses (HIR). By modifying previous screening protocols, we isolated two Arabidopsis mutants retaining VLFR and lacking HIR. Phytochrome A negatively or positively regulates phytochrome B signaling, depending on light conditions. These mutants retained the negative but lacked the positive regulation. Both mutants carry the novel phyA-302 allele, in which Glu-777 (a residue conserved in angiosperm phytochromes) changed to Lys in the PAS2 motif of the C-terminal domain. The phyA-302 mutants showed a 50% reduction in phytochrome A levels in darkness, but this difference was compensated for by greater stability under continuous far-red light. phyA-302:green fluorescent protein fusion proteins showed normal translocation from the cytosol to the nucleus under continuous far-red light but failed to produce nuclear spots, suggesting that nuclear speckles could be involved in HIR signaling and phytochrome A degradation. We propose that the PAS2 domain of phytochrome A is necessary to initiate signaling in HIR but not in VLFR, likely via interaction with a specific partner.

Phytochrome (P) was characterized in etiolated seedlings of wild-type, mutant and transgenic strains of Arabidopsis with the use of low-temperature (85 K) fluorescence spectroscopy and photochemistry. The position (lambda max) of the Pr emission spectrum, its intensity (F0) proportional to [P tot] and the extent of the Pr-->lumi-R phototransformation at 85 K (gamma 1) were shown to vary depending on the plant strains and tissues used, while the extent of the Pr-->Pfr transformation at 273 K (gamma 2) remained relatively constant. Depletion of phyA (fre1-1 in Nagatani et al., Plant Physiol. 102 (1993) 269-277, and fhy2-2 in Whitelam et al., Plant Cell 5 (1993) 757-768) resulted in a steep decrease of F0 to approximately equal to 10%. The phyB mutant (hy3-B064 in Reed et al., Plant Cell 5 (1993) 147-157) revealed a slight reduction (by approximately equal to 20%) of F0 while lambda max and gamma 1 remained practically unaffected. In phyAphyB mutuant no P emmission was observed. Overexpression of oat phyA (13k7 and 21k15 in Boylan and Quail, Proc. Natl. Acad. Sci. USA 88 (1991) 10806-10810) brought about an increase of F0 by two or three times, a shift of lambda max to 685 nm and an increase of gamma 1 to 0.3-0.4. On the contrary, an increase of F0 (up to 40%) in Arabidopsis and rice phyB overexpressors (ABO and RBO in Wagner et al., Plant Cell 3 (1991) 1275-1288) was followed by a decrease of gamma 1 values to 0.13-0.14. These data together with the results on phyB (lh) mutant of cucumber prove the existence of the two phyA populations with high (phyA') and low (phyA") photochemical activity at low temperatures. PhyB emits maximally in the same region as phyA in Arabidopsis (approximately equal to 683 nm) and at shorter wavelength (< 680 nm) in rice. It is characterized by low photochemical activity at 85 K (gamma 1 < or = 0.05) and can be attributed in this respect to the same pigment type as phyA".

Phytochrome is a key photoregulation pigment in plants which determines the strategy of their development throughout their life cycle. The major achievement in the recent investigations of the pigment is the discovery of its structural and functional heterogeneity: existence of a family of phytochromes (phyA-phyE) differing by the apoprotein was demonstrated. We approach this problem by investigating the chromophore component of the pigment with the use of the developed method of in vivo low-temperature fluorescence spectroscopy of phytochrome. In etiolated plants, phytochrome fluorescence was detected and attributed to its red-light absorbing form (Pr) and the first photoproduct (lumi-R), and a scheme of the photoreaction in phytochrome, a distinction of which is the activation barrier in the excited state, was put forward. It was found that the spectroscopic and photochemical characteristics of Pr depend on the plant species and phytochrome mutants and overexpressors used, on localization of the pigment in organs and tissues, plant age, effect of preillumination and other physiological factors. This variability of the parameters was interpreted as the existence of at least two phenomenological Pr populations, which differ by their spectroscopic characteristics and activation parameters of the Pr --> lumi-R photoreaction (in particular, by the extent of the Pr --> lumi-R photoconversion at low temperatures, gamma1): the longer-wavelength major and variable by its content in plant tissues Pr' with gamma1 = 0.5 and the shorter-wavelength minor relatively constant Pr" with gamma1 < or = 0.05. The analysis of the phytochrome mutants and overexpressors allows a conclusion that phytochrome A (phyA), which dominates in etiolated seedlings, is presented by two isoforms attributed to Pr' and Pr" (phyA' and phyA", respectively). Phytochrome B (phyB) accounts for less than 10% of the total phytochrome fluorescence and belongs to the Pr" type. It is also characterized by the relatively low extent of the Pr photoconversion into the far-red-light absorbing physiologically active phytochrome form, Pfr. Fluorescence of the minor phytochromes (phyC-phyE) is negligible. The recently discovered phytochrome of the cyanobacterium Synechocystis also belongs to the phenomenological Pr" type. PhyA' is a light-labile and soluble fraction, while phyA" is a relatively light-stable and, possibly, membrane (protein)-associated. Experiments with transgenic tobacco plants overexpressing full-length and C- and N-terminally truncated oat phytochrome A suggest that phyA' and phyA" might differ by the post-translational modification of the small N-terminal segment (amino acid residues 7-69) of the pigment. PhyA' is likely to be active in the de-etiolation processes while phyA" together with phyB, in green plants as revealed by the experiments on transgenic potato plants and phytochrome mutants of Arabidopsis and pea with altered levels of phytochromes A and B and modified phenotypes. And finally, within phyA', there are three subpopulations which are, possibly, different conformers of the chromophore. Thus, there is a hierarchical system of phytochromes which include: (i) different phytochromes; (ii) their post-translationally modified states and (iii) conformers within one molecular type. Its existence might be the rationale for the multiplicity of the photoregulation reactions in plants mediated by phytochrome.

Fluorescence emission and polarization spectra of the phycobilisomes (PBS) of the blue-green alga Nostoc muscorum were measured at 20, -73 and -196 degrees C while exciting at the absorption maximum of each pigment in the PBS. The emission spectra were deconvoluted into a number of Gaussian components and energy migration coefficients and quantum yields of fluorescence for the 8 forms of the phycobilins constituting the PBS were calculated. The overlap integrals and the critical and real distances for the energy transfer in the donor-acceptor pairs were evaluated. The general scheme of the energy transfer in the PBS is proposed according to which there is a homogeneous energy migration within each pigment form and a following effective heterogeneous migration directed from the short wavelength forms via the intermediate ones to the terminal long wavelength acceptors. The transfer passes one or more steps of the energy "staircase" which is formed by the excited levels of the forms. The backward "uphill" energy transfer does not take place. These data and the estimates of the real distances of the energy transfer allowed us to make a conclusion on the regular arrangement of the pigments in the PBS, to determine the distances between the chromophores and their localization in a pigment molecule and the distances between the chromophores of different pigments and thus to specify the structure of the PBS.

Red luminescence of purple membranes from Halobacterium halobium cells was found out, and its emission, excitation and polarization spectra were investigated. Simultaneous parallel measurements of absorption and luminescence changes in one sample brought about by the action of light were also carried out. The bands in the spectra can be attributed to a number of bacteriorhodpsin (BR) forms: BR(595,520), BR(650,575),BR(600-620), BR(700,625), BR(730,660) BR(780,695), where the number above is the position of the luminescence maxima, below--that of absorption. Proceding from the quantum yield of the luminescence (10(-3)) and of photoreaction (10(-1)) of BR, the photoisomerization rate constant of the latter was estimated (10(11) sec(-1). The temperature dependence of the luminescence quantum yield points to the existence of two or three quenching processes with different activation energies. BR phosphorescence was not observed in the region 500-1100 nm. High degree (36%) os luminescence polarization shows that there is no homogeneous energy transfer between BR molecules, or there is regularity in orientation of their dipoles. Energy migration from the bulk of carotenoids to BR was not found. However limited heterogeneous transfer between the different BR forms cannot be ruled out. The absence (or limitation) of migration indicated that there is a spatial separation of the chromophores. Data on possible participation of triplet states in the BR photoconversions are discussed.

The second derivative of absorption, fluorescence and fluorescence excitation spectra of chlorophyll a in concentrated solutions and films was investigated. More than 14 forms of pigment aggregates, which can be divided into two types--with narrow 8-10nm) and wide (25-40nm) low temperature (-196 degrees C) spectra bands, were found. For the most part of the aggregated forms, the position and half width of the bands, as well as the Stokes shift and relative quantum yield were determined. The comparison of the spectral characteristics points to the indentity of the aggregates and corresponding native forms of Chl. a. It is shown that the universal relationship between absorption and fluorescence bands in applicable to the aggregates of the two types and the energy of resonance interaction between monomers in the aggregates is evaluated.

By means of high sensitive spectrofluorometer the fluorescence spectra have been measured of normal chloroplasts and those with blocked photosystem 2 activity due to photoinhibition or treatment with 0.6 M tris-buffer. At room temperature fluorescence spectra of inactivated chloroplasts are similar to the spectrum of normal chloroplasts measured at low light intensity. Under excitation by intense light a decrease of intensity at 685 nm is appeared (about 3-4 times) in the fluorescence spectra of inactivated chloroplasts as compared to the spectrum of normal chloroplasts. The sharp intensity decrease of maxima at 685 and 695 nm (3-4 times) and small decrease at 680 and 730 nm (by 30-50%) are observed in low temperature fluorescence spectra of inactivated chloroplasts. Thus, the damage of photosystem 2 reaction centres is not accompanied by the preferential decrease of the only fluorescence band. The similarity of fluorescence difference spectra of chloroplasts distinguished by the state of photosystem 2 reaction centre, and the complex structure of difference spectra indicate that the variable fluorescence of chloroplasts during the induction is due to the emission of bulk chlorophyll alpha of the photosystem 2.

A new area of the investigation of visual and bacterial rhodopsins--fluorescence spectroscopy of the pigments is discussed. Fluorescence properties are considered in relation to photochemical transformations of the pigments at low temperatures. A number of fluorescent states of the pigments are described. It is shown that the excited states of bacteriorhodopsin and visual rhodopsin are characterized by a series of common features. The analysis of general properties of the pigments excited states allows a conclusion that the singlet excited states take part in the photoreaction. The photoreaction scheme is discussed in which structural changes of the chromophore take place already in the excited state.

Fluorescence characteristics of all-trans-bacteriorhodopsin (BR570), 13-cis-BR (BR560), and the intermediate of the BR cycle (P585) are obtained by measuring variable fluorescence in the course of their phototransformations. The fluorescence spectra (lambda greater than 580 nm) have a maximum at 720, 730 and 740 nm, their respective excitation spectra with a maximum of 578, 565 and 585 nm coincide with the absorption spectra of the three states. The emission quantum yield for P585 is approximately 10(-4) at -62 degrees C and 4 +/- 2 10(-4) for BR560 at -196 degrees C. The effect of sharp increase of BR570 fluorescence quantum yield (from less than or equal to 10(-4) to approximately 10(-3)) under action of light at -196 degrees C (photoactivation or induction of emission) is found which points to the existence of photoprocess parallel to the photoconversion of BR570 into P600 (formula: see text). The emission of BR560 is not photoactivated. The photoactivated state BR570 is not identical with pseudo-BR of Gillbro et al., (1977) which has a maximum in the excitation spectrum at 597 nm. The greater part of the red fluorescence of the membrane belongs to the minor nonphotoactive forms of BR (B578, B630) with relatively high fluorescence yield (greater than or equal to 10(-2) at -196 degrees C). The fluorescence characteristics of BR are discussed in the framework of a scheme in which the existence of an intermediate excited level common for the emission and photoconversion and a ground level common for the initial state and photoproduct is suggested.