Photooxidation of cytochrome b559 and the electron donors in chloroplast Photosystem II

The effect of light on the reaction center of Photosystem II was studied by differential absorption spectroscopy in spinach chloroplasts.

At − 196 °C, continuous illumination results in a parallel reduction of C-550 and oxidation of cytochrome b₅₅₉ high potential. With flash excitation, C-550 is reduced, but only a small fraction of cytochrome b₅₅₉ is oxidized. The specific effect of flash illumination is suppressed if the chloroplasts are preilluminated by one flash at 0 °C.

At − 50 °C, continuous illumination results in the reduction of C-550 but little oxidation of cytochrome b₅₅₉. However, complete oxidation is obtained if the chloroplasts have been preilluminated by one flash at 0 °C. The effect of preillumination is not observed in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea.

A model is discussed for the reaction center, with two electron donors, cytochrome b₅₅₉ and Z, acting in competition. Their respective efficiency is dependent on temperature and on their states of oxidation. The specific effect of flash excitation is attributed to a two-photon reaction, possibly based on energy-trapping properties of the oxidized trap chlorophyll.     

Effect of background illumination on evoked potentials of the visual system in the turtle

We discovered an enhancing effect of background illumination on amplitude and total duration of electrical reactions of the tectum of the midbrain and pallial thickening in response to a flash and electrical stimulation of the optic nerve. This effect is analogous to the phenomenon of photic potentiation known for the visual system of mammals. Changes of evoked potentials in the contralateral pallial thickening and tectum were fairly stable and survived throughout the course of the entire period of illumination (up to 30 min). The effect was intensified with an increase of illumination intensity. Intensification of response on the part of the tectum and pallial thickening during steady illumination was accompanied by a weakening of slow background electrical activity. During the action of background illumination, the excitability of the tectum rose considerably with direct electrical stimulation. The influence of prolonged illumination on responses of the tectum and pallial thickening was more clearly detected at submaximal strengths of stimulation of the optic nerve or tectum. All of the effects of photic potentiation are confined to centers contralateral to the illuminated eye. This indicates the absence of diffuse effects on excitability of the turtle brain.A. N. Severtsov Institute of Evolutionary Morphology and Ecology of Animals, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 1, No. 2, pp. 219–224, September–October, 1969.     

Mathematical model of two pathways of light perception by the photoreceptor cell

On the basis of the concept postulating polyfunctional role of rhodopsin in the photoreceptor cell excitation a mathematical model of two pathways of excitation was formulated. One pathway involves the appearance of Ca2+ ions in the cell, which block the sodium channels. The second one results in cGMP splitting which also brings about the blocking of sodium channels. A change in the concentration of acting agents and development of cell excitation under illumination was calculated. The calcium branch is shown to be rapid and direct, but of a low sensitivity. The cGMP branch is a slow one but has a high intensification coefficient. The action time of the calcium branch does not depend on illumination, while the time of cGMP branch decreases with the illumination rise and is in an inverse proportion to the square root of the flash intensity. Under repeated illumination the amplitude of calcium response decreases, and its time remains constant. In cGMP branch the time of the response to the second flash decreases. The amplitude of the second response depends on the intensity of the first flash, it can be smaller or larger than the amplitude of the first response.     

Novel form of adaptation in mouse retinal rods speeds recovery of phototransduction

Photoreceptors of the retina adapt to ambient light in a manner that allows them to detect changes in illumination over an enormous range of intensities. We have discovered a novel form of adaptation in mouse rods that persists long after the light has been extinguished and the rod's circulating dark current has returned. Electrophysiological recordings from individual rods showed that the time that a bright flash response remained in saturation was significantly shorter if the rod had been previously exposed to bright light. This persistent adaptation did not decrease the rate of rise of the response and therefore cannot be attributed to a decrease in the gain of transduction. Instead, this adaptation was accompanied by a marked speeding of the recovery of the response, suggesting that the step that rate-limits recovery had been accelerated. Experiments on knockout rods in which the identity of the rate-limiting step is known suggest that this adaptive acceleration results from a speeding of G protein/effector deactivation.     

Quenching of a room temperature fluorescence band at 693 nm during photoactivation of the water-splitting system of Photosystem II in flashed barley leaves

The modifications of the room temperature fluorescence spectrum during the photoactivation of the water-splitting system by continuous illumination were investigated in flashed barley leaves. A blue shift of the chlorophyll fluorescence band was detected during the first 2 min of illumination. During this shift, a decrease of the fluorescence intensity around 693 nm could be demonstrated in difference spectra and in second derivative spectra. This decrease is interpreted as a quenching of PS II fluorescence during the photoactivation. A relative fluorescence increase around 672 nm also occurred during the same period and is thought to reflect rapid light-induced chlorophyll formation. The flashed leaves contained small amounts of photoactive photochlorophyllide which could be removed by a short flash of intense white light given before continuous illumination. The fact that such flash had only weak effect on the 693 nm fluorescence decrease, whereas it strongly reduced the amplitude of the 672 nm fluorescence increase, favours the above interpretations.Abbreviations chl chlorophyll - PS II Photosystem II - PS I Photosystem I     

Modulation of leaf elongation, tiller appearance and tiller senescence in spring barley by far-red light

Supplemental far-red (FR) illumination of light-grown grass seedlings inhibits tiller production while enhancing leaf elongation. Although much is known about FR enhancement of internode elongation in dicots, relatively little research has been conducted to determine the effects of FR on monocot development. In growth chamber experiments, fibre optics were used to direct supplemental FR to elongating leaf blades, main stem bases and mature leaf blades of light-grown barley (Hordeum vulgare L.) seedlings. Our objective was to identify specific sites of perception for FR enhancement of leaf elongation and inhibition of tiller production, and to assess potential FR effects on tiller senescence. Far-red illumination of elongating leaves or of the main stem base reduced the total number of tillers per plant, primarily by reducing secondary and tertiary tiller production, and enhanced leaf elongation. However, leaf elongation was less sensitive to stem base treatments than to illumination of the elongating blade. Increased leaf length resulted from increased leaf elongation rate, while the duration of leaf elongation was unaffected. Exposure of mature leaf blades to FR had no effect on tillering or leaf elongation. None of the FR treatments led to tiller senescence. Localization of FR perception in vertically oriented tissues such as elongating blades and stem bases permits early detection of reflected light from neighbouring plants, allowing rapid response to impending competition.     

Light adaption of the cyclic GMP phosphodiesterase of frog photoreceptor membranes mediated by ATP and calcium ions

The light-activated guanosine 3',5'-cyclic monophosphate (cyclic GMP) phosphodiesterase (PDE) of frog photoreceptor membranes has been assayed by measuring the evolution of protons that accompanies cyclic GMP hydrolysis. The validity of this assay has been confirmed by comparison with an isotope assay used in previous studies (Robinson et al. 1980. J. Gen. Physiol. 76: 631-645). The PDE activity elicited by either flash or continuous dim illumination is reduced if ATP is added to outer segment suspensions. This desensitization is most pronounced at low calcium levels. In 10(-9) M Ca++, with 0.5 mM ATP and 0.5 mM GTP present, PDE activity remains almost constant as dim illumination and rhodopsin bleaching continue. At intermediate Ca++ levels (10-7-10-5M) the activity slowly increases during illumination. Finally, in 10(-4) and PDE activity is more a reflection of the total number of rhodopsin molecules bleached than of the rate of the rhodopsin bleaching. At intermediate or low calcium levels a short-lived inhibitory process is revealed by observing a nonlinear summation of responses of the enzyme to closely spaced flashes of light. Each flash makes PDE activity less responsive to successive flashes, and a steady state is obtained in which activation and inactivation are balanced. It is suggested that calcium and ATP regulation of PDE play a role in the normal light adaption processes of frog photoreceptor membranes.     

Effect of flash photoreactivation on Escherichia coli recA induction by ultraviolet light

Excision-deficient Escherichia coli, carrying the gene for the photolyase on a multicopy plasmid, were irradiated with ultraviolet (UV) light then photoreactivated by illumination delivered from a camera flash unit. Such instantaneous illumination monomerizes only cyclobutane pyrimidine dimers already bound by the photolyase. Whereas the lethal effect of UV light and the number of C-to-T transition-type mutations induced by UV irradiation were both significantly reduced by subsequent irradiation with a single flash of light, single-flash photoreactivation did not reverse the induction of the recA gene by UV light. The results indicate, therefore, that non-photoreactivable DNA lesions play a role in recA induction.     

Specific Features of Phosphate Transformation on Rhodobacter sphaeroides Chromatophores

ATP production has been shown to take place on illumination of Rhodobacter sphaeroides chromatophores by a single light flash, i.e., in the absence of a proton gradient (which would form as a result of electron transport should a second flash occur). ATP synthesis was accompanied by H₂O₂ formation. Simultaneous formation of ATP and H₂O₂ is indicative of oxidative activation of phosphate during ATP synthesis, as in model systems with isolated chlorophyll. These data provide a theoretical background for selecting illumination parameters in laboratory and industrial photobioreactors used for cultivation of photosynthetic bacteria in biotechnological processes.     

On the O2 flash yields of two cyanophytes

We explored O₂ flash yield in two cyanophytes, Anacystis nidulans and Agmenellum quadruplicatum. On a rate-measuring electrode, a single flash gave a contour of O₂ evolution with a peak at about 10 ms which was maximum (100) for 680 nm background light. On 625 nm illumination the peak was smaller (62) but was followed by an increased tail of O₂ attributed to enhancement of the background. After a period of darkness, repetitive flashes (5 Hz) gave a highly damped initial oscillation in individual flash yields which finally reached steady state at 94% of the yield for 680 nm illumination. When O₂ of repetitive flashes was measured as an integrated flash yield the results was distinctive and similar to that for a continuous light 1 (680 nm). An apparent inhibition of respiration which persisted into the following dark period was taken as evidence for the Kok effect. With a concentration-measuring electrode, integrated flash yield vs. flash rate showed the same nonlinear behavior as O₂ rate vs. intensity of light 1. We draw three conclusions about the two cyanophytes. (a) The plastoquinone pool is substantially reduced in darkness. (b) Because of a high ratio of reaction centers, reaction center 1 / reaction center 2, for the two photoreactions, saturating flashes behave as light 1. (c) Because repetitive flashes are light 1, they also give a Kok effect which must be guarded against in measurements designed to count reaction centers.     

Perturbations at the chloride site during the photosynthetic oxygen-evolving cycle

Photosystem II (PSII) catalyzes the oxidation of water to O₂ at the manganese-containing, oxygen-evolving complex (OEC). Photoexcitation of PSII results in the oxidation of the OEC; four sequential oxidation reactions are required for the generation and release of molecular oxygen. Therefore, with flash illumination, the OEC cycles among five S _n states. Chloride depletion inhibits O₂ evolution. However, the binding site of chloride in the OEC is not known, and the role of chloride in oxygen evolution has not as yet been elucidated. We have employed reaction-induced FT-IR spectroscopy and selective flash excitation, which cycles PSII samples through the S state transitions. On the time scale employed, these FT-IR difference spectra reflect long-lived structural changes in the OEC. Bromide substitution supports oxygen evolution and was used to identify vibrational bands arising from structural changes at the chloride-binding site. Contributions to the vibrational spectrum from bromide-sensitive bands were observed on each flash. Sulfate treatment led to an elimination of oxygen evolution activity and of the FT-IR spectra assigned to the S₃ to S₀ (third flash) and S₀ to S₁ transitions (fourth flash). However, sulfate treatment changed, but did not eliminate, the FT-IR spectra obtained with the first and second flashes. Solvent isotope exchange in chloride-exchanged samples suggests flash-dependent structural changes, which alter protein dynamics during the S state cycle. Supported by NSF MCB 03-55421.     

Involvement of ATP in activation and inactivation sequence of phosphodiesterase in frog rod outer segments

Cyclic GMP phosphodiesterase in frog rod outer segments is activated after flash illumination and is inactivated when left in the dark. ATP reduces the initial peak activity caused by dim flashes (with 50 microM ATP being required for a half-maximal effect) and also accelerates inactivation (with 2 microM ATP being required for a half-maximal effect). An acceleration of inactivation caused by ATP addition is 3-7-fold, depending on the preparation, and ATP effect can be observed even 1 min after a dim flash is given. The accelerated inactivation is also flash intensity-dependent. A low intensity of light causes more rapid inactivation than does a high intensity of light. ATP appears to control phosphodiesterase activity in various ways.     

Some manifestations of energy storage in algae and chloroplasts

Summary Dark-adapted plants must be activated by a flash of light before they can evolve oxygen in response to a subsequent flash. The chemiluminescence of chloroplasts, evoked by a change of pH or of oxidation potential, also requires prior illumination. Delayed fluorescence and thermoluminescence are by definition dependent on prior illumination.An attempt is made to explain these well known phenomena on a common basis. The resulting model accommodates many characteristics of the phenomena and suggests some new experiments.Dedicated to Professor C. B. van Niel on the occasion of his 70 th birthday.     

Representing word context effect in Chinese character perception — an application of information theory

The perception of a letter in the context of a word is easier than in the context of a random letter sequence. It appears that our knowledge about words can influence our perception process. McClelland and Rumelhart (1981) propose an interactive activation model to account for the interaction between our knowledge about words and our visual input. They use their model to explain how these interactions facilitate perception. In their account, word context effect is a constant independent of the identity of the words. In this paper, we propose the use of informatin theory to quantify word context effect. In this way, the strength of word context effect will depend on the identity of the words. We apply the method to quantify word context effect in Chinese words. This knowledge is encoded in an artificial neural network using the interactive activation and competition model. The network is used to recognize Chinese characters and we are able to achieve a high recognition rate.     

Direct quantification of the four individual S states in Photosystem II using EPR spectroscopy

EPR spectroscopy is very useful in studies of the oxygen evolving cycle in Photosystem II and EPR signals from the CaMn(4) cluster are known in all S states except S(4). Many signals are insufficiently understood and the S(0), S(1), and S(3) states have not yet been quantifiable through their EPR signals. Recently, split EPR signals, induced by illumination at liquid helium temperatures, have been reported in the S(0), S(1), and S(3) states. These split signals provide new spectral probes to the S state chemistry. We have studied the flash power dependence of the S state turnover in Photosystem II membranes by monitoring the split S(0), split S(1), split S(3) and S(2) state multiline EPR signals. We demonstrate that quantification of the S(1), S(3) and S(0) states, using the split EPR signals, is indeed possible in samples with mixed S state composition. The amplitudes of all three split EPR signals are linearly correlated to the concentration of the respective S state. We also show that the S(1) --> S(2) transition proceeds without misses following a saturating flash at 1 degrees C, whilst substantial misses occur in the S(2) --> S(3) transition following the second flash.     

Light-induced extracellular changes of calcium and sodium concentrations in the Planarian ocellus

Ion-selective double-barreled microelectrodes inserted into a planarian ocellus were used to monitor the ocellus potential and the changes in extracellular concentrations of Ca2+ (Ca(o)) and Na+ (Na(o)) caused by a 0.5-sec light flash or sustained (120s) illumination. Ca(o) and Na(o) were slightly decreased following a flash. Sustained illumination caused a biphasic change in Ca(o) (a rapid decrease followed by a slow increase) and a tonic decrease in Na(o). When Na+ in the planarian saline was replaced by Li+ or choline+, the increase in Ca(o) was prevented: sustained illumination induced only a decrease in Ca(o). These results suggest that illumination induces influxes of both Ca+ and Na+ into planarian photoreceptors, and that the Ca2+ influx is rapidly followed by a Na-dependent Ca2+ efflux due to Na-Ca exchange.     

Double effects of red light on the development of myopia

Color vision is known to play an important role in the discrimination and perception of object characteristics. More recently, there are a few trials found that red light stimulation was related with the development of myopia, though the mechanism is unclear. We assume that red light has two effects on the development of myopia. Individuals who are more sensitive to red light tend to develop myopia under white illumination and are unlikely to be myopic under red light: red light can help to inhibit the progress of myopia. By contrast, individuals who are equal sensitive to red light and green light are susceptible to be myopia under red light and keep stable refraction under white light. Thus whether red light illumination leads to myopia depends on the relative sensitivity of L and M cones. If confirmed, this hypothesis may lead to the formulation of new pathogenesis and new therapeutic approaches to myopia.     

Cyanogen-induced gamma-glutamyl to imidazole cross-link in carbonic anhydrase. A unique mode of inhibition

The release of protons from intact cells of Rhodopseudomonas capsulata after either 4μs flashes or during brief periods of continuous illumination has been measured with the indicator, cresol red. The half-time for H⁺-release after a flash was 35 ms and the extent, 1H⁺ per 134 bacteriochlorophyll. Myxothiazol completely inhibited the flash-induced H⁺-release and antimycin A reduced it by 37%. The proton-releasing reaction is discussed with reference to the protonmotive Q-cycle. During continuous illumination the rapid phase of H⁺ release is followed by a lag and then by another period of acidification, suggesting that other protolytic reactions may be in operation.     

Cyclic GMP levels and membrane current during onset, recovery, and light adaptation of the photoresponse of detached frog photoreceptors

We have used a preparation of isolated, intact rod photoreceptors to correlate the effects of flash illumination on the intracellular cyclic GMP content and the membrane current. We find that the recovery of cyclic GMP levels after brief flash illumination requires approximately twice as much time as the recovery of the membrane current. In contrast, the subsecond kinetics of the cyclic GMP response to light are faster than the kinetics of membrane current suppression. Both cyclic GMP and the membrane current show graded responses to a wide range of flash intensities; however, in a low Ca2+-Ringer's solution, dim flashes can trigger a decrease in cyclic GMP concentration with no corresponding decrease in membrane current. These results suggest that either other factors can regulate the membrane current, or that measurements of total cellular cyclic GMP do not accurately reflect dynamic changes in cyclic GMP concentration in the vicinity of the light-regulated channel. Changes in cyclic GMP concentration in the presence of background illumination exhibit adaptational behavior similar to that observed in a light-adapted photoresponse: acceleration in the response kinetics and a decrease in response amplitude. That this result is observed in rods depleted of internal Ca2+ suggests a Ca2+-independent mechanism by which background illumination can accelerate the cyclic GMP response.