The accelerating action of illumination in recovery of Arbacia eggs from exposure to ultraviolet radiation

Light of wave lengths 0.30µ to 0.50µ, accelerates return of the cleavage rate of Arbacia eggs to normal, after delay by exposure to ultraviolet radiation (wave lengths 0.238µ to 0.31µ). Recovery is apparently complete. Wave lengths 0.30µ to 0.50µ have no effect on the cleavage rate of normal eggs, nor does such illumination previous to dosage with ultraviolet radiation influence subsequent recovery. Acceleration of recovery of the egg occurs before fertilization as well as after. The effects of ultraviolet radiation and recovery therefrom are essentially the same in nucleated "white halves" as in the intact eggs. This phenomenon in the Arbacia egg seems basically comparable to photoreactivation of bacteria and fungi.     

An LED-based UV-B irradiation system for tiny organisms: System description and demonstration experiment to determine the hatchability of eggs from four Tetranychus spider mite species from Okinawa

We developed a computer-based system for controlling the photoperiod and irradiance of UV-B and white light from a 5 × 5 light-emitting diode (LED) matrix (100 × 100 mm). In this system, the LED matrix was installed in each of four irradiation boxes and controlled by pulse-width modulators so that each box can independently emit UV-B and white light at irradiances of up to 1.5 and 4.0 W m⁻², respectively, or a combination of both light types. We used this system to examine the hatchabilities of the eggs of four Tetranychus spider mite species (T. urticae, T. kanzawai, T. piercei and T. okinawanus) collected from Okinawa Island under UV-B irradiation alone or simultaneous irradiation with white light for 12 h d⁻¹ at 25 °C. Although no eggs of any species hatched under the UV-B irradiation, even when the irradiance was as low as 0.02 W m⁻², the hatchabilities increased to >90% under simultaneous irradiation with 4.0 W m⁻² white light. At 0.06 W m⁻² UV-B, T. okinawanus eggs hatched (15% hatchability) under simultaneous irradiation with white light, whereas other species showed hatchabilities <1%. These results suggest that photolyases activated by white light may reduce UV-B–induced DNA damage in spider mite eggs and that the greater UV-B tolerance of T. okinawanus may explain its dominance on plants in seashore environments, which have a higher risk of exposure to reflected UV-B even on the undersurface of leaves. Our system will be useful for further examination of photophysiological responses of tiny organisms because of its ability to precisely control radiation conditions.     

The effect of ultraviolet and white light on growth rate,lysis, and phage production of Bacillus megatherium

Cultures of megatherium 899a, growing under different conditions, were exposed to ultraviolet or white light. 1. Cultures exposed to ultraviolet light and then to white light continue to grow at the normal rate. Cultures exposed to ultraviolet light and then placed in the dark grow at the normal rate for varying lengths of time, depending on conditions, and then lyse with the liberation of from 5 to 1000 phage particles per cell, depending on the culture medium. 2. Increasing the time of exposure to ultraviolet light results in an increase in the fraction of cells which lyse in the dark. The lysis time decreases at first, remains constant over a wide range of exposure, and then increases. The lysis can be prevented by visible light after short exposure, but not after long exposures. 3. The time required for lysis is independent of the cell concentration. 4. Effect of temperature. After exposure to ultraviolet the cell concentration increases about 4 times at 20°, 30°, or 35°C., but only 1.5 to 2.0 times at 40–45°. This is due to the fact that the growth rate of the culture reaches a maximum at 38° while the lysis rate increases steadily up to 45°. 5. Terramycin decreases the growth rate and lysis rate in proportion. 6. At pH 5.1, the cultures continue to grow slowly in the dark after exposure to ultraviolet light. 7. Megatherium sensitive cells infected with T phage lyse more rapidly than ultraviolet-treated 899a, and visible light does not affect the lysis time. The results agree with the assumption that exposure to ultraviolet results in the production of a toxic (mutagenic) substance inside the bacterial cell. This substance is inactivated by white light.     

THE EFFECT OF UNILATERAL ULTRAVIOLET LIGHT ON THE DEVELOPMENT OF THE FUCUS EGG

1. When Fucus eggs which have been fertilized for a sufficient length of time are irradiated unilaterally with monochromatic ultraviolet light (λ2804 Å) of adequate dosage, 97–100 per cent form rhizoids on the halves of the eggs away from the source of radiation (see Figs. 1 and 2). 2. The responsiveness of the eggs increases gradually after fertilization and does not reach a maximum until about 7 hours at 15°C. (see Fig. 3). The first rhizoids begin to form in a population at about 12 hours after fertilization. The responsiveness remains maximal until at least 11 hours after fertilization. 3. It is suggested that the low responsiveness of a population of eggs at an earlier period is due to recovery from the effects of irradiation before the rhizoids begin to form. 4. The response of eggs to λ2804 Å is proportional, over a wide range, to the logarithm of the dosage (see Fig. 1). Dosage was regulated by the duration of exposure during the period of maximum response. 5. High dosages of λ2804 Å, of the order of 10,000 ergs per mm.², cause the rhizoids to form fairly precisely away from the source of radiation (see Fig. 2). Twice this dosage inhibits rhizoid formation altogether without causing cytolysis. 6. Other wave-lengths which have also been shown to be effective are: 3660, 3130, 2654, 2537, 2482, and 2345 Å. Only exploratory measurements have been made to test the effectiveness of these wave-lengths, but they show that much greater energy is necessary to obtain a strong response with λ3130 and 3660 Å, especially the latter. The wave-lengths shorter than 2804 Å, on the other hand, show the same order of effectiveness as λ2804 Å. Some may be more effective. 7. A beam of λ2804 Å which is incident on a single layer of Fucus eggs is completely extinguished at 2, 3, 6, or 6½ hours after fertilization. About 85 per cent of a beam of λ3660 Å is extinguished. The wave-length 3660 Å is thus not so completely absorbed as λ2804 Å, but the difference in proportion absorbed by the egg is not nearly so great as the difference in effectiveness.     

Retardation of Regeneration and Division of Blepharisma by Ultraviolet Radiation and Its Photoreversal

Regeneration of Blepharisma undulans variety japonicus from which the hypostome has been removed is retarded by dosages of 3000 to 4600 ergs/mm.² at wavelength 2654A most strongly when the fragment is exposed soon after cutting. Dosages greater than 4600 ergs/mm.² prevent regeneration. Regeneration is also retarded strongly when the Blepharisma are cut soon after irradiation. Starvation retards regeneration and potentiates the effect of ultraviolet radiations. Division after regeneration of Blepharisma is also retarded by ultraviolet radiations about equally, regardless of when the Blepharisma are cut indicating a more lasting effect of the radiations upon the cells. Blepharisma cut after irradiation usually recover from the effects of the radiations sooner than uncut individuals given the same dosage. Retardation of division by ultraviolet radiation is subject to photoreversal by visible light, especially in a nitrogen atmosphere, provided the ultraviolet dose is not excessive. Visible light alone if prolonged, retards regeneration or may even kill the cut fragments of Blepharisma.     

The ultraviolet action spectrum for stomatal opening in broad bean

The ultraviolet action spectrum for stomatal opening was measured using epidermal peels from leaves of broad bean (Vicia faba). The spectrum was calculated from hyperbolic fluence response curves using 11 wavelengths ranging from 275 to 459 nm. The action spectrum exhibits a major peak at approximately 280 nm and a minor peak at approximately 360 nm. The response at 280 nm is about three times greater than the response at 459 nm. Under the conditions utilized (i.e. the absence of saturating red light), stomatal opening saturated at extremely low fluence rates: <0.2 μmol m⁻² s⁻¹ at 280 nm, and approximately 1.0 μmol m⁻² s⁻¹ at 459 nm. The threshold for blue-light-induced stomatal opening was approximately 0.02 μmol m⁻² s⁻¹. In light-mixing experiments, the addition of 280 nm light to saturating 650 nm (red) light caused additional stomatal opening, which is indicative of separate photoreceptors. In contrast, adding 280 nm of light to saturating 459 nm (blue) light did not increase stomatal opening, suggesting that they both excite the same receptor. The results with white light were similar to those with blue light. We infer that ultraviolet light acts via the blue light photoreceptor rather than through photosynthesis. The additional absorbance peak at 360 nm suggests that the chromophore is either a flavin or a cis-carotenoid, both of which exhibit peaks in this region. It is proposed that the chromophore can be excited either directly by blue light or by energy transferred from the protein portion of the protein-pigment complex after it absorbs 280 nm light.     

Photolability of Photosynthesis in Two Separate Mutants of Scenedesmus obliquus: Preferential Inactivation of Photosystem I

Two separate mutants of the green alga, Scenedesmus obliquus, are described in which photosynthesis is sensitive to moderate intensities of white light (100 mw cm⁻²). Heterotrophic cultures of both mutants lose photosynthetic activity when exposed to white light; the site of at least the initial phase of this inactivation is within photosystem I. Although all whole cell and cell-free reactions typical of photosystem I examined are inhibited by irradiation, the principal component of photosystem I affected is P-700. In light-sensitive-4 the inactivation of P-700 activity is restored during the subsequent dark period. This recovery is prevented by various antibiotics and by anaerobic conditions. In light-sensitive-41 P-700 activity is recovered only after a complete cell division and new growth. Irradiation periods which inhibit photosynthesis in both mutants are without effects upon the activity or presence of ferredoxin, ferredoxin-NADP⁺ oxidoreductase, plastocyanin, cytochrome f(552), cytochrome b-562 or cytochrome b-559.

Prolonged irradiation of cells of light-sensitive-41 causes the disappearance of photosystem II activity, α-tocopherol, and plastoquinone. Some decrease of both the chlorophylls and carotenoids occurs but there is no preferential deletion of any particular carotenoid.

     

Ultraviolet-Stimulated KHCO(3) Efflux from Rose Cells: Regulation of Cytoplasmic pH

Suspension-cultured cells of Rosa damascena that have been irradiated with ultraviolet light (254 nanometers, 2.1 × 10⁴ joules per square meter) rapidly lose K⁺ and HCO₃⁻ ions to the medium. If the HCO₃⁻ is derived from respiratory CO₂ inside the cell, then loss of HCO₃⁻ should be accompanied by an acidification of the cytoplasm. Estimates of the pH of control and ultraviolet-irradiated cells by ³¹P-nuclear magnetic resonance spectroscopy indicated that, following irradiation, the pH of both cytoplasm and vacuole dropped by 0.2 to 0.3 units. This change was not as great as was predicted from the observed HCO₃⁻ loss. Analysis of nitrogenous compounds in the cell suggested that reduction of nitrate and synthesis of γ-aminobutyric acid absorbed some of the protons formed by the synthesis and dissociation of bicarbonate.     

Photobiology of diagravitropic maize roots

Light-induced modification of gravitropism in etiolated roots of Zea mays cv Bear × W38 is a low fluence response mediated by phytochrome. This cultivar has a threshold of 10⁻⁶ mol m⁻² and becomes saturated with 10⁻² mol m⁻² of red light. The maximum light-mediated response of 32 degrees downward from horizontal occurs in roots 10 to 30 millimeters in length, 120 to 165 minutes after irradiation. Reciprocity is valid from 2 to at least 9,000 seconds and the response can be about 90% reversed by far red light. Photoreversibility is lost (`escape' occurs) about 20 minutes after red irradiation but appears to be regained 60 to 80 minutes later. A red light-induced (or synchronized) nutation in the apparent curvature rather than unusual escape characteristics may explain these results.     

ON THE RATE OF OXYGEN CONSUMPTION BY FERTILIZED AND UNFERTILIZED EGGS : IV. CHAETOPTERHS AND ARBACIA PUNCTULATA

1. Unfertilized eggs of Chaetopterus consume about 2.4 mm.³ O₂ per hour per 10 mm.³ eggs at 21°C. 2. In the 1st hour after fertilization, the fertilized eggs consume oxygen at about 53 or 54 per cent of this rate, which is about 1.3 mm.³ O₂ per hour per 10 mm.³ eggs at 21°C. 3. For the first 6 hours after fertilization, at 21°C., the curve of the rate of oxygen consumption is slightly asymmetrically sigmoid. The prefertilization rate is regained between 4½ and 5 hours after fertilization. Soon after 6 hours, ciliary activity begins, and the rate of oxygen consumption rises rapidly. 4. The unfertilized eggs of Arbacia punctulata consume about 0.36–0.5 mm.³ O₂ per hour per 10 mm.³ eggs at 21°C. The absolute determination is difficult as these eggs are highly sensitive to shaking in the manometer vessels, and these difficulties are discussed. 5. The fertilized eggs of Arbacia punctulata consume oxygen at the rate of about 2.0 mm.³ O₂ per hour per 10 mm.³ 21°C. At 1 hour after fertilization the rate is already rising. 6. A comparison of the absolute rates of oxygen consumption, and the changes in rate at fertilization of these and a number of other eggs, together with a theoretical discussion, and a discussion of discrepancies in measurements on the eggs of Arbacia punctulata, is contained in the fifth paper of this series (21).     

UVB radiation affects the mobility of epidermal growth factor receptors in human keratinocytes and fibroblasts

Growth factor receptors transmit biological signals for the stimulation of cell growth in vitro and in vivo and their autocrine stimulation may be involved in tumorigenesis. It is therefore, of great value to understand receptor reactions in response to ultraviolet (UV) light which certain normal human cells are invaribly exposed to during their growth cycle. UV irradiation has recently been shown to deplete antioxidant enzymes in human skin. The aims of the present study were a) to compare the lateral mobility of epidermal growth factor receptors (EGF-R) in cultured human keratinocytes and human foreskin fibroblasts, b) to investigate effects of ultraviolet B radiation on the mobility of EGF-R in these cells, and c) study the response of EGF-R on addition of antioxidant enzymes. The epidermal growth factor receptors were labeled with rhodaminated EGF, the lateral diffusion was determined and the fraction of mobile EGF-R assessed with the fluorescence recovery after photobleaching (FRAP). We found that human keratinocytes display a higher basal level of EGF-R mobility than human skin fibroblasts, viz. with diffusion coefficients (D ± standard error of the mean, SEM) of 4.2±0.2 × 10^–10 cm²/s, and 1.8±0.2 × 10^–10 cm²/s, respectively. UVB-irradiated fibroblasts showed an almost four-fold increase in the diffusion coefficient; D was 6.3±0.3 × 10^–10 cm²/s. The keratinocytes, however, displayed no significant increase in receptor diffusion after irradiation; D was 5.1±0.8 × 10^–10 cm²/s. In both cell types the percentage of EGF-R fluorescence recovery after photobleaching, i.e. the fraction of mobile receptors, was significantly increased after irradiation. In keratinocytes it increased from 69% before irradiation to 78% after irradiation. Analogous figures for fibroblasts were 61% and 73%. The effect of UVB on fibroblast receptors was abolished by prior addition of superoxide dismutase (SOD) and catalase (CAT). It is concluded that UVB radiation of fibroblasts and keratinocytes can affect their biophysical properties of EGF-R. The finding that addition of antioxidant enzymes prevented the UVB effect in fibroblasts may indicate the involvement of reactive oxygen metabolites.Abbreviations CAT Catalase - D Lateral diffusion coefficient - EDTA Ethylenediaminetetraacetic acid - EGF Epidermal growth factor - E-MEM Eagle's minimum essential medium - FCS Fetal calf serum - FRAP Fluorescence recovery after photobleaching - KRG Krebs-Ringer phosphate buffer - PBS Phosphate-buffered saline - R Mobile fraction - ROS Reactive oxygen species - SEM Standard error of the mean - SOD Superoxide dismutase - UVA Ultraviolet light-A (315-400 nm) - UVB Ultraviolet light-B (280-315 nm)     

Exposing broiler eggs to green,red and white light during incubation

Previous work has shown that exposing broiler eggs to white light during incubation can improve hatchability and post-hatch animal welfare. It was hypothesized that due to how different wavelengths of light can affect avian physiology differently, and how pigmented eggshells filter light that different monochromatic wavelengths would have differential effects on hatchability and post-hatch animal welfare indicators. To determine, we incubated chicken eggs (n=6912) under either no light (dark), green light, red light or white light; the light level was 250 lux. White and red light were observed to increase hatch of fertile (P<0.05) over dark and green light incubated eggs. White, red and green light exposure during incubation improved (P<0.05) the proportion of non-defect chicks over dark incubated eggs. Post-hatch 45-day weight and feed conversion was not affected by light exposure of any wavelength (P>0.05). Fear response of during isolation and tonic immobility was reduced (P<0.05) in broilers incubated under white or red light when compared with either green or dark broilers. Broilers incubated with white or red light had lower (P<0.05) composite asymmetry scores and higher (P<0.05) humoral immunity titers than dark incubated broilers, however, green light broilers did not differ (P>0.05) from dark incubated broilers. All light incubated broilers had lower (P<0.05) plasma corticosterone and higher (P<0.05) plasma serotonin concentrations than dark incubated broilers. These results indicate that white light and red light that is a component of it are possibly the key spectrum to improving hatchability and lower fear and stress susceptibility, whereas green light is not as effective. Incubating broiler eggs under these spectrums could be used to improve hatchery efficiency and post-hatch animal welfare at the same time.     

Phytochrome Control of Cell Wall-bound Hydroxyproline Content in Etiolated Pea Epicotyls

The red light inhibition of growth of the intact pea (Pisum sativum L. cv. Alaska) third internode was correlated with an increase in the content of cell wall-bound hydroxyproline. These changes were detected 3 hours after irradiation, and possibly at 1 hour. Far red light reversed the effects of red light. The iron chelator α,α′-dipyridyl reversed the red light effects on both growth and hydroxyproline content. Using segments incubated in vitro, no phytochrome-mediated change in hydroxyproline content could be observed, perhaps because of an overwhelming wounding response. If plants were irradiated in situ and grown for 8 hours before excision and incubation of segments, some enhancement of hydroxylation by red light was detectable both colorimetrically and radioisotopically. The red light inhibition of segment growth was reversed by α,α′-dipyridyl. These results are examined in reference to the role of extensin in normal and induced growth cessation.     

STUDIES ON CELL METABOLISM AND CELL DIVISION : III. OXYGEN CONSUMPTION AND CELL DIVISION OF FERTILIZED SEA URCHIN EGGS IN THE PRESENCE OF RESPIRATORY INHIBITORS

1. The effects of a number of respiratory inhibiting agents on the cell division of fertilized eggs of Arbacia punctulata have been determined. For eggs initially exposed to the reagents at 30 minutes after fertilization at 20°C., the levels of oxygen consumption prevailing in the minimum concentrations of reagents which produced complete cleavage block were (as percentages of the control): In 0.4 per cent O₂-99.6 per cent N₂, 32; in 0.7 per cent O₂-99.3 per cent CO, 32; in 1.6 x 10^–4 M potassium cyanide, 34; in 1 x 10^–3 M phenylurethane, 70; in 4 x 10^–3 M 5-isoamyl-5-ethyl barbituric acid, 20; in 3 x 10^–4 M iodoacetic acid, 53. 2. The carbon monoxide inhibition of oxygen consumption and cell division was reversed by light. The percentage inhibition of oxygen consumption by carbon monoxide in the dark is described by the usual mass action equation with K, the inhibition constant, equal to approximately 60, as compared to values of 5 to 10 for yeast and muscle. In 20 per cent O₂-80 per cent CO in the dark there was a slight stimulation of oxygen consumption, averaging 20 per cent. 3. Spectroscopic examination of fertilized and unfertilized Arbacia eggs reduced by hydrosulfite revealed no cytochrome bands. The thickness and density of the egg suspension was such as to indicate that, if cytochrome is present at all, the amount in Arbacia eggs is extremely small as compared to that in other tissues having a comparable rate of oxygen consumption. 4. Three reagents poisoning copper catalyses, potassium dithio-oxalate (10^–2 M), diphenylthiocarbazone (10^–4 M), and isonitrosoacetophenone (2 x 10^–3 M) produced no inhibition of division of fertilized Arbacia eggs. 5. These results indicate that the respiratory processes required to support division in the Arbacia egg may perhaps differ in certain essential steps from the principal respiratory processes in yeast and muscle.     

UV-Stimulated K Efflux from Rose Cells: Counterion and Inhibitor Studies

Irradiation of a washed suspension of cultured rose (Rosa damascena var. Gloire de Guilan) cells with about 1,680 joules per square meter of short wave ultraviolet (UV) light (254 nanometers) caused K⁺ to appear in the external medium. Short-term tracer (⁸⁶Rb⁺) experiments confirmed the earlier suggestion (Wright, Murphy 1978 Plant Physiol 61: 434-436) that UV increases the efflux of K⁺; there was also a small decrease in influx of K⁺. There was a partial recovery of fluxes from the effects of UV radiation, but no net accumulation of K⁺ within 16 to 18 hours after the irradiation. The K⁺ appearing in the medium was matched by an equivalent amount of HCO₃⁻; it was suggested that HCO₃⁻ was the principal counterion for the K⁺ flux induced by UV. Inhibitors of ATP synthesis (10⁻⁵ molar carbonyl cyanide m-chlorophenyl hydrazone; 0.05 millimolar KCN plus 0.75 millimolar salicylhydroxamic acid) strongly reduced the UV-stimulated K⁺ leakage, suggesting that the leakage was dependent in some way on ATP concentration inside the cells. The UV-induced K⁺ leakage was also dependent on temperature and the presence of Ca²⁺ in the external medium.     

Quasielastic Light-Scattering Study on Changes in Sizes of Native White Membranes after Addition of Retinal

Aqueous suspensions of native white membranes from Halobacterium halobium, strain JW2N, have been studied by quasielastic light scattering. The intensity autocorrelation functions of polarized scattered light from suspensions of white membranes themselves and of white membranes after reconstitution with retinal were measured at various K², K being the magnitude of the scattering vector. The first cumulant or the average decay rate of the correlation function was obtained by a cumulant expansion method. The first cumulant for the white membranes increased after retinal was added to the suspension. The first cumulants obtained before and after the addition of retinal were almost independent of pH in the range 7 to 11, and of temperature in the range 15° to 40°C after T/η scaling, η being the solvent viscosity. This suggests that photocycling in reconstituted membranes, induced by the probe laser-beam, did not cause any detectable change in spectra, and that the membrane flexibility, if present, was independent of the above conditions, so that the spectral changes after the addition of retinal could be attributed mostly to the changes in the sizes of the membranes. A theoretical formulation for the first cumulant for a rigid disk-like scatterer (Fujime, S. and K. Kubota, 1985, Biophys. Chem., 23:1-13.) was applied to the analysis of the spectra. The results suggest that the radii of the membrane patches decreased by several percent after the addition of retinal.     

Isotope Label-Aided Mass Spectrometry Reveals the Influence of Environmental Factors on Metabolism in Single Eggs of Fruit Fly

In order to investigate the influence of light/dark cycle on the biosynthesis of metabolites during oogenesis, here we demonstrate a simple experimental protocol which combines in-vivo isotopic labeling of primary metabolites with mass spectrometric analysis of single eggs of fruit fly (Drosophila melanogaster). First, fruit flies were adapted to light/dark cycle using artificial white light. Second, female flies were incubated with an isotopically labeled sugar (¹³C₆-glucose) for 12 h – either during the circadian day or the circadian night, at light or at dark. Third, eggs were obtained from the incubated female flies, and analyzed individually by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS): this yielded information about the extent of labeling with carbon-13. Since the incorporation of carbon-13 to uridine diphosphate glucose (UDP-glucose) in fruit fly eggs is very fast, the labeling of this metabolite was used as an indicator of the biosynthesis of metabolites flies/eggs during 12-h periods, which correspond to circadian day or circadian night. The results reveal that once the flies adapted to the 12-h-light/12-h-dark cycle, the incorporation of carbon-13 to UDP-glucose present in fruit fly eggs was not markedly altered by an acute perturbation to this cycle. This effect may be due to a relationship between biosynthesis of primary metabolites in developing eggs and an alteration to the intake of the labeled substrate – possibly related to the change of the feeding habit. Overall, the study shows the possibility of using MALDI-MS in conjunction with isotopic labeling of small metazoans to unravel the influence of environmental cues on primary metabolism.     

Properties of Mitomycin C-sensitive Mutants of Escherichia coli K-12

Strains hypersensitive to mitomycin C (MC) were isolated from Escherichia coli K-12 after treatment with nitrosoguanidine. Of 43 MC-sensitive strains tested for their ultraviolet light (UV) sensitivity and for their ability to reactivate UV-inactivated λ phage, 38 were found to be insensitive to UV irradiation and to be able to reactivate UV-irradiated bacteriophage λ. Some properties of the MC-sensitive, uvr⁺ mutants were analyzed. Synthesis of deoxyribonucleic acid (DNA) in MC-sensitive, uvr⁺ mutants was inhibited at a lower concentration of MC than in the wild-type strain. Mutant cells, labeled with ³H-thymidine and then exposed to MC, released radioactivity as low molecular weight compounds. The amount of radioactivity released was the same as that from the wild-type strain. MC-sensitive, uvr⁺ mutants, as well as the corresponding wild-type strain, were equally susceptible to induction of prophage 80 by UV irradiation. However, MC induction of prophage was achieved in MC-sensitive, uvr⁺ mutants at a lower concentration of the antibiotic than in the wild-type strain. Genetic experiments indicated that a gene controlling MC sensitivity is located close to that determining lactose fermentation of E. coli. It is situated on episome F′13, and the wild type is dominant to the MC-sensitive allele.     

Effect of light period on egg-discharge of gametophyte clones of <Emphasis Type="BoldItalic">Undaria pinnatifida</Emphasis> (Phaeophyta)

A sporeling culture method using gametophyte clones developed in the early 1990s led to egg discharge occurring in the dark 5 min after the start of the dark period under growth under a 11:13 L-D photoperiod. The course of egg discharge could be disturbed by light, with irradiance as low as 5–6 μmol photon m⁻² s⁻¹ causing 75–80% of the discharged eggs to detach from the oogonia and consequently to die within several hours. In order to enhance outgrowth rate of young sporophytes, a study was conducted to test the effect of controlling darkness in the period 2–3 h after dusk. When the slides were transferred from the standard 11:13 L-D regime to continuous light, eggs were discharged 5 min after the end of the light phase and peaked 5–l5 min later on first day after transfer, indicating that the female gametes “remember” the light-dark regime. This suggests the existence of an endogenous circadian rhythm. During the second and third days, very few eggs were discharged throughout the 11 h of the normal light phase of the L-D regime, indicating the inhibitory effect of continuous light and that the rhtyhm is easily damped by light.     

NHP2 downregulation counteracts hTR‐mediated activation of the DNA damage response at ALT telomeres

About 10% of cancer cells employ the “alternative lengthening of telomeres” (ALT) pathway instead of re‐activating the hTERT subunit of human telomerase. The hTR RNA subunit is also abnormally silenced in some ALT⁺ cells not expressing hTERT, suggesting a possible negative non‐canonical impact of hTR on ALT. Indeed, we show that ectopically expressed hTR reduces phosphorylation of ssDNA‐binding protein RPA (p‐RPA^S33) at ALT telomeres by promoting the hnRNPA1‐ and DNA‐PK‐dependent depletion of RPA. The resulting defective ATR checkpoint signaling at telomeres impairs recruitment of the homologous recombination protein, RAD51. This induces ALT telomere fragility, increases POLD3‐dependent C‐circle production, and promotes the recruitment of the DNA damage marker 53BP1. In ALT⁺ cells that naturally retain hTR expression, NHP2 H/ACA ribonucleoprotein levels are downregulated, likely in order to restrain DNA damage response (DDR) activation at telomeres through reduced 53BP1 recruitment. This unexpected role of NHP2 is independent from hTR’s non‐canonical function in modulating telomeric p‐RPA^S33. Collectively, our study shines new light on the interference between telomerase‐ and ALT‐dependent pathways and unravels a crucial role for hTR and NHP2 in DDR regulation at ALT telomeres.