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PubMed1978

Inhibition of postreplication repair and the enhancement of induction of SV40 virus from transformed hamster kidney cells.

The induction by ultraviolet light of simian virus 40 (SV40) from two SV40--transformed hamster kidney cell lines is enhanced by caffeine. In order to investigate the mechanism responsible for this enhancement, the effect of caffeine on postreplication repair of DNA damaged by UV light was studied utilizing alkaline sucrose-gradient sedimentation. Caffeine at concentrations of 0.5, 1.0 or 2.0 mM inhibited the filling of gaps during postreplication repair. In addition, caffeine was found to potentiate cell killing by mitomycin C, an alkylating agent, and to enhance SV40 induction by mitomycin C. We postulate that the persistence of gaps in DNA, caused by the presence of caffeine, results in the enhancement of SV40 virus induction.

Caffeine↗
PubMed1992

Neurophysiological Correlates of Colour Induction on White Surfaces.

Coloured light surrounding a white surface of about equal luminance makes the white surface appear illuminated with an unsaturated light of the complementary colour. In an attempt to discover the neurophysiological basis of such colour induction, we recorded from spectrally opponent cells of the parvocellular layers of the lateral geniculate nucleus (P-LGN) of anaesthetized macaques. Only cells with wide-band (W) spectral sensitivity in the short (S) or long wavelength (L) part of the spectrum (WS, WL) are excited by white spots of light centred on their receptive field. Cells with narrow-band (N) spectral sensitivity (NS, NL) and light-inhibited (LI) cells are inhibited by white light. Therefore, it is likely that the code for white is contained in a balanced excitation of the W cells. The effects of continuous illumination of remote surrounds with different wavelengths on the responses to achromatic light stimuli were investigated. Responses [on minus maintained discharge rate (MDR) or on-minus-off] were determined for white spots (1 - 3 degrees diameter) flashed on the receptive field centre, presented either alone or in the presence of an annular surround of equal luminance (inner diameter 5 degrees; outer diameter 20 degrees ). During red surround illumination the responses of WL cells to white spots tended to be reduced as were those of WS cells during blue surround illumination. Surround illumination with the opponent colour had more variable effects, neither WS nor WL cells showing a significant alteration of their mean response to white during surround illumination with opponent light. Response alterations were to a large extent due to changes in MDR, which increased in WS cells during blue surround illumination and in WL cells during red surround illumination. It is argued that the surround effects on centre responses are due to intraocular stray light rather than lateral connections in the retina. The surround effects also depended to some extent on the size of the test spot. LI cells and the very rare parvocellular panchromatic on-cells showed no chromatic response changes during coloured surround illumination. Inasmuch as the excitation of WS cells, either alone or in combination with NS cell activation, is involved in coding for green and blue, and that of WL cells, in combination with NL cell activation, is involved in coding for red and yellow in perception, the shift of excitation towards one or the other W cell group indicates relatively more red or green signals in the white response, consistent with and in the same direction as colour induction. In addition, the summed population response of WS and WL cells is decreased during surround illumination with any colour including white. This is related to brightness decrease during surround illumination in perception.

Journal Article↗
PubMed1984

Analysis of mRNA synthesis following induction of the Escherichia coli SOS system.

Escherichia coli responds to impairment of DNA synthesis by inducing a system of DNA repair known as the SOS response. Specific genes are derepressed through proteolytic cleavage of their repressor, the lexA gene product. Cleavage in vivo requires functional RecA protein in a role not yet understood. We used mRNA hybridization techniques to follow the rapid changes that occur with induction in cells with mutations in the recA operator or in the repressor cleavage site. These mutations allowed us to uncouple the induction of RecA protein synthesis from its role in inducing the other SOS functions. Following induction with ultraviolet light, we observed increased rates of mRNA synthesis from five SOS genes within five minutes, maximum expression ten to 20 minutes later and then a later decline to near the initial rates. The presence of a recA operator mutation did not significantly influence these kinetics, whereas induction was fully blocked by an additional mutation in the repressor cleavage site. These experiments are consistent with activation of RecA protein preceding repressor cleavage and derepression of SOS genes. The results also suggest that the timing and extent of induction of individual SOS genes may be different.

Bacterial Proteins↗
PubMed1992

The Role of Ribulose-1,5-Bisphosphate Regeneration in the Induction Requirement of Photosynthetic CO(2) Exchange under Transient Light Conditions.

The temporally variable light environment of natural plant canopies presents distinct limitations to carbon assimilation, partially as a result of the photosynthetic induction requirement that develops when leaves are shaded. This study was undertaken with soybean (Glycine max L.) leaves to further identify factors contributing to the activation state of the fast component of induction during low photosynthetic photon flux density (PPFD) periods. Determination of pool sizes of carbon reduction cycle intermediates at low light and upon return to saturating light indicated that different limitations to photosynthetic activity arise over the time course of a 10-minute low PPFD period. Photosynthetic activity upon reillumination was limited by the regeneration of ribulose 1,5-P(2). There was an increase in the levels of fructose 1,6-P(2), sedoheptulose 1,7-P(2), triose-P, ribose 5-P, and ribulose 5-P pools, indicating inactivation of stromal enzymes, most notably fructose 1,6-bisphosphatase, sedoheptulose 1,7-bisphosphatase, and ribulose 5-P kinase. The fast-induction component was the most important factor limiting assimilation during rapid, brief light transients, during which the decay of the slow component was minimal. This may be particularly significant for upper leaves in soybean canopies that generally experience very rapid light transients.

Journal Article↗
PubMed1978

Temperature and photocontrol of onoclea spore germination.

Germination of Onoclea sensibilis L. spores is controlled by light and temperature. Temperatures of 30 C can induce maximal germination in the dark to a level of 60 to 95% of that induced by a saturating dose of red light (0.38 joules/square meter) providing the spores are placed at the elevated temperature immediately after being sown. Maximum dark germination occurs with a minimum exposure of 16 to 24 hours at 30 C, suggesting that the temperature treatment is required for the induction of germination rather than for the germination process per se. Interaction of temperature and light for induction of germination shows nonadditive behavior. Germination induced by light and temperature applied consecutively never exceeded that which could be induced by a saturating dose of red light alone. Imbibition of the spores at 25 C in the dark for 12 or more hours prior to incubation at 30 C results in a loss of thermosensitivity. Dose response curves for red light induction of germination after varying times of imbibition at 25 C show no concomitant loss of sensitivity of the spores to red irradiation. This suggests that the mechanism and/or pathway of thermoinduction of germination differs from that of photoinduction. The loss of thermosensitivity as a result of presoaking at 25 C can be prevented if the spores are imbibed at 25 C in osmotic agents such as 0.3 molar mannitol or 0.1 gram per liter of polyethylene glycol 400 or in 0.08% dimethylsulfoxide or 10 micrograms per milliliter of herbicide SAN 9789 (4-chloro-5-(methylamino)-2-(alpha,alpha,alpha-trifluoro-m-tolyl-3-(2H)pyridazinone). The latter two substances are hypothesized to act upon membranes. These results suggest that the degree of hydration and possibly changes in membrane properties play a role in the change in sensitivity of Onoclea spores to temperature.

Journal Article↗
PubMed1994

Interactions between Light and the Circadian Clock in the Regulation of CAT2 Expression in Arabidopsis.

In Arabidopsis seedlings germinated and grown in continuous light, CAT2 mRNA abundance peaks 1 d after imbibition, consistent with the role of catalase in detoxifying H2O2 generated during the [beta]-oxidation of fatty acids stored in the seed. A second peak of CAT2 mRNA abundance, of lower amplitude than the initial peak, appears 6 d after imbibition and may be associated with the development of photosynthetic competence and induction of photorespiration. This second peak in steady-state CAT2 mRNA abundance is regulated by light and is not seen in etiolated seedlings. CAT2 mRNA accumulation is induced by exposure to high-fluence blue or far-red light but not by red light. In addition, light induction is unaffected by several mutations that block blue light-mediated inhibition of hypocotyl elongation (blu1, blu2, blu3, hy4), suggesting phytochrome involvement. When etiolated seedlings are transferred to continuous white light, CAT2 mRNA rapidly (within 30 min) accumulates. It is interesting that in these seedlings CAT2 mRNA abundance undergoes pronounced oscillations with a circadian (24 h) periodicity, indicating control by the endogenous circadian clock. No such oscillations are detected in CAT2 mRNA abundance in etiolated seedlings prior to illumination. Control of CAT2 expression by the circadian clock is also seen in 5-week-old plants grown in a light-dark cycle and transferred either to continuous dark or to continuous light; in continuous light the circadian oscillations in CAT2 mRNA abundance persist for at least five circadian cycles, indicating the robustness of this circadian rhythm.

Journal Article↗
PubMed1991

Propofol and thiopentone for caesarean section revisited: maternal effects and neonatal outcome.

In 56 women undergoing elective caesarean section, general anaesthesia was induced with either propofol 1% or thiopentone 2.5% followed by 50% nitrous oxide in oxygen and isoflurane 0.75% until delivery. In the thiopentone group the arterial pressure rose following tracheal intubation and skin incision, while in the propofol group there was a significant tendency to hypotension immediately following induction of anaesthesia. There were differences in electroencephalogram (EEG) between the groups, while laryngoscopy, intubation and surgical stimulation had no effect on EEG pattern. Recovery after anaesthesia did not differ between groups. None of the patients had recall of the intraoperative period, but 53% of patients induced with propofol showed signs of light anaesthesia between induction and delivery. Neonates in the propofol group had lower Apgar scores 1 min after birth than those in the thiopentone group, but these differences were no longer significant at 5 min. No differences were noted in neurobehavioural status at 1, 4 and 24 h.

Journal Article↗
PubMed2006

Cyclic AMP inhibits p38 activation via CREB-induced dynein light chain.

The mitogen-activated protein kinase p38 plays a critical role in inflammation, cell cycle progression, differentiation, and apoptosis. The activity of p38 is stimulated by a variety of extracellular stimuli, such as the proinflammatory cytokine tumor necrosis factor alpha (TNF-alpha), and subjected to regulation by other intracellular signaling pathways, including the cyclic AMP (cAMP) pathway. Yet the underlying mechanism by which cAMP inhibits p38 activation is unknown. Here we show that the induction of dynein light chain (DLC) by cAMP response element-binding protein (CREB) is required for cAMP-mediated inhibition of p38 activation. cAMP inhibits p38 activation via the protein kinase A-CREB pathway. The inhibition is mediated by the CREB target gene Dlc, whose protein product, DLC, interferes with the formation of the MKK3/6-p38 complex, thereby suppressing p38 phosphorylation activation by MKK3/6. The inhibition of p38 activation by cAMP leads to suppression of NF-kappaB activity and promotion of apoptosis in response to TNF-alpha. Thus, our results identify DLC as a novel inhibitor of the p38 pathway and provide a molecular mechanism by which cAMP suppresses p38 activation and promotes apoptosis.

Animals↗
PubMed2000

Functional analysis of a nitrite reductase promoter from birch in transgenic tobacco.

Nitrate assimilation is a highly regulated process in higher plants, and the regulatory cues governing gene expression in this pathway include both external and internal factors. In birch (Betula pendula Roth) the expression of nitrate reductase (NR) and nitrite reductase (NiR) genes is co-regulated by light and nitrate at the transcriptional level. In order to identify cis-acting DNA-elements involved in light and nitrate induction of the birch NiR gene, a 0.9 kb 5' flanking region of the NiR gene was isolated, analysed on the DNA level, and the transcription start site was determined. Deletion analysis of the birch NiR promoter region fused to the GUS reporter gene (uidA) in transgenic tobacco (Nicotiana tabacum) revealed the presence of light- and nitrate-responsive promoter fragments. The responsive fragments showed different activities in leaves and roots. Further, gel mobility shift assays using nuclear proteins from leaves detected a specific DNA-binding activity to the sequence between -146 and -267 bp that was induced in darkness and disappeared in the light. The deletion analysis has shown that this region is critical for light inducibility of the birch NiR gene in leaves.

Journal Article↗
PubMed1998

Identification of a functional respiratory complex in chloroplasts through analysis of tobacco mutants containing disrupted plastid ndh genes.

The plastid genomes of several plants contain homologues, termed ndh genes, of genes encoding subunits of the NADH:ubiquinone oxidoreductase or complex I of mitochondria and eubacteria. The functional significance of the Ndh proteins in higher plants is uncertain. We show here that tobacco chloroplasts contain a protein complex of 550 kDa consisting of at least three of the ndh gene products: NdhI, NdhJ and NdhK. We have constructed mutant tobacco plants with disrupted ndhC, ndhK and ndhJ plastid genes, indicating that the Ndh complex is dispensible for plant growth under optimal growth conditions. Chlorophyll fluorescence analysis shows that in vivo the Ndh complex catalyses the post-illumination reduction of the plastoquinone pool and in the light optimizes the induction of photosynthesis under conditions of water stress. We conclude that the Ndh complex catalyses the reduction of the plastoquinone pool using stromal reductant and so acts as a respiratory complex. Overall, our data are compatible with the participation of the Ndh complex in cyclic electron flow around the photosystem I complex in the light and possibly in a chloroplast respiratory chain in the dark.

Chlorophyll↗
PubMed1979

Cold storage of isolated class C chloroplasts: optimal conditions for stabilization of photosynthetic activities.

Preservation of photosynthetic activities (photophosphorylation, electron transport, fluorescence induction, 0.3-second delayed light emission) of isolated broken (class C) chloroplasts by low temperature storage was investigated under a wide range of conditions in order to optimize long time activity retention.The more labile functions (photophosphorylation and electron transport) required very low temperatures (below -79 C) and relatively high (above 20%, v/v) concentrations of cryoprotectives for satisfactory stabilization. Fluorescence induction and delayed light emission were less sensitive, especially during the 1st month of storage.Taking into account the effect of cryoprotectives on absolute activities prior to freezing, optimum activity retention was observed with a medium containing ethylene glycol (30%, v/v) and a storage temperature of -100 C or below. In this case, given fast thawing and high chloroplast concentration, practically 100% preservation of all of the photosynthetic activities investigated was obtained for at least 10 months, even with very simple freezing and storage procedures.The same optimal medium at somewhat higher temperatures (-79 C and to a lesser extent at -41 C) caused a dramatic uncoupling effect: photophosphorylation was inhibited in a few hours, while electron transport increased 3- to 5-fold. The enhanced electron transport was stable for almost a month and then declined sharply. This uncoupling effect was specific only to ethylene glycol.

Journal Article↗
PubMed1977

Induction of sister chromatid exchanges in xeroderma pigmentosum cells after exposure to ultraviolet light.

The role of DNA repair mechanisms in the induction of sister chromatid exchanges (SCE) after exposure to ultraviolet radiation was investigated in xeroderma pigmentosum cells. Cells from different excision-deficient XP strains, representing the 5 complementation groups in XP, A, B, C, D and E, and from excision-proficient XP variant strains were irradiated with low doses of UVR (0-3.5 J/m2). The number of SCE was counted after two cycles in the presence of BUdR. In cells of the complementation groups A, B, C and D the number of SCE was significantly higher than in UV-exposed control cells. The frequencies of SCE in group E cells and in XP varient cells were not different from those in control cells. Treatment with caffeine (0-200 microgram/ml) did not result in a different response of variant cells compared with normal cells. A simple correlation between SCE frequency and residual excision-repair activity was not observed. The response of the excision-repair deficient cells suggest that unrepaired damage, produced by UVR is involved in the production of SCE.

Caffeine↗
PubMed2001

Implications of adenylate kinase-governed equilibrium of adenylates on contents of free magnesium in plant cells and compartments.

On the basis of the equilibrium of adenylate kinase (AK; EC 2.7.4.3), which interconverts MgATP and free AMP with MgADP and free ADP, an approach has been worked out to calculate concentrations of free magnesium (Mg(2+)), based on concentrations of total ATP, ADP and AMP in plant tissues and in individual subcellular compartments. Based on reported total adenylate contents, [Mg(2+)] in plant tissues and organelles varies significantly depending on light and dark regimes, plant age and developmental stage. In steady-state conditions, [Mg(2+)] in chloroplasts is similar in light and darkness (in the millimolar range), whereas in the cytosol it is very low in the light and increases to about 0.4 mM in darkness. During the dark-to-light transition (photosynthetic induction), the [Mg(2+)] in chloroplasts falls to low values (0.2 mM or less), corresponding to a delay in photosynthetic oxygen evolution. This delay is considered to result from lower activities of Mg-dependent enzymes in the Calvin cycle. In mitochondria, the changes in [Mg(2+)] are similar but smoother. On the other hand, when the transition from light to darkness is considered, an initial increase in [Mg(2+)] occurs in both chloroplasts and mitochondria, which may be of importance for the control of key regulatory enzymes (e.g. mitochondrial malic enzyme and pyruvate dehydrogenase complex) and for processes connected with light-enhanced dark respiration. A rationale is presented for a possible role of [MgATP]/[MgADP] ratio (rather than [ATP(total)]/[ADP(total)]) as an important component of metabolic cellular control. It is postulated that assays of total adenylates may provide an accurate measure of [Mg(2+)] in plant tissues/cells and subcellular compartments, given that the adenylates are equilibrated by AK.

Adenosine Diphosphate↗
PubMed2006

Cross talk between a fungal blue-light perception system and the cyclic AMP signaling pathway.

Blue light regulates many physiological and developmental processes in fungi. In Trichoderma atroviride the complex formed by the BLR-1 and BLR-2 proteins appears to play an essential role as a sensor and transcriptional regulator in photoconidiation. Here we demonstrate that the BLR proteins are necessary for carbon deprivation induced conidiation, even in the absence of light, pointing to the existence of an unprecedented cross talk between light and carbon sensing. Further, in contrast to what has been found in all other fungal systems, clear BLR-independent blue-light responses, including the activation of protein kinase A (PKA) and the regulation of gene expression, were found. Expression of an antisense version of the pkr-1 gene, encoding the regulatory subunit of PKA, resulted in a nonsporulating phenotype, whereas overexpression of the gene produced colonies that conidiate even in the dark. In addition, overexpression of pkr-1 blocked the induction of early light response genes. Thus, our data demonstrate that PKA plays an important role in the regulation of light responses in Trichoderma. Together, these observations suggest that the BLR complex plays a general role in sensing environmental cues that trigger conidiation and that such a role can be separated from its function as a transcription factor.

Cyclic AMP↗
PubMed1976

Ultraviolet light carcinogenesis in hairless mice: cell kinetics during induction and progression of squamous cell carcinoma as estimated by the double-labeling method.

The double-labeling method for studying cell cycle kinetics was applied to uninvolved epidermis of hairless mice (irradiated and unirradiated), and at various stages of ultraviolet light (UVL)-induced squamous cell carcinoma. The growth fraction (GF) was determined by continuous labeling with tritiated thymidine (3HTdR). The data indicate that the double-labeling technique is an acceptable method for studying cell kinetics in UVL-induced squamous cell carcinoma. The GF in normal epidermis (unirradiated and irradiated), hyperplastic epidermis, and lesions of squamous cell carcinoma is 1. The duration of cell generation (Tc) and DNA synthesis (Ts) periods decreased progressively with induction and progression of squamous cell carcinoma. For normal epidermis (irradiated mice) the Tc and Ts were 91 and 6.3 hr, respectively. In the hyperplastic epidermis, the Tc and Ts were reduced to 44 and 4.5 hr, respectively, while even lower values were obtained for the tumor (Tc = 16 hr, Ts = 3.1 hr). The labeling index (LI) in normal epidermis was 7% and increased progressively in hyperplastic epidermis (10%) and squamous cell carcinoma (19%). Epidermal cell differentiation in hyperplastic and tumor tissues appears to have been delayed (GF = 1), since cells above the basal layer, which in normal epidermis keratinize, retain the ability to proliferate, as evidenced by extensive incorporation of [3H]TdR in these cells. The results suggest that tumor production is associated with a progressive shortening of the cell cycle and delayed keratinization of epidermal cells.

Animals↗
PubMed1997

Mutation induction and mutation frequency decline in halogen light-irradiated Escherichia coli K-12 AB1157 strains.

The effects of halogen light irradiation on reversion of argE3-->Arg+ in E. coli K12 strain AB1157 and its mfd- mutant, and on mutation frequency decline (MFD) after transiently incubating irradiated bacteria under non-growing conditions were studied. The induction of mutations, the mutational specificity, and the MFD effect had the same characteristic features as those seen in E. coli B strains after irradiation with 254 nm UV light. MFD which is due to repair of premutagenic lesion in the transcribed strand of glnU gene and prevents mutations leading to supB formation, was not observed in halogen light-induced mutations in the mfd-1 strain. Overproduction of UmuD'C proteins led to a large increase in mutation frequency, which was much greater in mfd- than in mfd+ strains. In bacteria irradiated with halogen light and incubated immediately in a rich medium to express mutations, the formation of supB predominated strongly over that of supE(ochre) in mfd- cells but was at a similar level in mfd+ cells. Introduction of zcf117::Tn10 to AB1157 strain makes cells more sensitive to halogen light irradiation, whereas introduction of mfd-1 does not.

Arginine↗
PubMed1985

Ultraviolet-light exposure induces a heritable sensitivity to the induction of SCE by mitomycin-C.

The dose-response relationship for mitomycin-C (MMC)-induced sister-chromatid exchange (SCE) has been determined in the progeny of Chinese hamster lung fibroblasts (V79) exposed to 5.0 J/m2 ultraviolet light-C (UVC, 254 nm) and in the progeny of non-UVC-irradiated controls. Progeny of UVC-irradiated cultures exhibited sensitivity to MMC-induced SCE at doses of MMC that were not detectably lethal. This sensitivity was manifest as an increase in SCE per cell in a large proportion of the cells derived from UVC-exposed cultures and thus appears not to result from the expression of a rare event such as mutation.

Animals↗