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  • 1
    ISSN: 1432-1335
    Keywords: Regenerating liver ; Cell cycle ; O6-Methylguanine DNA transferase ; DNA alkylation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary O6-Methylguanine DNA transferase activity was investigated in liver proteins obtained at various intervals after partial hepatectomy and/or after hydroxyurea-induced synchronization of the liver cell cycle. Liver proteins were incubated with 3H-methylated calf thymus DNA as previously described by Pegg et al. (1981). The loss of O6-methylguanine was measured by radiochromatography of DNA hydrolysates. The extent of O6-methylguanine repair differed during the cell cycle: the activity increased in late G1, reached a maximum in early S phase and declined in late S phase and G2M. These results indicate that hepatocytes are endowed with an increased DNA repair capacity for this promutagenic lesion during the period of highest transformation sensitivity in the cell cycle. Though increased, however, this repair potential does not, because of its exhaustibility, appear to be sufficient to prevent initiation of transformation after high doses of alkylating carcinogens.
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  • 2
    ISSN: 1432-2048
    Keywords: Chlorophyll (gene expression) ; Gene expression and light ; Light and gene expression ; Light-harvesting chlorophyll a/b-binding protein ; Phytochrome ; Ribulose-1.5-bisphosphate carboxylase ; Sinapis (light, mRNA)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The amount of in-vitro translatable mRNA of the light-harvesting chlorophyll a/b-binding protein (LHCP) of photosystem II strongly increases in darkness (D) after a 5-min red-light pulse while continuous illumination of mustard seedlings with far-red (FR), red or white light leads only to a slight increase in the amount of translatable LHCP-mRNA. No increase can be observed after a long-wavelength FR (RG9-light) pulse. However, a FR pretreatment prior to the RG9-light pulse strongly increase LHCP-mRNA accumulation in subsequent D. This is not observed in the case of the mRNA for the small subunit of ribulose-1.5-bisphosphate carboxylase. The increase of LHCP-mRNA in D after a FR pretreatment can be inhibited by a reillumination of the seedlings with FR. The inhibition of LHCP-mRNA accumulation during continuous illumination with FR and the strong increase in D following a FR illumination was found to be independent of chlorophyll biosynthesis since no correlation between chlorophyll biosynthesis and translatable LHCP-mRNA levels could be detected. Even strong changes in the amount of intermediates of chlorophyll biosynthesis caused by application of levulinic acid or 5-aminolevulinic acid did not affect LHCP-mRNA levels. Therefore, we conclude that the appearance of LHCP-mRNA is inhibited during continuous illumination, even though illumination leads to a storage of a light singal which promotes accumulation of translatable LHCP-mRNA in D.
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  • 3
    ISSN: 1432-2048
    Keywords: Chloroplast (photooxidation, proteins) ; mRNA (chloroplast proteins) ; Nitrate reductase ; Photooxidation (chloroplast) ; Sinapis (plastid photooxidation)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract It was inferred from previous findings that a plastid-derived factor (plastidic factor) is involved in the transcriptional control of nuclear genes coding for proteins destined for the chloroplast. Photooxidative damage to the plastid destroys the ability of the organelle to give off this factor. Cytosolic enzyme levels are not impaired if plastids are damaged, and morphogenesis of seedlings is normal. The only exception found so far is nitrate reductase, a cytosolic enzyme, which is regulated by the cellas if it were a plastidic protein. In the present study we have shown that the plastids in the mesophyll of mustard (Sinapis alba L.) cotyledons, damaged by 3 h photooxidation in red light (6.8 W·m-2) and then returned to darkness or to continuous, non-photooxidative far-red light (cFR), recover from photooxidative damage. The rate of recovery is stimulated by phytochrome (operationally, cFR). Since the cytosolic enzyme nitrate reductase is affected by the different treatments in principally the same way as the levels of plastidic enzymes, we conclude that it is recovery of the plastids' ability to give off the plastidic factor rather than structural recovery which leads to recovery of gene expression and protein (and chlorophyll) re-accumulation. The extent of recovery varied according to the enzyme and this variation could be explained by different plastidic-factor requirements for gene expression. This explanation was confirmed by measurements of translatable mRNAs. It was found that LHCP-gene expression (light-harvesting chlorophyll a/b-binding protein of photosystem II) is far more sensitive to photooxidative damage of the plastids than SSU-gene expression (small subunit of ribulose-1.5-bisphosphate carboxylase). Correspondingly, recovery is expressed to a much greater extent in the latter than in the former case.
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  • 4
    ISSN: 1432-2048
    Keywords: Gene expression (control points) ; Phytochrome, and ribulose-1,5-bisphosphate carboxylase ; Ribulose-1,5-bisphosphate carboxylase ; Sinapis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract We have measured levels of ribulose-1,5-bisphosphate carboxylase (RuBPCase) and levels of in-vitro-translatable mRNA for the small subunit (SSU) of RuBPCase up to 96 h after sowing in mustard (Sinapis alba L.) cotyledons, in order to investigate to what extent the rate of enzyme synthesis is related to the level of SSU-mRNA. Both enzyme and mRNA level are controlled strongly by phytochrome, but the rate of RuBPCase accumulation was found to be unrelated to the level of translatable SSU-mRNA. As an example, it was found that the amount of SSU-mRNA in far-red light (FR)-grown mustard seedlings doubles between 54 and 84 h after sowing while the rate of RuBPCase accumulation remains constant over this period. Since the holoenzyme shows zero turnover during this period it is concluded that the rate of enzyme synthesis remains constant although the level of SSU-mRNA increases strongly. Following an FR→dark transition, with different levels of physiologically active phytochrome (Pfr) established at the end of the light period, no correlation was found between the time course of mRNA levels in darkness and the rate of enzyme synthesis. Rather, the data indicate that there is at least one translational or post-translational regulatory step which is also phytochrome-dependent. It is concluded that coarse control of the appearance of translatable SSU-mRNA is essential for RuBPCase to appear at a high rate but that fine tuning by phytochrome of the actual appearance of RuBPCase is not transcriptional.
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  • 5
    ISSN: 1432-2072
    Keywords: Drug discrimination ; Humans ; Stimulus effects ; Subjective effects ; Mood ; Marijuana ; Cannabinoids ; Carbon monoxide
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract The discriminative stimulus (DS) effects of smoked marijuana were studied by training marijuana smokers to discriminate between the effects of marijuana containing 2.7% △9-THC (M) and marijuana containing 0.0% △9-THC (P). In addition to measures of discrimination responding, subjective effects were assessed with standardized mood questionnaires. The post-smoking increase in expired air carbon monoxide (CO) level was used as an index of smoke inhalation. Relative to P cigarettes, M cigarettes increased heart rate and produced changes on eight mood scales. M cigarettes were rated as harsher and more potent than P cigarettes, and produced lower levels of CO than P cigarettes. The P-M discrimination was readily acquired by most subjects. The DS effects of marijuana showed a rapid onset, appearing within 90 s from the beginning of smoking. The DS effects were dose dependent, with 0.9% △9-THC marijuana producing primarily placebo-appropriate discrimination responding, and 1.4% △9-THC marijuana producing 100% drug-appropriate responding. This experimental paradigm could be used to determine whether the DS effects of smoked marijuana would generalize to those of other psychoactive drugs.
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  • 6
    ISSN: 1432-2072
    Keywords: Rhesus monkey ; Morphine dependence ; Naloxone ; Nalorphine ; Postdependent sensitization ; Abstinence ; Schedule-controlled responding
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract The effects of morphine, naloxone, and nalorphine on responding maintained under a variable-interval schedule of food presentation were assessed in rhesus monkeys before and after successive periods of daily morphine maintenance (15.0 mg/kg/day SC). Withdrawal from morphine dependence was accomplished gradually following the first two maintenance periods and abruptly following the third period. Schedule-controlled responding was disrupted when morphine maintenance was abruptly discontinued but not when the maintenance dosage was gradually reduced to zero. Tolerance to the acute effects of IV morphine on responding developed during morphine maintenance and dissipated after daily injections were discontinued. The effects of IV naloxone and IV nalorphine following each period of morphine maintenance were generally similar to their effects in initial determinations. These data indicate that tolerance-producing regimens of repeated daily injections with morphine do not necessarily produce enduring changes in the effects of opiate antagonists on schedule-controlled behavior. Additionally, gradual withdrawal from morphine maintenance can minimize the behavioral disruptions that attend abrupt abstinence.
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  • 7
    ISSN: 1432-2048
    Keywords: Nitrate reductase ; Nitrite reductase ; Phytochrome ; Signal (storage, transduction) ; Sinapis (phytochrome)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Application of nitrate leads to an induction of nitrate reductase (NR; EC 1.6.6.1) and nitrite reductase (NIR; EC 1.7.7.1) in the cotyledons of dark-grown mustard (Sinapis alba L.) seedlings, and this induction can strongly be promoted by a far-red-light pretreatment — operating through phytochrome — prior to nitrate application. This light treatment is almost ineffective — as far as enzyme appearance is concerned — if no nitrate is given. When nitrate is applied, the stored light signal potentiates the appearance of NR and NIR in darkness, even in the absence of active phytochrome, to the same extent as continuous far-red light. This action of previously stored light signal lasts for approx. 12 h. Storage of the light signal was measured for NR and NIR. The process shows enzyme-specific differences. Storage occurs in the absence as well as in the presence of nitrate, i.e. irrespective of whether or not enzyme synthesis takes place. The kinetics of signal transduction and signal storage indicate that the formation and action of the stored signal are a bypass to the process of direct signal transduction. Signal storage is possibly a means of enabling the plant to maintain the appropriate levels of NR and NIR during the dark period of the natural light/dark cycle.
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  • 8
    ISSN: 1432-2048
    Keywords: Nitrate reductase ; Nitrite reductase ; Phytochrome ; Plastidic signal ; Sinapis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract We compared the response of NO 3 - -induced nitrate-reductase (NR) and nitrite-reductase (NIR) levels in virtually carotenoid-free far-red-light-grown mustard (Sinapis alba L.) cotyledons following a photooxidative treatment of the plastids. The cytosolic localization of NR and the plastidic localization of NIR were confirmed with this approach. Emphasis was on a plastidic factor previously postulated to be involved obligatorily in the transcriptional control of nuclear genes coding for proteins destined for the chloroplast. Photooxidative damage of the plastid would be to destroy the ability of the organelle to send off this signal. Dependency of NIR and NR induction by NO 3 - on the plastidic factor is described in detail, and it is concluded that requirement for the plastidic factor is relatively high in the case of NR while factor requirement to allow induction is low in the case of NIR. The data indicate that in the case of NIR the photooxidative damage done to the plastid also affects accumulation of the enzyme directly. Since this effect is absent in the case of cytosolic NR, induction of NR is a particularly suitable system for further molecular studies of the plastidic factor and its mode of action.
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  • 9
    ISSN: 1432-2048
    Keywords: Ammonium toxicity ; Nitrate reductase (multiple forms) ; Phytochrome ; Plastidic factor ; Sinapis (nitrate reductase)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract In mustard (Sinapis alba L.) cotyledons, four different forms of nitrate reductase (NR) can be separated by anion-exchange chromatography. Two of these forms (NR1 and NR2) appear in the presence of NO 3 - while the other two (NR3 and NR4) appear as a response to the application of NH 4 + as the sole nitrogen source. In the presence of NH4NO3, NR3 appears to be superimposed on nitrate-induced NR1 and NR2 while the NH 4 + -induced appearance of NR4 is totally abolished in the presence of equimolar amounts of NO 3 - . The appearance of NR1, NR2 and NR3 is strongly stimulated by red light pulses which operate via the far-red-absorbing form of phytochrome (Pfr), whereas the appearance of NR4 requires continuous light (likewise operating through pytochrome). Continuous red light is more effective in this case than continuous far-red light. Analysis of the data shows that the mode of action of phytochrome (Pfr) is the same in the case of the appearances of NR1 and NR2, whereas it is quantitatively different in the case of NR3 and totally different in that of NR4. A ‘plastidic factor’ has previously been postulated to be obligatorily involved in the transcriptional control of nuclear genes encoding for proteins destined for the chloroplast. Photooxidative damage of the plastid is postulated to destroy the ability of the organelle to produce this signal. If the plastids are damaged by photooxidation, the action of nitrate and phytochrome on the appearance of NR is abolished. The plant cell regulates the appearance of nitrate-induced NR, which is cytosolic, as if it were a plastidic protein. The appearance of NR3 depends on the plastidic factor in principally the same way as that of NR1 and NR2 whereas NR4 is totally independent of the plastidic factor. The data document particular kinds of interaction between controlling factors (light, nitrate, ammonium, plastidic factor) which affect gene expression in plants. These intricacies of regulation have so far not been considered in molecular studies on NR-gene expression.
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  • 10
    ISSN: 1618-2650
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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