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141.
142.
Thomas L.J. Boehm Willi Kreis Dusan Drahovsky 《Biochimica et Biophysica Acta (BBA) - Gene Structure and Expression》1982,696(1):52-56
The incorporation of [14C]deoxycytidine, [3H]deoxyuridine, and [3H]thymidine, respectively into pyrimidine bases of DNA has been measured in rapidly proliferating P815 mouse mastocytoma cells in the presence of hydroxyurea. The incorporation of [14C]deoxycytidine-derived radioactivity into DNA cytosines is increased when compared to the incorporation into DNA thymines. The [3H]deoxyuridine-derived radioactivity is incorporated solely into DNA thymines and this incorporation is inhibited by hydroxyurea in a dose-dependent manner. This suggests an inhibitory effect of hydroxyurea on the thymidylate synthase which was proved in experiments in which the conversion of deoxyuridine monophosphate into deoxythymidine monophosphate catalysed by a crude enzyme preparation from P815 cells was inhibited in the presence of hydroxyurea. Enzymatic DNA methylation as measured by the conversion of incorporated [14C]deoxycytidine into 5-methylcytosines was not affected by hydroxyurea. 相似文献
143.
144.
Colonization of artificial substrates by stream insects: Influence of substrate size and diversity 总被引:5,自引:5,他引:0
A field experiment was performed in a New Mexico (U.S.A.) stream to investigate the relation between size and diversity of substrate and numbers and diversity of colonizing insects. Baskets with either small gravel, large gravel, or a 1 : 1 mixture of the two sizes were placed in a riffle area and colonization was monitored for 19 days.Colonization was rapid, and both total number of individuals and number of species colonizing the substrates had stopped increasing by the end of the experiment. Fewer individuals colonized baskets downstream, suggesting that the drift is a major source of insect colonists. Small substrate supported more individuals and more species than the larger stones. Total number of individuals and the number of species on the mixed substrate were between numbers on small and large substrate. These results do not support previous generalizations on the relation between substrate size and complexity and the structure of invertebrate communities in streams. Results of the study are compared with the findings of recent experimental studies and it is suggested that earlier generalizations on the role of substrate size and complexity need to be reexamined.Order of authors was determined by the flip of a coin. Reprint requests may be sent to either author. 相似文献
145.
K Gottlieb 《American journal of physical anthropology》1978,48(2):213-214
Studies examining an association between artificial cranial deformation and the presence of wormian bones in the lambdoid suture have been inconclusive. Cranial deformation, however, does not seem to have a direct effect of increasing the sutural complexity of the pars lambdica of the lambdoid suture and also increasing the mean number of lambdoidal wormian bones, given their presence. 相似文献
146.
Trypanosomatid protozoa: survey of acetylornithinase and ornithine acetyltransferase 总被引:1,自引:0,他引:1
Trypanosomatid protozoa (Crithidia deanei, C. deanei aposymbiotic, C. oncopelti, C. fasciculata, C. acanthocephali, Leptomonas seymouri, L. collosoma, L. samueli, Herpetomonas samuelpessoai, H. sp., H. megaseliae, H. muscarum muscarum, Leishmania donovani, L. braziliensis, Trypanosoma cruzi, T. conorhini and T. mega) were examined for the presence of acetylornithinase (EC 3.5.1.16) and ornithine acetyltransferase (EC 2.3.1.35). As a rule, species of the genus Crithidia presented one of the two enzymes for the conversion of acetylornithine into ornithine. Crithidia fasciculata and C. acanthocephali presented acetylornithinase, while C. deanei and C. oncopelti, species harboring symbionts, presented ornithine acetyltransferase. The enzyme was absent in the aposymbiotic strain of C. deanei, which suggests that the enzyme belongs to the symbiont. Among the other trypanosomatids examined only Herpetomonas samuelpessoai presented acetylomithinase. The participation of acetylornithinase and ornithine acetyltransferase in the metabolism of trypanosomatids is discussed in the light of their nutritional requirements and possession of enzymes of the arginineornithine metabolism. 相似文献
147.
The effect of exogenous adenine nucleotides on CO2 fixation and oxygen evolution was studied with mesophyll protoplast extracts of the C4 plant Digitaria sanguinalis. Exogenous ATP was found to stimulate the rate of pyruvate and pyruvate + oxalacetate induced CO2 fixation, as well as reverse the inhibition of CO2 fixation by carbonyl cyanide m-chlorophenyl hydrazone and several electron transport inhibitors. The ATP-dependent stimulation of CO2 fixation varied from 40 to 70 μmol CO2 fixed/mg chlorophyll per h, suggesting that ATP was crossing the chloroplast membranes at rates of 80–140 μmol/mg chlorophyll per h, since 2 ATP are required for each CO2 fixed. Fixation of CO2 could also be induced in the dark by exogenous ATP, in which case ADP accumulated outside the chloroplasts. This suggests that external ATP is exchanging for internal ADP. In contrast, ADP and AMP were found not to traverse chloroplast membranes, on the basis that neither nucleotide inhibited CO2 fixation or stimulated oxygen evolution that was limited by available ADP for phosphorylation. Further evidence that ATP can enter the chloroplasts was obtained by direct measurements of the increase in ATP in the chloroplasts due to addition of exogenous ATP in the dark. These studies yielded minimal rates of ATP uptake on the order of 30–40 μmol/mg chlorophyll per h. It is suggested that a membrane translocator exists that specifically transports ATP into the chloroplasts in exchange for ADP. The significance of these findings are considered with respect to the C4 pathway of photosynthesis. 相似文献
148.
Iain M. Campbell 《Phytochemistry》1975,14(3):683-693
Mathematical expressions governing incorporation and dilution values in a simple, three component biosynthetic system are established and used to track the influence of pathway structure and sampling time on the numerical value of these parameters. A more productive manner of using the incorporation concept is suggested. 相似文献
149.
The metabolism of d-gluconate-[1-14C] and -[6-14C] by segments from etiolated hypocotyls of Phaseolus mungo has been studied. The release of 14CO2 from gluconate-[1-14C] was greater than that from gluconate-[6-14C] in all parts of hypocotyls examined. Incorporation of the radioactivity from gluconate-[6-14C] into RNA, lignin and aromatic amino acid fractions was greater in the upper (younger) part of the hypocotyls. Incorporation into sugars was greater in the lower (more mature) parts. 相似文献
150.
The effect of exogenous proline on the activity of the glycolate pathway in Nicotiana tabacum cv. Xanthi n.c. An exogenous proline supply in the light provokes an increase in free glycine concentration in apical tissues or in leaf disks of vegetative Nicotiana tabacum L. cv. Xanthi n.c. This does not occur in the equivalent tissues of tobacco plants after floral induction, these being naturally rich in proline. In vegetative tobacco, we have tried to determine this specific action of exogenous proline. With 14C glycine, 14CO2 experiments (Pulse-chase) and glycine decarboxylase activity determinations, we observed that glycine-serine transformation was inhibited by proline supply. Presently it is important to determine if endogenous proline acts on the same reaction. 相似文献