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91.
Stable monolayers of contact-inhibited C3H 10T1/2 cells were used in multifraction radiation experiments to measure the oxygen enhancement ratio (OER) at low doses/fraction under conditions where cell cycle effects (repopulation, redistribution) were minimal. Consistent with there being a dose-dependent reduction in the OER at low doses, an extremely low OER of 1.34 was measured after 20 fractions of 1.7 Gy every 12 h. The sparing effects of fractionating radiation doses were not apparent for cells irradiated under hypoxic conditions (i.e., multifraction survivals were lower than acute single-dose values) until doses exceeding 15 Gy were reached. This result suggested a deficiency in the recovery from sublethal and/or potentially lethal damage might exist after hypoxic irradiations, thereby reducing the OER. The capacity to repair potentially lethal damage was found to be nearly the same after hypoxic as compared to aerobic irradiations. However, there was an apparent absence of sublethal damage repair by 10T1/2 cells between two hypoxic irradiations which could be a major contributing factor to the extremely low OER value measured in this multifraction schedule. 相似文献
92.
Construction of hybrid Tn501/Tn21 transposases in vivo: identification of a region of transposase conferring specificity of recognition of the 38-bp terminal inverted repeats. 总被引:1,自引:0,他引:1 下载免费PDF全文
In order to study the transposase enzymes of Class II prokaryotic transposable elements, we have constructed genes encoding hybrid transposase proteins. This was done by recombination in vivo between the tnpA genes of transposons Tn501 and Tn21. These hybrid genes can complement in trans a transposition-defective mutant of Tn501. The structures of the products of this complementation indicate whether the specificity of the hybrid transposase in recognising the 38 bp terminal inverted repeats is that of Tn501 or that of Tn21. The determinant of this specificity is in the N-terminal region of the transposase protein, between amino acids 28 and 216. The predicted amino acid sequences so far determined of transposases from the Class II family reveal an area of homology in this region. 相似文献
93.
Binding of a nuclear factor to a consensus sequence in the 5' flanking region of zein genes from maize 总被引:26,自引:6,他引:20 下载免费PDF全文
The genomic organization of the zein structural genes and of regulatory loci influencing their expression suggests that control of zein gene expression will involve interactions between cis elements in the flanking DNA sequences and products from trans-acting genes. The interaction between fragments from the 5' flanking region of a zein gene and specific, double-stranded oligonucleotides with crude nuclear extracts from maize endosperm have been studied by nitrocellulose filter binding, gel retention and DNase I footprinting assays. Specific binding of a nuclear factor was observed and the exact position of the protein binding site was determined. The 22-nt binding site included 14 bp of a 15-bp sequence conserved in all zein genes. 相似文献
94.
Proteoglycan metabolism associated with mouse metanephric development: morphologic and biochemical effects of beta-D-xyloside 总被引:3,自引:0,他引:3
Morphology and de novo incorporation of [35S]sulfate into proteoglycans were studied in fetal mouse kidneys at the onset of organogenesis. Branching morphogenesis and nephron development in organ culture and in vivo were associated with de novo synthesis of chondroitin-SO4 and heparan-SO4 proteoglycans. The role of proteoglycan metabolism in metanephrogenesis was then studied by analysis of the effects of p-nitrophenyl-beta-D-xylopyranoside (beta-D-xyloside) on renal development and proteoglycan metabolism. Incubation of fetal kidneys in beta-D-xyloside at concentrations of 1.0 and 0.5 mM, but not at 0.1 mM, caused inhibition of ureteric branching and markedly diminished synthesis of a large Mr 2.0 X 10(6) Da chondroitin-SO4 proteoglycan. Incorporation of [35S]sulfate was stimulated at all beta-D-xyloside concentrations, reflecting synthesis of xyloside initiated dermatan-35SO4 chains. In contrast to dramatic effects on chondroitin-SO4 synthesis and ureteric branching, beta-D-xyloside had no effect on heparan-SO4 synthesis or on development of the glomerulus and glomerular basement membrane. We thus characterize the proteoglycans synthesized early in the course of renal organogenesis and describe observations which suggest an association between metabolism of chondroitin-SO4 proteoglycan and development of the ureter. 相似文献
95.
Evidence for simultaneous derepression of messenger RNA and the guanine nucleotide exchange factor in fertilized sea urchin eggs 总被引:1,自引:0,他引:1
A M Colin B D Brown J N Dholakia C L Woodley A J Wahba M B Hille 《Developmental biology》1987,123(2):354-363
Translational control was studied in extracts of Lytechinus pictus eggs and zygotes. We showed that neither mRNA nor initiation factors alone limit translation in these lysates; rather they are together rate limiting. Added globin mRNA was translated in egg and zygote lysates but overall protein synthesis did not increase significantly as the added RNA competed with the endogenous message. The lysates mimicked the in vivo response, since microinjection of globin mRNA into L. pictus eggs similarly competed with endogenous mRNAs. A number of translational components were used to determine if they would stimulate protein synthesis in these lysates. The addition of globin polyribosomes increased the level of protein synthesis. The majority of this increase was due to reinitiation of the globin mRNA, and under these conditions the level of endogenous protein synthesis in both egg and zygote extracts did not change. The addition of crude initiation factors alone did not appreciably alter the rate of protein synthesis in the egg lysates. However, in the presence of added mRNA, these initiation factors stimulated translation two- to fourfold. Of all the initiation factors tested, only the guanine nucleotide exchange factor (GEF, eIF-2B, RF) significantly increased protein synthesis when globin mRNA was present. The addition of an unfractionated initiation factor preparation further stimulated protein synthesis in the presence of added GEF and mRNA, suggesting that a component other than mRNA and GEF was also limiting in these egg lysates. Other initiation factors, including eIF-2, eIF-4A, eIF-4B, and eIF-4F, did not substitute for the component in the unfractionated initiation factor preparation. We propose that alkalinization of the cytoplasm and the subsequent activation of initiation factors and mRNAs contribute to the large stimulation of protein synthesis in echinoid eggs after fertilization. Furthermore, we discuss the possibility that the increase in NADPH at the expense of NAD+, which occurs within 3 min after fertilization, may lead to the activation of GEF. 相似文献
96.
Precommitment states induced during HL-60 myeloid differentiation: possible similarities of retinoic acid- and DMSO-induced early events 总被引:3,自引:0,他引:3
The possible relationship of the pathways by which two inducers, retinoic acid and DMSO, cause myeloid differentiation of HL-60 promyelocytic leukemia cells was studied. HL-60 cells were first exposed to retinoic acid and then washed free of it. As reported previously, this brief exposure results in no subsequent G0 growth arrest or phenotypic differentiation. When these cells were subsequently exposed to DMSO, onset of G1/0 growth arrest but not phenotypic differentiation occurred within 24 h. Since in these cells retinoic acid or DMSO normally requires 48 h of continuous exposure for onset of significant G0 growth arrest and phenotypic differentiation, it appears that retinoic acid and DMSO induce similar early cellular events needed for subsequent G0 growth arrest but not for phenotypic differentiation. While onset of growth arrest and differentiation occur together when the cells are exposed for 48 h to retinoic acid, the present results indicate that their occurrence can be uncoupled by this split dosage to inducers. The results are discussed in terms of a previously hypothesized model of cellular response to the inducers. 相似文献
97.
Plasmodium falciparum: identification and localization of a knob protein antigen expressed by a cDNA clone 总被引:7,自引:0,他引:7
J G Culvenor C J Langford P E Crewther R B Saint R L Coppel D J Kemp R F Anders G V Brown 《Experimental parasitology》1987,63(1):58-67
Differential screening of cDNA libraries constructed from knobby and predominantly knobless Plasmodium falciparum isolates, identified the sequence SD17. Chromosome blotting experiments have shown that this sequence, which is located on chromosome 2 of most isolates, was deleted in the cloned parasite line E12 of the FCQ27/PNG isolate. Here we show that erythrocytes infected with the SD17-containing cloned line D10 have typical knob structures on their surfaces, whereas those infected with the line E12 lack knobs. An expression clone was constructed from SD17 and used to affinity purify antibodies from the sera of individuals living in areas of Papua New Guinea where malaria is endemic. The antibodies reacted in immunoblotting experiments with a single polypeptide that varied in Mr from 85,000 to 105,000 among different isolates. The antigen was not expressed in the knobless clone E12. Postembedding immunoelectron microscopy showed localization of the antigen over the knobs of FC27 and two other isolates, largely on the cytoplasmic side. We conclude that the parasite antigen corresponding to clone SD17 is a knob protein. 相似文献
98.
Z H Zhao B Watschinger C D Brown M M Beyer E A Friedman 《Hormones et métabolisme》1987,19(11):534-537
The variations of susceptibility to alloxan induced Diabetes in a total of seventeen rabbits was described. Our study was designed to explore dosage schedules which might improve rabbit responsiveness to and survival after alloxan treatment. A wide range of response to intravenously administered alloxan was observed. Permanent diabetes (blood glucose 350 mg/dl) was found in three rabbits after a single injection (60 mg/kg in one, 100 mg/kg in two). This effect has persisted for eight months. By contrast, two other rabbits injected with a single dose of alloxan (60 mg/kg) developed only transient hyperglycemia. Similarly, four other rabbits either did not respond or had an incomplete response after receiving a total dose of 120 mg/kg. These data suggest that there is extreme variability in individual rabbits susceptibility to the diabetogenic affects of alloxan. 相似文献
99.
100.
Locations of the hydrogenases of Methanobacterium formicicum after subcellular fractionation of cell extract. 总被引:3,自引:3,他引:0 下载免费PDF全文
The F420 hydrogenase of Methanobacterium formicicum was associated with membranes isolated by sucrose density gradient ultracentrifugation of cell extract. The methyl viologen hydrogenase was present in the soluble fractions. Column chromatography with phenyl-Sepharose CL-4B revealed that the F420 hydrogenase was strongly hydrophobic, suggesting that it associates with isolated membranes through hydrophobic interactions. 相似文献