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The interactions established at the 5'-splice site during spliceosome assembly are likely to be important for both precise recognition of the upstream intron boundary and for positioning this site in the active center of the spliceosome. Definition of the RNA-RNA and the RNA-protein interactions at the 5' splice site would be facilitated by the use of a small substrate amenable to modification during chemical synthesis. We describe a trans-splicing reaction performed in Saccharomyces cerevisiae extracts in which the 5' splice site and the 3' splice site are on separate molecules. The RNA contributing the 5' splice site is only 20 nucleotides long and was synthesized chemically. The trans-splicing reaction is accurate and has the same sequence, ATP, and Mg2+ requirements as cis-splicing. We also report how deoxy substitutions around the 5'-splice site affect trans-splicing efficiency. 相似文献
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C Cepeda CS Colwell JN Itri SH Chandler MS Levine 《Canadian Metallurgical Quarterly》1998,79(1):82-94
The present experiments were designed to examine dopamine (DA) modulation of whole cell currents mediated by activation of N-methyl-D-aspartate (NMDA) receptors in visualized neostriatal neurons in slices. First, we assessed the ability of DA, D1 and D2 receptor agonists to modulate membrane currents induced by activation of NMDA receptors. The results of these experiments demonstrated that DA potentiated NMDA-induced currents in medium-sized neostriatal neurons. Potentiation of NMDA currents occurred at three different holding potentials, although it was more pronounced at -30 mV. It was mediated by D1 receptors, because it was mimicked by D1 agonists and blocked by exposure to a D1 antagonist. Activation of D2 receptors produced inconsistent effects on NMDA-induced membrane currents. Either decreases, increases, or no effects on NMDA currents occurred. Second, we examined the contributions of intrinsic, voltage-dependent conductances to DA potentiation of NMDA currents. Blockade of K+ conductances did not prevent DA enhancement of NMDA currents. However, voltage-activated Ca2+ conductances provided a major contribution to DA modulation. The dihydropyridine L-type Ca2+ channel blockers, nifedipine, and methoxyverapamil (D-600), markedly reduced but did not totally eliminate the ability of DA to modulate NMDA currents. The D1 receptor agonist SKF 38393 also enhanced Ba2+ currents in neostriatal neurons. Together, these findings provide evidence for a complex interplay between DA, NMDA receptor activation and dihydropyridine-sensitive Ca2+ conductances in controlling responsiveness of neostriatal medium-sized neurons. 相似文献
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A study of determine the seroprevalence rate and clinical presentation of HIV-associated pulmonary tuberculosis was carried out in Jos between October 1990 and September 1991. Out of the 180 newly diagnosed pulmonary tuberculosis patients, 11(6.1%) were confirmed seropositive for HIV-1 and 2. The peak age range for both tuberculosis and HIV infection in both sexes is 20-40 years. The risk of HIV infection was associated with multiple sex partners and blood transfusion. There was no significant difference in the clinical presentation of pulmonary tuberculosis between HIV-seropositive and seronegative patients (P > 0.1). However, diarrhoea, lymphadenopathy and marked weight loss were found to be significantly associated with HIV infection (P < 0.05). The mean lymphocyte count of HIV seropositives was significantly lower than seronegatives (P < 0.01). 相似文献