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61.
62.
Alexandra Peacock-Villada Darren Yang Claudia Danilowicz Efraim Feinstein Nolan Pollock Sarah McShan Vincent Coljee Mara Prentiss 《Nucleic acids research》2012,40(20):10441-10451
RecA-family proteins mediate homologous recombination and recombinational DNA repair through homology search and strand exchange. Initially, the protein forms a filament with the incoming single-stranded DNA (ssDNA) bound in site I. The RecA–ssDNA filament then binds double-stranded DNA (dsDNA) in site II. Non-homologous dsDNA rapidly unbinds, whereas homologous dsDNA undergoes strand exchange yielding heteroduplex dsDNA in site I and the leftover outgoing strand in site II. We show that applying force to the ends of the complementary strand significantly retards strand exchange, whereas applying the same force to the outgoing strand does not. We also show that crystallographically determined binding site locations require an intermediate structure in addition to the initial and final structures. Furthermore, we demonstrate that the characteristic dsDNA extension rates due to strand exchange and free RecA binding are the same, suggesting that relocation of the complementary strand from its position in the intermediate structure to its position in the final structure limits both rates. Finally, we propose that homology recognition is governed by transitions to and from the intermediate structure, where the transitions depend on differential extension in the dsDNA. This differential extension drives strand exchange forward for homologs and increases the free energy penalty for strand exchange of non-homologs. 相似文献
63.
Ionophores (monensin, nigericin) capable of transporting both Na+ and K+ across cell membranes down their concentration gradients reduce the rate and total magnitude of serotonin uptake by platelets. The effect of the ionophores was time dependent, so that inhibition increased progressively until eventually uptake ceased entirely. Nigericin and monensin produced loss of platelet K+ and an equivalent molar uptake of Na+ thereby abolishing the normal transmembrane Na+ and K+ gradients. The time course of these ionophore-induced cation shifts at 37° C corresponded to the rate at which inhibition of serotonin transport developed. The ionophores did not affect total ATP concentration of platelets nor the metabolic pool of ATP formed from [14C] adenine. Nigericin and monensin released about 80% of platelet 14C and endogenous serotonin over a 30 min period, without release of platelet adenine nucleotides, calcium or β-glucuronidase. The ionophores did not elicit platelet aggregation nor did they prevent maximal aggregation brought about by ADP, collagen or A23187. Replacement of Na+ in the medium by K+ abolished serotonin uptake but only 10–20% of endogenous serotonin was released. In KCl medium the Na+ gradient was initially reversed, but quickly dissipated as Na+ reequilibrated with the extracellular fluid. At 37° C the ionophores did not affect either the rate of Na+ reequilibration or the efflux of [14C] serotonin. Na+ reequilibration was slower at 20° C and the ionophores significantly increased platelet Na+ loss and strongly inhibited the efflux of [14C] serotonin. The data support a mechanism of serotonin transport due to a Na+-dependent carrier-mediated process which need not be directly dependent on metabolic energy, but which does require metabolic energy to maintain normal gradients. 相似文献
64.
描述了采自中国四川的剑花蟹蛛Xysticus sicus Fox,1937,其雄性系首次发现。标本保存于河北大学博物馆。 相似文献
65.
白细胞介素-2中枢镇痛作用途径的探讨 总被引:3,自引:0,他引:3
抗IL-2受体α亚基的单克隆抗体不能阻断IL-2的中枢镇痛作用,以及丧失与IL-2受体β亚基结合能力的IL-2突变体仍具有提高大鼠痛阈的能力,这表明IL-2的中枢镇痛作用并不是通过IL-2受体所介导,亦表示IL-2的免疫和镇痛作用是通过不同的受体途径实现的。加之内源性阿片肽与IL-2分子有着共同的抗原决定基和结构相似性,提示IL-2可以与阿片受体直接结合产生中枢镇痛效应。从放射免疫法测定的IL-2侧脑室注射后不同时间大鼠脑内不同核团的内源性阿片肽含量,推测IL-2的中枢镇痛作用可能还与弓状核、室旁核、蓝斑等核团的β-EP和LEK有关。 相似文献
66.
Introduction
Difficulties with visual perception (VP) are often described in children with neurological or developmental problems. However, there are few data regarding the range of visual perceptual abilities in populations of normal children, or on the impact of these abilities on children''s day-to-day functioning.Methods
Data were obtained for 4512 participants in an ongoing birth cohort study (Avon Longitudinal Study of Parents and Children; ALSPAC). The children''s mothers responded to questions designed to elicit indications of visual perceptual difficulties or immaturity, when their children were aged 13 years. We examined associations with standardised school test results in reading and in mathematics at age 13–14 years (SATS-KS3), accounting for potential confounders including IQ.Results
Three underlying factors explained half the variance in the VP question responses. These correlated best with questions on interpreting cluttered scenes; guidance of movement and face recognition. The adjusted parameter estimates (95% CI) for the cluttered-scenes factor (0.05; 0.02 to 0.08; p<0.001) suggested positive associations with the reading test results whilst that for the guidance-of-movement factor (0.03; 0.00 to 0.06; p = 0.026) suggested positive association with the mathematics results. The raw scores were associated with both test results.Discussion
VP abilities were widely distributed in this sample of 13-year old children. Lower levels of VP function were associated with under-achievement in reading and in mathematics. Simple interventions can help children with VP difficulties, so research is needed into practicable, cost-effective strategies for identification and assessment, so that support can be targeted appropriately. 相似文献67.
68.
利用75%乙醇对向日葵根化学成分进行提取;再用萃取法将向日葵根化学成分乙醇提取物分为石油醚萃取物浸膏、乙酸乙酯萃取物漫膏、正丁醇萃取物浸膏;最后用常压硅胶色谱从石油醚萃取物漫膏中分离得到1个甾体类化合物A,经质谱、碳核磁共振和氢核磁共振谱鉴定,确定出其结构为β-谷甾醇。 相似文献
69.
α-乙酰乳酸脱羧酶产生菌的微波诱变 总被引:1,自引:0,他引:1
为获得α-乙酰乳酸脱羧酶的高产突变株,以产α-ALDC的枯草芽孢杆菌3226—5为出发菌株进行了诱变处理。经过微波(小火)物理诱变得到3株高产正突变株W181、W184、W195,经过多次传代实验,表明W181、W195是稳定的突变株。突变株W195的α-ALDC相对酶活(OD522)由出发菌株的0.35提高到0.617,提高了76%,突变株W181提高了66.9%。 相似文献
70.