Kinetic analysis of the coding properties of O6-methylguanine in DNA: the crucial role of the conformation of the phosphodiester bond

Production by N-nitroso compounds of O6-alkylguanine (O6-alkylG) in DNA directs the misincorporation of thymine during DNA replication, leading to G:C to A:T transition mutations, despite the fact that DNA containing O6-alkylG:T base pairs is less stable than that containing O6-alkylG:C pairs. We have examined the kinetics of incorporation by Klenow fragment (KF) of Escherichia coli DNA polymerase I of thymine (T) and of cytosine (C) opposite O6-MeG in the template DNA strand. Both T and C were incorporated opposite O6-MeG much slower than nucleotides forming regular A:T or G:C base pairs. Using various concentrations of dTTP, dCTP, or their phosphorothioate (Sp)-dNTP alpha S analogues, or a mixture of dTTP and dCTP, the progress of incorporation of a single nucleotide in a single catalytic cycle of a preformed KF-DNA complex was measured (pre-steady-state kinetics). The results were consistent with the kinetic scheme (Kuchta, R. D., Benkovic, P., & Benkovic, S. J. (1988) Biochemistry 27, 6716-6725): (1) binding of dNTP to polymerase-DNA; (2) conformational change in polymerase; (3) formation of phosphodiester between the dNTP and the 3'-OH of the primer; (4) conformational change of polymerase; (5) release of pyrophosphate. The results were analyzed mathematically to identify the steps at which the rate constants differ significantly between the incorporation of T and C. The only significant difference was the 5-fold difference in the rates of formation of the phosphodiester bond (for dTTP, kforward = 3.9 s-1 and kback = 1.9 s-1; for dCTP, kforward = 0.7 s-1 and kback = 0.9 s-1). These pre-steady-state progress curves were biphasic with a rapid initial burst followed by an apparently steady-state rise. Deconvolution of these curves gave direct evidence for the importance of the conformational change after polymerization by showing that the curves represented the sum of the rapid accumulation of the product of step 3 followed by the slow conversion of that to the product of step 5 (because of the rapidity of the release of pyrophosphate there was no significant accumulation of the product of step 4). The equilibrium constants for each step suggest that the greatest change in the Gibbs free energy occurs at the conformational change after polymerization and that while the formation of the phosphodiester bond to T is slightly exothermic, that to C is slightly endothermic.(ABSTRACT TRUNCATED AT 400 WORDS)     

采用区内C／I平衡的多区峰窝CDMA系统下行链路的性能

本文对采用区内Ｃ／Ｉ平衡的多区蜂窝ＣＤＭＡ系统下行链路的性能进行分析，给出两种区内Ｃ／Ｉ平衡算法并比较它们的性能，考虑呈对数正态分布的阴影和Ｒ＾－４规律的路径损失的综合影响，采用与传统不同的小区平均中断概率来评价系统的性能。     

锗γ能谱测量中的符合相加修正

本文介绍了锗γ能谱测量中符合相加修正的原理和方法,给出了修正公式。分别计算了不同几何条件下两个锗探测器对一些放射性核素的符合相加修正因子,估计了符合相加修正因子对活度测量结果的误差贡献。     

Y-89在新桥矿选铜厂的应用实践

新桥矿选铜厂1999年使用捕收剂Y－89后，选铜回收率提高了4．82％，获得了显著的经济效益。     

超细水泥封堵技术

注水开发后期日益出现的地质问题及油套管的严重损坏,制约了油田的开发效果。针对河南油田注水开发中存在的上述问题,通过对超细水泥堵剂配方、堵剂性能、适应性等方面的研究,可实现对油井的高含水层、注水井的停注无用层、油水井套管外窜槽、穿孔、丝扣处的漏失等方面的封堵,取得了显著效果。自1998年8月-2001年12月在河南油田应用61井次,有效率85.3%,其中油井36井次,年度累计增油1.441×104t,降水7.6123×104m3;水井封堵25井次,累计增注14.32×104m3,控制无效注水16.95×104m3,创效益1800多万元,投入产出比达1:11.36,具有广泛的推广应用价值。     

加氢裂化热高分油管线弯头开裂分析

对国产加氢裂化热高分油管线的弯头开裂原因进行了分析 ,对预防连多硫酸腐蚀提出了相应的防范措施 :选用碳含量 <0 .0 3 %并含有稳定化元素Ti、Nb的奥氏体不锈钢或双相钢 ;控制焊缝硬度不大于HB2 0 0并焊后热处理 ;12CrMo和 15CrMo耐应力腐蚀性能好 ;设备停运后马上进行清洗并用碱液中和     

Skeletal Isomerization and Inter—molecular Hydrogen Transfer Reactions in Catalytic Cracking

Bimolecular hydrogen transfer and skeletal isomerization the important secondary reac-tions among catalytic cracking reactions,which affect product yield distribution and product quality,Catalyst properties and operating parameters have great impact on bimolecular hydrogen transfer and skeletal isomerization reactions .Bimolecular hydrogen transfer activity and skeletal isomrization activity of USY-containing catalysts are higher thn that of ZSM-5-containing catalyst.Coke deposition on the active sites of catalyst may suppress bimolecular hydrogen transfer activity and skeletal isomer-ization activity of catlys in different degrees.Short raction time causes a decrease of hydrogen trans-fer reaction,but and increase of skeletal isomerization reaction compared to cracking reaction in catalytic cracking process.     

假钞的公众识别方法

一、假钞公众识别的必要性马克思在《资本论》中有一段名言：“一旦有适当的利润，资本就胆大起来。如果有10%的利润，它就保证到处使用。有20%的利润，它就活跃起来。有50%的利润，它就铤而走险。为了100%的利润，它就敢践踏一切人间法律。有300%的利润，它就敢犯任何罪行，甚至冒着绞首的危险。”伪造货币“创造”的利润之巨不言而喻，而其危害性也人所共知。轻者给个人造成损失，特别是经济损失。重者扰乱金融秩序，影响社会的稳定。在世界范围内，所有国家都将伪造货币视为严重犯罪，对伪造者处以极刑。对普通大众而言，了解假钞的公众…     

深基坑支护结构计算的有限元方法

针对软土地基的特性，本文提出了考虑土体抗力释放与结构模型随施工过程改变的新杆系有限元法，并 编制了求解支护结构内力与变形的计算程序，通过对事例分析．计算结果与实际情况非常吻合，证明了该方法的合理 性。     

A concise process technology for 3-D suspended radio frequency micro-inductors on silicon substrate

In this letter, a concise process technology is proposed for the first time to enable the fabrication of good quality three-dimensional (3-D) suspended radio frequency (RF) micro-inductors on bulk silicon, without utilizing the lithography process on sidewall and trench-bottom patterning. Samples were fabricated to demonstrate the applicability of the proposed process technology.