Taq酶体外合成DNA时模板错位机制的探讨

利用Taq DNA聚合酶体外合成DNA过程中,当反应体系中缺少与模板链互补配对的dNTP底物时,产物合成并不会在底物缺失位点处终止,聚合反应继续进行.为研究此复制缺陷现象,设计一系列模板用于DNA体外酶促合成.除了已知的碱基错配机制,笔者发现存在另一种"模板错位"机制,即模板中与底物非Watson-Crick互补配对的碱基位点首先进行收缩滑动,形成模板bulge结构后再继续进行酶促合成反应.这项研究有助于提高DNA样品合成保真度以及继续深入探索体外DNA合成的详细机制.     

Fidelity of DNA replication catalysed in vitro on a natural DNA template by the T4 bacteriophage multi-enzyme complex

More than 50 copies of a phi X174 DNA template can be made in 60 min in an in vitro DNA replication system consisting of seven purfied replication proteins isolated from T4 bacteriophage-infected cells. By transfecting with the DNA products and assaying for the reversion of specific amber mutants, the high degree of base-pairing fidelity in this system is revealed; the in vitro system is also shown to respond to the mutagenic effect of Mn2+ and to display strong base-pair context effects on fidelity, as expected from in vivo studies.     

Single-molecule studies of the effect of template tension on T7 DNA polymerase activity

T7 DNA polymerase catalyses DNA replication in vitro at rates of more than 100 bases per second and has a 3'-->5' exonuclease (nucleotide removing) activity at a separate active site. This enzyme possesses a 'right hand' shape which is common to most polymerases with fingers, palm and thumb domains. The rate-limiting step for replication is thought to involve a conformational change between an 'open fingers' state in which the active site samples nucleotides, and a 'closed' state in which nucleotide incorporation occurs. DNA polymerase must function as a molecular motor converting chemical energy into mechanical force as it moves over the template. Here we show, using a single-molecule assay based on the differential elasticity of single-stranded and double-stranded DNA, that mechanical force is generated during the rate-limiting step and that the motor can work against a maximum template tension of approximately 34 pN. Estimates of the mechanical and entropic work done by the enzyme show that T7 DNA polymerase organizes two template bases in the polymerization site during each catalytic cycle. We also find a force-induced 100-fold increase in exonucleolysis above 40 pN.     

以病毒的230 kb DNA分子为模板的DNA序列测定

目的：为确认Tn3-gpt已插入小鼠巨细胞病毒(mouse cytomegalovirus,MCMV)基因组的特定位置,改进大分子病毒DNA的测序方法,建立以230kb DNA为模板直接进行DNA序列测定的方法。方法：待测MCMV的Tr3-gpt插入突变株感染NIH3T3细胞后,从培养液中制备病毒颗粒并从中纯化得到基因组DNA,以此DNA为模板,合成的寡核苷酸为引物,用热循环方法按Promega公司的DNA Cycle Sequencing System试剂盒的方法进行测序。结果：获得纯度较高可直接作为模板进行测序的：DNA,测序所得放射自显影图片条带清晰明亮,间隔正常,分辨率较高,可顺利读出插入位点的MCMV的DNA序列,证实Tn3-gpt插入在特定的位置。结论：长达230kb的大分子病毒DNA可直接作为模板,用热循环测序方法进行测序,无需切割成小片段后才进行测序。     

齐墩果酸、鞣花酸与拉米夫定体外联合抗HBV作用研究

为了观察拉米夫定（3TC）分别与齐墩果酸（OA）、鞣花酸（EA）联合应用抗乙型肝炎病毒（HBV）的作用,在HepG2．2．15细胞培养液中分别加入适当浓度的3TC、OA、EA、3TC＋OA、3TC＋EA含药培养液,每4d换用含同等药物浓度的新鲜培养液,第8d末取上清液,用ELISA法检测上清液中HbsAg和HbeAg的量,计算各药对HepG2．2．15细胞分泌HbsAg和HbeAg的抑制率,并用金正均Q值法分析联合用药的抗HBV效果．结果表明,3TC与OA合用,对HbeAg的抑制表现为拮抗作用,在低浓度时对HbsAg的抑制为协同作用,其它浓度时均为相加作用．3TC与EA合用,对HbeAg、HbsAg的抑制均呈相加作用,但对HbeAg的相加作用在所取浓度范围内比较稳定（Q值在0．85-0．90之间）,对HbsAg的相加作用则随药物作用浓度的增大而逐渐减弱．     

The conformation alteration of mouse hepatic histones after reacting with nicotine in vitro

UV differential spectroscopy, fluorescence spectroscopy and circular dichroism (CD) spectroscopy assays have been applied to studying the conformation alteration of mouse hepatic histones H1 and H3 after reacting with nicotine in vitro. The results indicate that their conformation changes from regular form to random form with the increasing reaction dose of nicotine. The adduction of nicotine or its metabolites with histones H1 and H3 accounts for the conformation alteration. Nicotine may affect the structure, function and expression of genes of chromosome by changing the conformation of histones.     

The conformation alteration of mouse hepatic histones after reacting with nicotinein vitro

UV differential spectroscopy, fluorescence spectroscopy and circular dichroism (CD) spectroscopy assays have been applied to studying the conformation alteration of mouse hepatic histones H1 and H3 after reacting with nicotinein vitro. The results indicate that their conformation changes from regular form to random form with the increasing reaction dose of nicotine. The adduction of nicotine or its metabolites with histones H1 and H3 accounts for the conformation alteration. Nicotine may affect the structure, function and expression of genes of chromosome by changing the conformation of histones.