真核基因启动子非依赖的功能转录延伸复合物的体外组装

真核生物RNA聚合酶Ⅱ的持续合成能力对基因转录过程中每一个阶段,包括启动子脱离、转录暂停、转录终止以及转录偶联DNA损伤修复过程的调节至关重要.在RNA聚合酶Ⅱ介导的转录延伸过程中,其和模板DNA及转录产物RNA紧密结合,形成一个非常稳定的延伸三维复合物(elongationcomplex,EC).此特征性“泡”状结构的形成是RNA聚合酶Ⅱ持续合成能力所必需的.在不依赖启动子及众多转录起始因子的条件下,利用人工合成的RNA与DNA寡核苷酸,在体外组装形成具有功能转录活性的延伸复合物.结果表明,长度为9个核苷酸的RNA与模板DNA形成的杂合分子对转录延伸复合物的形成是必需的,而非转录模板DNA链的加入导致最终活性转录“泡”状复合物的形成,并可转录形成与模板相关的转录产物,进一步通过在模板DNA的特定位置引入一个乙酰氧乙酰氨基芴修饰基团,可特异性地阻断转录延伸过程,从而显示该系统在研究真核基因转录及转录偶联DNA损伤修复机制中的潜在应用价值.     

Mechanism of RNA synthesis initiation by the vesicular stomatitis virus polymerase

The minimal RNA synthesis machinery of non-segmented negative-strand RNA viruses comprises a genomic RNA encased within a nucleocapsid protein (N-RNA), and associated with the RNA-dependent RNA polymerase (RdRP). The RdRP is contained within a viral large (L) protein, which associates with N-RNA through a phosphoprotein (P). Here, we define that vesicular stomatitis virus L initiates synthesis via a de-novo mechanism that does not require N or P, but depends on a high concentration of the first two nucleotides and specific template requirements. Purified L copies a template devoid of N, and P stimulates L initiation and processivity. Full processivity of the polymerase requires the template-associated N protein. This work provides new mechanistic insights into the workings of a minimal RNA synthesis machine shared by a broad group of important human, animal and plant pathogens, and defines a mechanism by which specific inhibitors of RNA synthesis function.     

Domain exchange: chimeras of Thermus aquaticus DNA polymerase, Escherichia coli DNA polymerase I and Thermotoga neapolitana DNA polymerase

The intervening domain of the thermostable Thermus aquaticus DNA polymerase (TAQ: polymerase), which has no catalytic activity, has been exchanged for the 3'-5' exonuclease domain of the homologous mesophile Escherichia coli DNA polymerase I (E.coli pol I) and the homologous thermostable Thermotoga neapolitana DNA polymerase (TNE: polymerase). Three chimeric DNA polymerases have been constructed using the three-dimensional (3D) structure of the Klenow fragment of the E.coli pol I and 3D models of the intervening and polymerase domains of the TAQ: polymerase and the TNE: polymerase: chimera TaqEc1 (exchange of residues 292-423 from TAQ: polymerase for residues 327-519 of E.coli pol I), chimera TaqTne1 (exchange of residues 292-423 of TAQ: polymerase for residues 295-485 of TNE: polymerase) and chimera TaqTne2 (exchange of residues 292-448 of TAQ: polymerase for residues 295-510 of TNE: polymerase). The chimera TaqEc1 showed characteristics from both parental polymerases at an intermediate temperature of 50 degrees C: high polymerase activity, processivity, 3'-5' exonuclease activity and proof-reading function. In comparison, the chimeras TaqTne1 and TaqTne2 showed no significant 3'-5' exonuclease activity and no proof-reading function. The chimera TaqTne1 showed an optimum temperature at 60 degrees C, decreased polymerase activity compared with the TAQ: polymerase and reduced processivity. The chimera TaqTne2 showed high polymerase activity at 72 degrees C, processivity and less reduced thermostability compared with the chimera TaqTne1.