生物谷报道：启动环（trigger loop，TL）结构是真核生物和原核生物RNA聚合酶中的一个高度保守区域，但它在转录过程的确切功能目前还不甚了解。美国斯坦福大学的Kaplan等人的最新研究发现，TL在底物选择过程中发挥重要功能，是真菌毒素α鹅膏覃碱（α-amanitin）的直接作用位点。该研究结果发表于6月5日的《分子细胞》（Molecular Cell）上。
以往研究表明，TL结构直接与新合成RNA的3'端和NTP相互作用，其中一个保守的组氨酸残基直接与NTP底物相结合。Kaplan等人用其他氨基酸替代酿酒酵母RNA聚合酶Ⅱ中TL结构中的His1085残基，结果导致酿酒酵母表现出严重的生长缺陷，甚至无法存活。离体实验表明，His1085能够选择正确的NTP底物，该位点变异后，聚合酶Ⅱ掺入错误的NTP底物和2'dNTP底物的可能性大大增加。
α鹅膏覃碱能够抑制RNA聚合酶Ⅱ的延伸速率，降低其底物选择性，但His1085被替代后的RNA聚合酶Ⅱ对α鹅膏覃碱则具有较高的抗性。研究人员进一步研究了纯化的RNA聚合酶－α鹅膏覃碱复合体的晶体结构，发现α鹅膏覃碱和TL结构间存在直接的相互作用。因此，研究人员认为，α鹅膏覃碱通过直接作用于TL结构而降低RNA聚合酶Ⅱ的速率和保真度。（生物谷www.bioon.com）
生物谷推荐原始出处：
Molecular Cell，Vol 30, 547-556, 05 June 2008，Craig D. Kaplan, Roger D. Kornberg
The RNA Polymerase II Trigger Loop Functions in Substrate Selection and Is Directly Targeted by α-Amanitin
Craig D. Kaplan,1, Karl-Magnus Larsson,1 and Roger D. Kornberg1
1 Department of Structural Biology, Stanford University, Stanford, CA 94305, USA
Corresponding author
Summary
Structural, biochemical, and genetic studies have led to proposals that a mobile element of multisubunit RNA polymerases, the Trigger Loop (TL), plays a critical role in catalysis and can be targeted by antibiotic inhibitors. Here we present evidence that the Saccharomyces cerevisiae RNA Polymerase II (Pol II) TL participates in substrate selection. Amino acid substitutions within the Pol II TL preferentially alter substrate usage and enzyme fidelity, as does inhibition of transcription by α-amanitin. Finally, substitution of His1085 in the TL specifically renders Pol II highly resistant to α-amanitin, indicating a functional interaction between His1085 and α-amanitin that is supported by rerefinement of an α-amanitin-Pol II crystal structure. We propose that α-amanitin-inhibited Pol II elongation, which is slow and exhibits reduced substrate selectivity, results from direct α-amanitin interference with the TL.
