生物谷報道：一段時間以來,，科研人員致力于破譯基因轉(zhuǎn)錄所需的分子機器聚集到基因轉(zhuǎn)錄起始位點的順序,。以Wistar研究所研究人員為首的研究小組最近發(fā)現(xiàn)，基因-轉(zhuǎn)錄機器到達(dá)正確位點后,，并不是立刻開始行動,，而是在某種監(jiān)控下“蓄勢待發(fā)”，直到某種激發(fā)信號“通知”它沿著線狀DNA分子前進(jìn),。詳細(xì)內(nèi)容刊登于Molecular Cell雜志,。
成套關(guān)鍵基因面對壓力刺激或者其它致命性需要時，能夠被瞬時激活,。比如胚胎干細(xì)胞就擁有無數(shù)處于這種狀態(tài)的基因,。Wistar研究人員的新數(shù)據(jù)為闡明這種現(xiàn)象提供了一種機制。
研究人員將關(guān)注點放在泛素（ubiquitin）分子上,。泛素到達(dá)位點后似乎會抑制已經(jīng)聚集好的轉(zhuǎn)錄機器進(jìn)行轉(zhuǎn)錄,。泛素一旦離開，核心中攜帶RNA聚合酶II的轉(zhuǎn)錄機器獲得自由,，轉(zhuǎn)錄過程被啟動,。研究過程是在酵母（一種經(jīng)常被用于遺傳研究的模式生物）中進(jìn)行的。
文章高級作者,、Wistar研究所Shelley L. Berger博士說,，本次實驗中我們發(fā)現(xiàn)，聚合酶附著在基因上,，在泛素離開的時候被激活,。來自其它實驗室的證據(jù)提示，干細(xì)胞中可能有許多基因以這種方式處于平衡狀態(tài),，準(zhǔn)備驅(qū)動細(xì)胞采取各種各樣的發(fā)育途徑,，形成不同的組織。很有可能許多性命攸關(guān)的細(xì)胞功能依賴于一種由這種過程調(diào)節(jié)的快反應(yīng)。
早期工作中,，Berger與其同事希望通過添加或者移除一個小蛋白（泛素修飾的特異組蛋白）對基因表達(dá)進(jìn)行調(diào)控,。組蛋白被DNA緊密纏繞后形成的盤狀結(jié)構(gòu)是核小體，核小體就像是穿在DNA鏈上的珠子,。核小體鏈經(jīng)過進(jìn)一步的盤繞,、折疊而成染色質(zhì)——染色體的基本物質(zhì)。已知對組蛋白的特定修飾與基因的抑制或活化有關(guān),。早期研究獲得的一個關(guān)鍵發(fā)現(xiàn)是添加和移除泛素都是優(yōu)化轉(zhuǎn)錄所必需的,。
此次研究過程中，Berger等人抑制泛素的離開,，以求闡明離開步驟的重要性和所導(dǎo)致的結(jié)果,。文章第一作者Anastasia Wyce說，如果泛素不能離開組蛋白,，那么某種在正常情況下會參與啟動基因表達(dá)的酶不能活化,。聚合酶體停留在基因的起始位點，但不能被激活,。泛素似乎是一個檢查點,，離開后聚合酶才能完全發(fā)揮功能。
泛素是一種存在于真核生物中且在進(jìn)化中高度保守的小分子蛋白,，含76個氨基酸殘基,。在蛋白酶體中進(jìn)行的底物蛋白泛素化過程依賴一個復(fù)雜的系統(tǒng)，它包括泛素活化酶（E1）,、泛素-聯(lián)連酶（E2）和底物識別蛋白（E3酶）,。泛素在真核生物體內(nèi)廣泛存在,泛素化修飾是轉(zhuǎn)錄后的修飾方式之一;組蛋白是染色質(zhì)的主要成分之一,與基因的表達(dá)有密切關(guān)系.組蛋白的泛素化修飾與經(jīng)典的蛋白質(zhì)的泛素調(diào)節(jié)途徑不同,不會導(dǎo)致蛋白質(zhì)的降解,但是能夠招募核小體到染色體、參與X染色體的失活,、影響組蛋白的甲基化和基因的轉(zhuǎn)錄.組蛋白的去泛素化修飾同樣與染色質(zhì)的結(jié)構(gòu)及基因表達(dá)密切相關(guān).組蛋白的泛素化和磷酸化,、乙酰化,、甲基化修飾之間還存在協(xié)同和級聯(lián)效應(yīng),。
原始出處:
Molecular Cell, Vol 27, 275-288, 20 July 2007
Article
H2B Ubiquitylation Acts as a Barrier to Ctk1 Nucleosomal Recruitment Prior to Removal by Ubp8 within a SAGA-Related Complex
Anastasia Wyce,1,2 Tiaojiang Xiao,3 Kelly A. Whelan,1 Christine Kosman,1,2 Wendy Walter,1 Dirk Eick,4 Timothy R. Hughes,5 Nevan J. Krogan,6 Brian D. Strahl,3 and Shelley L. Berger1,
1 Gene Expression and Regulation Program, The Wistar Institute, Philadelphia, PA 19104, USA
2 University of Pennsylvania School of Medicine, Philadelphia, PA 19104, USA
3 Department of Biochemistry and Biophysics, University of North Carolina School of Medicine, Chapel Hill, NC 27599, USA
4 Institute of Clinical Molecular Biology and Tumour Genetics, GSF-National Research Center for Environment and Health, Marchioninistrasse 25, D-81377 Munich, Germany
5 Banting and Best Department of Medical Research, University of Toronto, Toronto, ON M5G 1L6, Canada
6 Department of Cellular and Molecular Pharmacology, University of California, San Francisco, San Francisco, CA 94143, USA
Corresponding author
Shelley L. Berger
[email protected]
Histone modifications play an important role in transcription. We previously studied histone H2B ubiquitylation on lysine 123 and subsequent deubiquitylation by SAGA-associated Ubp8. Unlike other histone modifications, both the addition and removal of ubiquitin are required for optimal transcription. Here we report that deubiquitylation of H2B is important for recruitment of a complex containing the kinase Ctk1, resulting in phosphorylation of the RNA polymerase II (Pol II) C-terminal domain (CTD), and for subsequent recruitment of the Set2 methyltransferase. We find that Ctk1 interacts with histones H2A and H2B, and that persistent H2B ubiquitylation disrupts these interactions. We further show that Ubp8 enters the GAL1 coding region through an interaction with Pol II. These findings reveal a mechanism by which H2B ubiquitylation acts as a barrier to Ctk1 association with active genes, while subsequent deubiquitylation by Ubp8 triggers Ctk1 recruitment at the appropriate point in activation.
