近日來自浙江大學(xué)生物技術(shù)研究所和美國(guó)俄亥俄州立大學(xué)的研究人員在新研究中發(fā)現(xiàn)了雙生病毒克服植物甲基化修飾以及抑制轉(zhuǎn)錄水平基因沉默的新機(jī)制。相關(guān)研究論文發(fā)表在病毒學(xué)和病原生物學(xué)領(lǐng)域?qū)W術(shù)期刊PLoS Pathogens(IF：9.079）上,。
浙江大學(xué)長(zhǎng)江學(xué)者特聘教授周雪平和美國(guó)俄亥俄州立大學(xué)的David M. Bisaro教授為這篇文章的共同通訊作者,。周雪平教授實(shí)驗(yàn)室的謝艷副教授和楊秀玲博士為該論文的共同第一作者。這一項(xiàng)目得到了國(guó)家基礎(chǔ)研究計(jì)劃（973）和國(guó)家自然科學(xué)基金重點(diǎn)項(xiàng)目的資助,。
雙生病毒是植物病毒中唯一一類具有孿生顆粒形態(tài)的單鏈環(huán)狀DNA病毒,，可對(duì)番茄、棉花,、木薯,、豆類、小麥,、玉米等多種重要經(jīng)濟(jì)作物上引起毀滅性的危害,。全球50多個(gè)國(guó)家的經(jīng)濟(jì)作物均受到這一病毒的影響，而受到廣泛關(guān)注,。然而由于雙生病毒病蔓延迅速,，其控制存在很大的難度。
DNA甲基化是植物,、動(dòng)物以及真菌中比較保守的一類表觀遺傳修飾,，在調(diào)控基因表達(dá)、基因組印記,、X染色體失活,、轉(zhuǎn)錄水平基因沉默(Transcriptional gene silencing, TGS)以及防御DNA病毒的侵染中發(fā)揮重要作用。植物可以通過DNA甲基化修飾及抑制TGS來抵御雙生病毒的侵染,，干擾病毒的復(fù)制和轉(zhuǎn)錄,；而雙生病毒需要逃脫植物的甲基化修飾及TGS才能致病，因此雙生病毒克服植物甲基化修飾及TGS的機(jī)制一直是病毒學(xué)領(lǐng)域的研究熱點(diǎn),。
在這篇文章中,，研究人員以伴隨有衛(wèi)星DNA的中國(guó)番茄黃曲葉病毒（TYLCCNV）為研究對(duì)象解析了病毒克服植物甲基化修飾及TGS反應(yīng)的分子機(jī)制。該研究發(fā)現(xiàn)雙生病毒TYLCCNV并不能有效的抑制甲基化或TGS,，病毒基因組DNA在病毒侵染的植物中普遍發(fā)生甲基化,，通過對(duì)病毒全基因組的甲基化水平測(cè)定，發(fā)現(xiàn)甲基化主要集中于病毒的啟動(dòng)子區(qū),；但當(dāng)雙生病毒TYLCCNV與病毒的衛(wèi)星DNA共同侵染時(shí),，衛(wèi)星DNA能夠顯著降低TYLCCNV全基因組的甲基化水平；衛(wèi)星DNA能夠有效抑制TGS,，并使本氏煙中轉(zhuǎn)錄水平沉默的GFP轉(zhuǎn)基因表達(dá),，而衛(wèi)星DNA抑制TGS主要由其編碼的βC1蛋白發(fā)揮作用,；βC1還能回復(fù)TGS本氏煙中轉(zhuǎn)錄水平沉默的GFP轉(zhuǎn)基因的表達(dá)，能夠激活擬南芥中F-box等內(nèi)源表觀沉默位點(diǎn)的表達(dá),，并顯著降低擬南芥基因組的甲基化水平,。進(jìn)一步的研究表明βC1能夠在體內(nèi)與甲基循環(huán)中的關(guān)鍵酶S-腺苷高半胍氨酸水解酶(SAHH)互作，βC1與SAHH互作后能夠使SAHH的活性降低80%左右,。SAHH是甲基循環(huán)過程中參與甲基化介導(dǎo)的TGS的核心組份,，βC1通過與SAHH的互作來降低SAHH的活性，從而達(dá)到抑制甲基化和TGS的目的,。
該研究對(duì)詮釋作物抵御雙生病毒侵染及雙生病毒逃避作物防御的分子機(jī)制具有重要意義,，并為植物抗病毒提供了新理論和新策略。（生物谷Bioon.com）
doi:10.1371/journal.ppat.1002329
PMC:
PMID:
Suppression of Methylation-Mediated Transcriptional Gene Silencing by βC1-SAHH Protein Interaction during Geminivirus-Betasatellite Infection
Xiuling Yang1#, Yan Xie1#, Priya Raja2¤, Sizhun Li2, Jamie N. Wolf2, Qingtang Shen1, David M. Bisaro2*, Xueping Zhou1*
DNA methylation is a fundamental epigenetic modification that regulates gene expression and represses endogenous transposons and invading DNA viruses. As a counter-defense, the geminiviruses encode proteins that inhibit methylation and transcriptional gene silencing (TGS). Some geminiviruses have acquired a betasatellite called DNA β. This study presents evidence that suppression of methylation-mediated TGS by the sole betasatellite-encoded protein, βC1, is crucial to the association of Tomato yellow leaf curl China virus (TYLCCNV) with its betasatellite (TYLCCNB). We show that TYLCCNB complements Beet curly top virus (BCTV) L2- mutants deficient for methylation inhibition and TGS suppression, and that cytosine methylation levels in BCTV and TYLCCNV genomes, as well as the host genome, are substantially reduced by TYLCCNB or βC1 expression. We also demonstrate that while TYLCCNB or βC1 expression can reverse TGS, TYLCCNV by itself is ineffective. Thus its AC2/AL2 protein, known to have suppression activity in other geminiviruses, is likely a natural mutant in this respect. A yeast two-hybrid screen of candidate proteins, followed by bimolecular fluorescence complementation analysis, revealed that βC1 interacts with S-adenosyl homocysteine hydrolase (SAHH), a methyl cycle enzyme required for TGS. We further demonstrate that βC1 protein inhibits SAHH activity in vitro. That βC1 and other geminivirus proteins target the methyl cycle suggests that limiting its product, S-adenosyl methionine, may be a common viral strategy for methylation interference. We propose that inhibition of methylation and TGS by βC1 stabilizes geminivirus/betasatellite complexes.
