探討酵母菌中R-loop結構與端粒重組之關聯

Characterization of the relationship between R-loop and telomere recombination in Saccharomyces cerevisiae

高郁雯(Yu-Wen Kao)

指導教授 ： 林敬哲

國立臺灣大學/醫學院/生物化學暨分子生物學研究所/碩士(2014年)

https://doi.org/10.6342/NTU.2014.10454

摘要

端粒是位於染色體末端的結構，在細胞功能中扮演重要的角色。除了避免染色體間的末端融合，還維持染色體的穩定性並協助複製的完成，也因此缺少端粒會造成細胞死亡或老化。在酵母菌Saccharomyces cerevisiae中，其端粒為250至300鹼基對(base pair)的TG1-3/C1-3A重複核酸序列組成，由於末端複製的問題，端粒長度隨著DNA複製次數增加而逐漸縮短，直到無法再維持細胞生長，則進入老化(senescence)階段。缺乏端粒酶活性的酵母菌細胞會利用端粒DNA重組(telomere recombination)的方式逃過老化的命運，而我們先前的研究發現到TERRA (telomere RNA transcript)會參與端粒重組的調控。TERRA是由RNA polymerase II由sub-telomeric區域往染色體末端所轉錄出一長片段non-coding的RNA，我們先前研究發現到TERRA會形成DNA：RNA hybrid的結構並促進老化及端粒重組發生。因此我們想知道R-loop結構，包含DNA：RNA hybrid及單股DNA的三股結構是否參與在端粒重組的調控，如果是的話，又是如何調控？首先，我們分析了在已知會影響R-loop生成的蛋白突變時所產生的端粒重組，觀察到過量表現RNase H1或RNase H2會延後老化及端粒重組，同樣地，在過量表現核酸螺旋酶，Sen1及Pif1，或是拓撲異構酶I (topoisomerase I)也都會延遲端粒重組，這些結果顯示R-loop結構與端粒重組有相關性。接著也利用DNA damage agents來阻礙TERRA的轉錄，發現確實會延遲端粒重組的發生。另外，我們發現到R-loop結構可能會經由引發類似於transcription-coupled repair (TCR)的機制而促使端粒重組的發生，當TCR起始蛋白Rad26發生突變則會延遲端粒重組發生，與之會形成複合體的Def1蛋白突變也會得到一樣的結果，兩者同時突變卻沒有更顯著的影響，表示兩者參與在同樣的端粒重組調控，而這樣的調控可能會被Rad28所抑制。在我的實驗當中觀察到TERRA會形成R-loop的結構進而促進端粒重組，且初步確認端粒重組會由TCR相關蛋白所調控。

關鍵字

端粒 ； 端粒重組

並列摘要

Telomere, the end structure of chromosome, plays an important role in cellular function that allows complete replication and maintains the integrity of chromosomes. Due to the end-replication problem, the telomere length gets shorter and shorter after DNA replications. The critically short telomere cannot support cell growth that eventually causes senescence. In S. cerevisiae cells lacking telomerase, telomere recombination is activated to bypass senescence. Our previous results show that telomere RNA transcript (TERRA) plays a role in regulating telomere recombination. TERRA is a long non-coding RNA that is transcribed by RNA polymerase II. We found TERRA associates with telomeres in a form of DNA:RNA hybrid to regulate senescence and telomere recombination. Here, we test if an R-loop structure is involved in telomere recombination. Using yeast Saccharomyces cerevisiae as a model system, we analyzed telomere recombination phenotype in telomerase-deficient cells with mutation of genes that affect R-loop formation. We found overexpression of RNase H1 or H2 delayed senescence and telomere recombination. Expressing excess amounts of helicases, Sen1 and Pif1, or topoisomerase I also delay telomere recombination. These results suggest a role of R-loop structure in telomere recombination. DNA damage agents that block transcription appeared to delay telomere recombination. Our results suggest for a role of TERRA in telomere recombination. Then, we want to figure out the regulation of telomere recombination. We found that Rad26, the initiator of transcription-coupled repair, is involved in telomere recombination. And so is Def1, which forms complex with Rad26. Specifically, we found TERRA in a form of R-loop is involved in regulating telomere recombination. And, telomere recombination may be regulated similarly through transcription-coupled repair mechanism. Since the telomere structure in yeast and human are virtually the same, it is likely that similar mechanism might be involved in regulating the formation of ALT (alternative lengthening of telomeres) cells during senescence in mammalian cells.

並列關鍵字

telomere ； R-loop ； telomere recombination

參考文獻

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