PpMRG1藉由辨識H3K36me3參與光調控mRNA選擇性剪接

Abstract

我們以小立碗藓（Physcomitrella patens）為模式植物，研究植物在照到光之後所引發信使核糖核酸前驅物(pre-mRNA)選擇性剪接(alternative splicing, AS)的現象。選擇性剪接可將相同的信使核糖核酸前驅物剪接成不同的信使核糖核酸，此機制可以改變蛋白質組成來調控生長發育各階段的反應。實驗室先前的研究發現植物照到光之後發生頻率最高的選擇性剪接，是內插子保留(intron retention, IR)的現象，而且有很多內插子保留的調控發生在核糖體蛋白(ribosomal proteins)的信使核糖核酸前驅物上。由於過去曾有動物系統的文獻指出轉錄的過程中，信使核糖核酸選擇性剪接伴隨著組蛋白修飾(histone modification)同時進行。我參考動物系統的文獻並加以測試，以暸解植物中信使核糖核酸選擇性剪接的分子調控機制。我的研究發現，小立碗藓照光之後在受光調控而保留內插子的基因座上，組蛋白3第36號離氨酸被加上三個甲基 （H3K36me3)的機制受到調控；小立碗藓的MRG1 (MORF-Related Gene 1, PpMRG1) 蛋白會辨識H3K36me3並參與許多核糖體蛋白信使核糖核酸前驅物的剪接，選擇性保留信使核糖核酸前驅物的內插子。我們也嘗試找出MRG1對植物向光性的影響，發現PpMRG1的突變株對紅光誘導的向光性不敏感。這些結果結合動物系統的研究，我推測在紅光的處理後，H3K36me3的累積在個別基因座的變化，間接影響許多內插子保留現象發生在核糖體蛋白的信使核糖核酸前驅物上，可能使得最終核糖體蛋白結構改變，或使大小次單元的核糖體蛋白重組，而對特定現有的信使核糖核酸進行轉譯，使植物可以快速的對光線的變化作出反應。

Plants perceive dynamic light conditions and optimize their growth and development accordingly by mainly regulating gene expression at multiple layers. Alternative splicing (AS), a widespread mechanism in eukaryotes that post-transcriptionally generates two or more mRNAs from the same pre-mRNA, is rapidly controlled by light. However, detailed mechanism of light-regulated AS is still not clear. In this study, I demonstrate that histone 3 lysine 36 trimethylation (H3K36me3) rapidly and differentially responds to light at specific gene loci with light-regulated intron retention (IR) of their transcripts in the moss Physcomitrella patens. However, the level of H3K36me3 following exposure to light is inversely related to that of IR events. P. patens MORF-Related Gene 1 (PpMRG1), a chromatin adaptor, bound with higher affinity to H3K36me3 in light conditions than in darkness and was differentially targeted to gene loci showing light-responsive IR. Transcriptome analysis indicated that PpMRG1 functions in light-mediated splicing regulation. We also found PpMRG1 associated with a hnRNP-K like splicing regulator to potentially control AS. Furthermore, PpMRG1 was also involved in red-light-mediated phototropic responses. my results suggest that light regulates histone methylation, which leads to alterations of AS patterns. The chromatin adaptor PpMRG1 potentially participates in light-mediated AS, revealing that chromatin-coupled regulation of pre-mRNA splicing is an important aspect of the plant’s response to environmental changes.