| 摘要: | 溫室效應導致了全球各地出現極端氣候，如極端溫度、暴雨和乾旱等諸多環境逆境，這些逆境已嚴重影響農作物的存活率及產量，農作物中如水稻皆為固著性生物，因此了解植物如何抵抗逆境，並加以提高逆境耐受能力為改良作物生存率和提高產量的重要方向之一。一般而言，植物在面對逆境時，內部快速的基因調控影響了後續諸多蛋白的表達，為適應及對抗逆境的重要關鍵，而基因調控可初步分為轉錄階層與後轉錄階層，針對轉錄階層如轉錄因子如何決定植物在逆境時的生存策略已有許多研究，後轉錄階層則是相對較少，本篇文章即是以後轉錄調控裡mRNA降解的初始步驟-deadenylation (Poly(A) tail的分解)為研究主題。
酵母菌以及哺乳類研究已指明Poly(A) tail的降解主要由CCR4-NOT1複合體中的CAF1以及CCR4所負責，且在生化與分生的基礎研究上提出了許多嚴謹的機制，說明這些酵素藉由影響Poly(A) tail的長度，改變了mRNA的穩定度，使mRNA走向降解的途徑或使轉譯效率降低，導致標的蛋白產量下降而改變了細胞原有的表型。目前在植物的研究裡，我們雖然得知CCR4和CAF1的表現量會影響逆境下的表型，但相關機制不若酵母菌和人類明朗，本篇文章以水稻CAF1和CCR4為主，探討以下有趣的發現：一、同為複合體的一員且具有酵素活性，植物CAF1相較於CCR4演化出較多的同源性基因，且CAF1成員之間在胺基酸的相似度上有較高的差異性。二、植物CCR4缺乏其他物種所包含的LRR domain但演化出Mynd domain，使其可以和CAF1結合。三、正常環境下，只有水稻CAF1B-GFP會出現在細胞降解mRNA的P-bodies結構中，然而後續發現CAF1H-GFP在受熱後也會出現顆粒狀結構。四、不同CAF1成員在環境逆境下有不同的表達量，如只有CAF1H在受熱時的表現模式與熱休克蛋白類似。五、在實驗設計的熱處理下，CAF1H會影響水稻幼苗的生長。這些發現使我們了解水稻CAF1成員雖皆有相同酵素活性，但對環境逆境可能有不同作用機制，並據此假設水稻CAF1H在熱逆境時，會結合其他熱誘導的mRNA結合蛋白質，進而出現在P-bodies中，惟CAF1H的酵素活性是否發揮作用和其作用階段、標的基因有哪些仍有待未來研究。
;The greenhouse effect has led to extreme climate around the world, such as extreme temperatures, heavy rain, drought and so on. These severe environmental conditions have seriously affected the survival and yield of crops. Understanding how crops like rice are resistant to environmental stresses is one of the important directions for improving survival and yields under different types of stresses. In general, rapid gene regulation affecting the subsequent protein production is one of important strategy to adapt and against environmental stresses in plants. Moreover, gene regulation can be preliminarily classified into transcriptional level and post-transcriptional level. In transcriptional level, for example, many studies have determined mechanisms how transcription factors affect stress tolerances. However, in contrast to transcriptional level, mechanisms of plants in post-transcriptional level is not well known. Thus, our subject of this article focuses on the mechanism of deadenylation, an initial step of mRNA turnover, in rice.
Studies in yeasts and mammals have indicated the deadenylation is mainly mediated by subunits of CCR4-NOT1 complex, CCR4 and CAF1. Also, it’s thought these deadenylation enzymes affects mRNA stability via poly(A) tail shortening, leading mRNA turnover or reducing translation efficiency. Although in plants, it has been proven that CCR4 and CAF1 affect the stress tolerance, related mechanisms aren’t clear. In this article, we provide some interesting findings as follows. First, not like its partner CCR4, the expansion of CAF1 homologs are distinct in rice. Among these rice CAF1 members, the divergence of protein sequences is obvious. Second, plant CCR4 members lacking LRR domain contain Mynd domain to interact with CAF1 in evolution. Third, only CAF1B-GFP located in P-bodies under normal condition, but CAF1H-GFP also aggregated as granule structures under heat stress. Fourth, expression patterns of each CAF1 member are divergent under different types of treatment. For example, only CAF1H expression is induced as well as small Hsps under heat stress. Fifth, CAF1H affected heat tolerance and seedling growth under heat stress in our experimental condition. These findings promote us a model which supposes CAF1H is recruited to P-bodies by other heat-induced mRNA binding proteins under heat stress. However, it should be further examined whether the function of CAF1H is involved in deadenylation and which genes are targets. |
