褪黑激素是由人類松果腺所分泌的一種荷爾蒙，不僅存在於脊椎動物，在原蟲、原生動物、真菌以及植物體中，皆發現褪黑激素的存在。但現今褪黑激素的研究只在動物中較為完整，已知褪黑激素可調節一些生理功能，例如情緒、睡眠、生物週期、免疫功能以及具有清除自由基的能力。Arylalkylamine N-acetyltransferase (AANAT) 是合成褪黑激素的關鍵酵素，而在酵母菌Saccharomyces cerevisiae中，一個AANAT的可能同源基因 (scAANAT) 已被證實具有與脊椎動物的AANAT類似之酵素活性，但此基因在酵母菌中的功能尚未明瞭，目前為止也無相關研究報導。本研究利用遺傳學方法，以scAANAT基因的大量表逹和剔除菌株作為研究材料，來探討在不同逆境下scAANAT基因在酵母菌細胞中所可能扮演之角色。結果發現，酵母菌中大量表現scAANAT基因會使其生長較遲緩。另外，當scAANAT被剔除後，酵母菌抗過氧化氫能力增強；scAANAT太過量表現時，酵母菌抗過氧化氫能力則不增反減，所以在過氧化氫逆境下，scAANAT扮演抑制生長的角色。在另一方面，scAANAT與酵母菌抗硫酸銅和氯化鈉逆境反應則無顯著關聯性。另外，本論文也利用生物資訊手法，進行資料庫搜尋及序列比對，找尋可能的植物AANAT或相關基因，結果卻找到了一群植物特有的GNAT (GCN5-related N-acetyltransferase) 類的乙醯基轉移酶，但其在植物體內真正的功能仍需進一步以分子生物學及轉殖植物研究策略加以探討。 Melatonin is an almost ubiquitous substance, which has been detected not only in vertebrates, but also in monera, protista, fungi, and plants. Little is known to date about its function except in vertebrates. In vertebrates, melatonin functions as a biological modulator of mood, sleep, circadian rhythms, immunologic function and free radical scavenger. Arylalkylamine N-acetyltransferase (AANAT) is the rate-limiting enzyme in melatonin synthesis. Although previous study has shown that a putative AANAT in yeast Saccharomyces cerevisiae, designated as scAANAT, possesses typical arylalkylamine N-acetyltransferase activity in vitro, the in vivo function of scAANAT remains uncharacterized. Using reverse genetic approach, this work aimed to study possible role of scAANAT in growth and stress response of yeast. Unexpectedly, the results showed that over-dose expression of scAANAT in yeast led to growth retardation under normal growth condition and deduction of resistance to hydrogen peroxide stress. On other hand, scAANAT is not involoved in yeast response to copper and salinity stresses. In addition, we employed a bioinformatic approach to explore putative plant AANAT homologous genes, and a group of novel GNATs unique in plants has been identified by database search. The results and possible studies necessary for further elucidating the role of melatonin and AANATs in growth and stress response in yeast and plants are discussed.
