葉綠體DNA

葉綠體DNA(Chloroplast DNA,cpDNA)為真核生物細胞葉綠體內的DNA。葉綠體等质粒体中具有一獨立於細胞核基因組,且含有核糖體轉譯合成自身的蛋白質[1][2],為一半自主的胞器,葉綠體DNA最早於1959年透過生化實驗發現[3],並於1962年透過電子顯微鏡實際觀測確認[4]。1986年菸草地錢英语Marchantia polymorpha的葉綠體DNA被定序發表,為最早完成定序的葉綠體基因組[5][6],目前已有上百種植物與藻類英语List of sequenced plastomes的质粒体被定序發表。

模式植物阿拉伯芥的葉綠體DNA基因組

基因結構编辑

葉綠體DNA為環狀,長度一般介於120至170kb之間[7][8][9],分子量為8000萬至1.3億Da[10],多數植物的葉綠體包含約120個基因[11][12],大多編碼光合作用所需的蛋白(包括RuBisCO的大次單元)與基因表現所需的蛋白[13],如4種rRNA、約30種tRNA、21種核糖體蛋白以及葉綠體RNA聚合酶的4個次單元[13]。絕大多數生物的葉綠體DNA均不分段,雙鞭毛蟲門的藻類則為罕見例外,此類生物的葉綠體DNA包括約40段質體,每個質體長2至10kb,包含1至3個基因,也有些質體不編碼任何基因[14]

多數葉綠體DNA包括兩段反向重複序列(IRa與IRb),將葉綠體DNA分成大單拷貝區(LSC)與小單拷貝區(SSC)兩區域[9]。反向重複序列長4至25kb,其中植物的一般在20至25kb之間[15],IRa與IRb的序列一般會因協同演化英语Concerted evolution而非常近似[14]。植物葉綠體DNA的反向重複序列演化上相當保守[9][15]藍菌基因組與灰藻紅藻的葉綠體基因組中也有與其同源的序列,顯示此序列在演化上的起源很早,在葉綠體出現前即已存在[14]。有些葉綠體(豌豆與數種紅藻[14])的基因組丟失了反向重複序列[15][16]紫菜屬英语Porphyra紅藻葉綠體則有一個重複序列發生了倒轉,使IRa與IRb變為同向排列[14]。反向重複序列可能可幫助維持葉綠體DNA的穩定[16]

陸生植物新葉的葉綠體中一般有約100個葉綠體DNA,老葉中則僅剩15至20個[17],多包裹成擬核,一個葉綠體中常有數個擬核[10]。葉綠體DNA雖不與組蛋白結合[18],但紅藻的葉綠體DNA可編碼和組蛋白相似的組蛋白樣葉綠體蛋白(HC)與自身結合[19]。較原始的紅藻Cyanidioschyzon merolae英语Cyanidioschyzon merolae(屬溫泉紅藻綱)的葉綠體擬核集中在葉綠體基質的中央,綠藻與植物的葉綠體擬核則均勻分布於基質中[19]

 
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菸草的葉綠體DNA基因組,標示於內側的基因位於DNA的同一股上,標示於外側者則位於另一股,基因中的缺口為內含子

參見编辑

參考文獻编辑

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