摘要:擬南芥與水稻是植物基因組中的模式生物,植物的基因功能及基因結構間的相關聯系一直是研究的重點,通過對這2種模式生物進行基因敲除來確定基因功能是目前對植物基因研究最為普遍的做法。目前,關于基因功能突變體的數據資源的介紹并不充分,針對這種情況,本文介紹了基因功能研究的大規(guī)模隨機敲除突變的常見載體標簽及其方法,并給出了與其相應的相關數據庫的介紹。
關鍵詞:基因敲除;數據庫;模式生物;標簽
中圖分類號: Q754文獻標志碼: A文章編號:1002-1302(2014)02-0039-04
收稿日期:2013-07-18
作者簡介:潘曉寒(1988—),女,江蘇宜興人,碩士研究生,研究方向為生物進化。E-mail:huajianji00@163.com。目前,基因研究已逐漸由結構基因組學向功能基因組學領域展開。擬南芥和水稻作為模式生物,在植物中率先展開基因功能的研究[1]。目前,對基因功能的研究方法有許多種,而建立敲除突變體庫是最常見的方法[2]?;蚯贸侵赣梅椒ㄊ怪参锘蚴Щ睿缓笸ㄟ^觀察表型來確定基因結構和功能的關系。最初,γ射線、化學誘變劑EMS等物理化學誘變方法被用來制作突變體[3],之后同源重組[4]、基因沉默、RNA干擾[5]等方法也被用來制作突變體,但這些方法都不足以在多細胞生物體中構建能夠包含所有基因的突變體庫。自農桿菌在單子葉植物中的轉化技術獲得成功后,利用外源序列對植物基因組的大規(guī)模插入來構建突變體庫成為最常用也是最可靠的辦法[6-7]。根據插入元件的不同,大規(guī)模外源基因的插入建立的突變體庫可以大致分為3種:T-DNA插入構建的突變體庫[8-9]、轉座子插入構建的突變體庫[10-11]以及反轉座子插入構建的突變體庫[12-13]。目前在全世界范圍內已建立了大量的上述3種元件作為突變工具的水稻和擬南芥的突變數據[14-15]。
隨著研究深入,更多的突變體庫在全世界范圍內建立起來[16],在擬南芥中更是實現了幾乎全部基因都有敲除突變的高覆蓋率[2]。在其他植物(如玉米、馬鈴薯、小麥、苜蓿等)中也都有插入突變數據庫的存在[17-19],不過數據尚不夠充分。本文介紹了一些重要的擬南芥和水稻突變體庫目前的規(guī)模和制造突變株的方法,為需要突變株種子以及數據的學者提供方便。
1大規(guī)模外源基因插入構建突變體庫的幾種常用方法
1.1T-DNA插入構建突變體庫
T-DNA突變體庫的建立首先需要制作載體Ti質粒??剐曰虮徽先胼d體,然后導入農桿菌中。取植物的愈傷組織進行誘導和繼代培養(yǎng),將繼代培養(yǎng)的植物愈傷組織放入農桿菌培養(yǎng)液之中,使其感染農桿菌,最后轉入選擇培養(yǎng)基培養(yǎng)。在進行完2次選擇培養(yǎng)后,將長出的抗性愈傷組織通過組織分化形成植株,對植株進行轉基因檢測來確?;蚯贸ぷ鞯耐瓿?。提取植物DNA,用TAIL-PCR等方法得到旁鄰序列,測序比對全基因組序列得到T-DNA插入的具體位置[20]。
1.2轉座子插入構建突變體庫
轉座子系統分為一元系統和雙元系統2種。雙元系統是將Ac轉座子和Ds轉座子分別插入到2個T-DNA載體之中[21],侵染植株,讓2種植株雜交得到F1代,又通過自交得到F2代,在F2代中對Ac轉座酶進行篩選。為了穩(wěn)定Ds的插入位置使其不再轉移,需要對Ac轉座子的存在進行排除。一元系統簡化了雙元系統,同時將Ac的轉座酶編碼序列和Ds轉座子載入一個T-DNA載體上,通過轉座酶的存在使Ds跳躍移位。因此在第一代中就能得到Ds跳躍的植株[22-23]。當Ds轉座子轉移出原插入位點后,則可以通過篩選將轉座酶基因去除,得到穩(wěn)定遺傳突變株。目前作為轉座子插入的轉座子有玉米轉座子Ac/Ds、En/Spm以及金魚草轉座子Tam3。
1.3反轉座子標簽插入法
1999年,Sato等利用水稻逆轉座子Tos17基因敲除體系分離了6個水稻knl-型同源異型框基因,發(fā)現了水稻矮化突變基因OSH15。Tos17從此成為植物基因水稻中的一個內源反轉座子突變載體[24],主要被用來在水稻中進行突變體數據庫的研究[25]。Tos17在自然條件下約有4個拷貝數,在組培的條件下激活,可有5~30個拷貝數插入,分化成植株后就失活,因此Tos17插入引起的突變可以穩(wěn)定遺傳[26]。Tos17的拷貝數隨著組織培養(yǎng)時間延長而增多,可以通過控制組織培養(yǎng)時間來控制轉座的拷貝數[27]。
2相關植物突變數據庫的介紹
2.1擬南芥突變數據庫
下面分別介紹了水稻和擬南芥的一些常用的重要突變數據庫,表1列出了質粒中所存在的各種元件及其作用。
縮寫名稱作用19S CaMV Pro花椰菜花葉病毒19S啟動子35S CaMV Pro花椰菜花葉病毒35S啟動子Amp氨芐青霉素抗性基因F1 oriF1噬菌體復制起始位點GAL4/VP16酵母轉錄激活蛋白Gal4基因/單純皰疹病毒蛋白VP16蛋白基因GFP綠色熒光蛋白基因GUS轉β-葡糖醛酸酶基因Hph潮霉素抗性基因aph(4)-IaHyg潮霉素抗性基因aph(4)-IbI2水稻α微管A1基因第二內含子MAS Pro甘露堿合成酶雙向啟動子Nos pro農桿菌胭脂堿啟動子Nos Ter農桿菌胭脂堿終止子NptⅡ新霉素磷酸轉移酶,抗卡那霉素基因OsTubA1水稻α微管A1基因OsTubA1 Pro水稻α微管A1基因啟動子OsTubA1 Ter水稻α微管A1基因終止子ployAployA尾巴pUC oripUC質粒的復制起點pUC18 patial sequencepUC18質粒部分序列Sul1磺胺藥物抗性基因
2.1.1SALK T-DNA 數據庫SALK實驗室是目前用 T-DNA 插入的方法建立的擬南芥基因組插入突變數據庫中突變最為可觀的實驗室。數據庫使用傳統的T-DNA載體,對擬南芥生態(tài)型col進行基因敲除工作。目前完成137 259個轉基因植株,敲除擬南芥96%以上基因。這個插入數目在擬南芥中已經接近飽和[2]。SALK實驗室使用的質粒載體是pROK2,這是一個由pBIN19改良后的質粒,擁有卡那霉素抗性基因[28]。圖1給出了SALK實驗室的載體結構。
2.1.2RATM(Riken)Ac/Ds轉座子敲除數據庫RATM數據庫是一個采用Ds轉座子對擬南芥進行基因敲除的數據庫[29]。這個基因敲除數據庫已有17 671個突變株。突變體庫采用雙元載體的方法,將含有Ac轉座酶序列的T-DNA插入突變株與含有Ds轉座子的T-DNA插入突變株雜交,得到突變株種子,然后對種子進行植株培養(yǎng),再自交對種子進行篩選,剔除那些含有Ac轉座酶的不穩(wěn)定植株。圖2給出了RTAM轉座子標簽的結構。
2.1.3GABI-Kat T-DNA數據庫GABI-Kat數據庫是一個使用T-DNA對擬南芥生態(tài)型col-0進行基因敲除的數據庫,在T-DNA插入數據庫數目僅次于SALK T-DNA 數據庫。目前已有130 000條側翼序列標簽以及70 578左右的突變株系。其中被敲除的基因數量達62.5%[30]。質粒是pAC161、pADIS1、pAC160和pGABI1[31],通過加入增強子作為激活標簽。F1代抗性植株的種子在F2代時可能會丟失插入的T-DNA,因此所有的二代種都必須經過檢驗確定 T-DNA 插入。目前這個數據庫的T-DNA保留率在78%左右[32]。圖3給出了GABI-Kat載體pAC161的結構。
2.2水稻突變數據庫
2.2.1POSTECH RISD T-DNA數據庫RISD數據庫是由韓國postech中心植物功能基因組實驗室構建的以傳統的 T-DNA 為載體的數據庫, 大約有47 932個T-DNA插入突
變株[33-34]。另外在T-DNA的插入過程中,需通過組培的階段,因而產生反轉座子Tos17的新插入,生成一部分新的突變株。使用的載體有pGA2707、pGA2717以及激活標簽載體pGA2715、pGA2772[35]。圖4給出postech實驗室T-DNA載體pGA2707的結構。
2.2.2SHIP T-DNA數據庫SHIP數據庫是中國科學院上海植物生態(tài)生理研究所建立的T-DNA插入突變體庫,以水稻粳稻中花11作為受體品種。對插入到基因的突變株植株進行分離,使用的質粒為pSMR-J18R,這是水稻基因突變研究較常見的載體,少數植株采用質粒pCAMBIA1301[36]。圖5給出SHIP實驗室T-DNA載體pSMR-J18R的結構。
2.2.3RTIM Tos17突變數據庫RTIM數據庫由日本農業(yè)資源研究所(NIAS)開展構建,采用的水稻株系是粳稻品種日本晴[37]。該數據庫利用Tos17反轉座子來制造突變數據庫[38],突變株中反轉座子插入位點數量較高,通常每個突變株都帶有10~12個反轉座子插入。
3植物突變數據庫的研究前景
基因功能的研究繼基因結構研究之后對生物自身信息進一步解密,直接關系到生物表型和遺傳信息之間的聯系,因此尤為被關注。目前,植物基因功能研究正如火如荼地展開,單個基因進行敲除后進行培育獲得的穩(wěn)定遺傳的純和突變株系為后續(xù)展開的基因功能的研究提供了很好的研究模型。例如,在植物信號傳導、抗逆性研究上突變株起到了不可替代的作用。此外,基因功能獲得突變株也和功能缺失突變株系一樣在研究中得到重視。本文介紹的這些數據庫大部分都提供突變株種子,以滿足研究者們對基因功能研究的需要。
隨著模式生物基因組研究的深入,各種非模式生物的基因組研究也開始進行。同一個基因結構在不同植物是否擁有相同的功能,基因結構進化的同時是否帶入了功能的演變,基因在不斷進化分化的同時,功能又受到怎樣的影響,基因數量的演變與功能的存在有何關系,都是在植物功能數據庫進一步完善后所需解決的問題。另外,載體的構建及抗性基因的篩選等步驟,都將在未來進一步精簡,更多的為了專門的研究而提出的新方法也將陸續(xù)出現。
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