葉綠體,，是植物細(xì)胞的綠色太陽(yáng)能發(fā)電機(jī),，也曾是自主的生物。約十億年前,，這一切發(fā)生了改變，當(dāng)時(shí)它們被更大的細(xì)胞所吞噬，但沒(méi)有被消化掉,。從那時(shí)起,，它們就已經(jīng)失去了自主權(quán)。隨著時(shí)間推移,，它們的大部分遺傳信息找到了進(jìn)入細(xì)胞核的方式,；今天，葉綠體離開(kāi)宿主將不能存活,。馬克斯普朗克植物分子生理學(xué)研究所Ralph Bock研究團(tuán)隊(duì)發(fā)現(xiàn),，葉綠體基因采取一種直接路線進(jìn)入細(xì)胞核，在細(xì)胞核內(nèi),，盡管存在結(jié)構(gòu)差異,，它們還是能被正確讀取。
藍(lán)細(xì)菌（Cyanobacteria,，藍(lán)綠藻類原核生物）是最古老的生命形式,，似乎是植物細(xì)胞內(nèi)綠色葉綠體的先驅(qū)。它們沒(méi)有真正的細(xì)胞核,，但它們的遺傳物質(zhì)由四個(gè)標(biāo)準(zhǔn)部件組成,，正如人類、植物與動(dòng)物的那樣,。因此,，編碼葉綠體DNA的基因在細(xì)胞核也可被讀取，現(xiàn)在,，早期進(jìn)化期間仍存在于細(xì)胞器中的許多基因的確專門位于細(xì)胞核基因組,。一直尚未知它們是如何按自己的方式的?？赡艿?種機(jī)制：葉綠體DNA片段直接輸送入細(xì)胞核或以mRNA形式輸入然后再反轉(zhuǎn)錄回DNA,。
DNA直接輸送似乎在葉綠體中占主導(dǎo)地位，但這條通路有兩個(gè)問(wèn)題,。第一個(gè)問(wèn)題存在于啟動(dòng)子,，它是確保基因被識(shí)別成本該識(shí)別成的那樣的DNA序列,。它們都位于基因上游,，招募基因轉(zhuǎn)錄所必需的蛋白質(zhì)。但是,，葉綠體啟動(dòng)子不能被細(xì)胞核蛋白所識(shí)別,，以至于DNA讀取機(jī)制必須跳過(guò)這些新來(lái)的基因。第二個(gè)就是基因序列的正確處理,?；蛴蓭讉€(gè)模塊組成,，被非編碼DNA區(qū)域（內(nèi)含子）所分隔，因?yàn)閮?nèi)含子阻礙蛋白質(zhì)合成,，需要把它們從mRNA中移除,，這就是所說(shuō)的剪接過(guò)程。整個(gè)過(guò)程,，以正確蛋白合成為終點(diǎn),，一旦發(fā)生就只能恢復(fù)一次。另外,，細(xì)胞核內(nèi)的mRNA加工處理不同于葉綠體內(nèi)的,，一直以來(lái)，葉綠體內(nèi)含子似乎是細(xì)胞核內(nèi)葉綠體基因正確讀取不能克服的障礙,。
盡管這樣,，還是會(huì)形成功能蛋白。甚至認(rèn)為內(nèi)含子幫助剪接酶自我折疊成穩(wěn)定的RNA結(jié)構(gòu),，因此指導(dǎo)酶到正確位置,。同時(shí)，RNA結(jié)構(gòu)似乎幫助核糖體尋找蛋白質(zhì)合成的正確起始點(diǎn),。
因?yàn)榛蜉斎爰?xì)胞核內(nèi)是一個(gè)極其緩慢的進(jìn)化過(guò)程,，可能需要花數(shù)百萬(wàn)年時(shí)間，現(xiàn)在研究其所含機(jī)制是不可能的,。但是,，研究人員現(xiàn)在在實(shí)驗(yàn)室已設(shè)法快速輸入此基因。因?yàn)榧?xì)胞遭受高選擇壓力,，葉綠體基因進(jìn)入細(xì)胞核的遷移成為生存所必需的,，因此它能隨時(shí)可見(jiàn)。結(jié)果發(fā)現(xiàn),，輸入在沒(méi)有RNA參與的情況下發(fā)生,，顯然DNA跳過(guò)細(xì)胞葉綠體直接進(jìn)入細(xì)胞核內(nèi)。（生物谷bioon.com）
doi:10.1016/j.cub.2012.03.005
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Experimental Reconstruction of the Functional Transfer of Intron- Containing Plastid Genes to the Nucleus
Ignacia Fuentes, Daniel Karcher, Ralph Bock
Background Eukaryotic cells arose through the uptake of bacterial endosymbionts and their gradual conversion into cell organelles (mitochondria and chloroplasts). In this process, a massive transfer of genes from the genome of the endosymbiont to the nuclear genome of the host cell occurred. Whereas intron-free organellar genescould conceivably enter the nucleus as DNA pieces and become functional nuclear genes, the transfermechanisms of organellar genescontaining prokaryotic-type group I or group II introns are not clear. Results We describe an experimental system that allows us to screen for functional endosymbiotic genetransferof intron-containing chloroplast genes to the nuclear genome. To distinguish between DNA-mediated and RNA/complementary DNA-mediated transfer, we have constructed an antibiotic resistance gene that is interrupted by a chloroplast group II intron and whose expression is dependent upon both intron removal andgenetransfer from the chloroplast genome to the nuclear genome. Screening chloroplast-transformed tobacco plants for the acquisition of the antibiotic resistance via genetransfer to the nucleus, a large number of transfer events were selected. We show that all events involved the direct DNA-mediatedtransfer of the intron-containing chloroplast gene into the nuclear genome. Gene activity in the nucleus is brought about by utilization of cryptic splice sites within chloroplast intron sequences resulting in appearance of a contiguous reading frame. Conclusion Our data pinpoint mechanisms for the functionaltransfer of organellar genes to the nucleus and demonstrate that intron possession is not an insurmountable obstacle to DNA-mediated endosymbioticgenetransfer.
