圖1. 擬南芥是一種，廣泛用於基礎生物學發現的植物物種。由於這種多用途之測試對象的協助，美國冷泉港實驗室(CSHL)的科學家們，現在已經挖掘出一種，有助於控制遺傳過程的秘密。
Arabidopsis thaliana is a plant species widely used to make fundamental biological discoveries. With the help of this versatile test subject, CSHL scientists have now dug up the secrets of a process that helps control inheritance.
When organisms pass their genes on to future generations, they include more than the code spelled out in DNA. Some also pass along chemical markers that instruct cells how to use that code. The passage of these markers to future generations is known as epigenetic inheritance. It’s particularly common in plants. So, significant findings here may have implications for agriculture, food supplies, and the environment.
當生物體將其基因傳遞給後代時，它們不僅包含DNA中，拼寫出的遺傳密碼。有些也傳遞，指令細胞如何使用那遺傳密碼的化學標記。此些標記對後代的遷移，被通稱為漸成論(表觀)的遺傳。這在植物中，特別常見。因此，在這裡的重大發現，可能對農業、糧食供應及環境具有諸多意涵。
圖2. 此影片顯示了，在細胞分裂過程中，DNA甲基化減少I (DDM1(綠色)：Decreased DNA Methylation I。是一種，編碼促進DNA甲基化之核小體重塑因子的植物基因)的運作情況。當染色體複製時，DDM1纏著於染色體上，來為添加重要化學標記做好準備。之後，當細胞一分為二時，DDM1退出，而另一種蛋白質H3.3(紅色)與染色體結合，來防止添加過量標記。 (請參閱原文短片)
This video shows DDM1 (green) at work during cell division. As chromosomes are duplicated, DDM1 latches onto chromosomes to allow for vital chemical markers to be tacked on. Then, as the cell divides in two, DDM1 exits, and another protein, H3.3 (red), binds to chromosomes to prevent excess markers from being added.
Cold Spring Harbor Laboratory (CSHL) Professors and HHMI Investigators Rob Martienssen and Leemor Joshua-Tor have been researching how plants pass along the markers that keep transposons inactive. Transposons are also known as jumping genes. When switched on, they can move around and disrupt other genes. To silence them and protect the genome, cells add regulatory marks to specific DNA sites. This process is called methylation.
冷泉港實驗室(CSHL)教授及霍華德休斯醫學研究所(HHMI：Howard Hughes Medical Institute)研究員，Rob Martienssen及Leemor Joshua-Tor 一直進行研究，植物如何傳遞，保持也被通稱為跳躍基因之轉位子不活躍的標記。當被打開時，它們能四處移動並擾亂其他基因。為了使它們靜默及保護基因體，細胞將調節性標記，添加到特定的DNA位點。此過程被稱為甲基化。
Martienssen and Joshua-Tor have now shown how protein DDM1 makes way for the enzyme that places these marks on new DNA strands. Plant cells need DDM1 because their DNA is tightly packaged. To keep their genomes compact and orderly, cells wrap their DNA around packing proteins called histones.
目前，Martienssen及Joshua-Tor已經證實了，DDM1蛋白質如何，為在新DNA股上，放置這些標記的酵素開路。植物細胞需要DDM1，因為其DNA被緊密封包。為了保持其基因體緊密有序，細胞將其DNA纏繞於，被稱為組蛋白的封包蛋白質周遭。
“But that blocks access to the DNA for all sorts of important enzymes,” Martienssen explains. Before methylation can occur, “you have to remove or slide the histones out of the way.”
Martienssen解釋：「不過，對各種重要酵素而言，那阻礙了接觸DNA的途徑。」在甲基化能發生之前，「必須移除或使此組蛋白滑動離開此途徑。」
Martienssen and former CSHL colleague Eric Richards first discovered DDM1 30 years ago. Since then, researchers have learned it slides DNA along its packing proteins to expose sites needing methylation. Martienssen likens the movement to a yo-yo gliding along a string. The histones “can move up and down the DNA, exposing parts of the DNA at a time, but never falling off,” he explains.
30年前，Martienssen及前CSHL同僚Eric Richards，首度發現DDM1。打從那時起，研究人員們已經得悉，它沿著其封包蛋白質滑動DNA，來暴露需要甲基化的位點。Martienssen將此移動比喻為，沿著繩子滑動的溜溜球。他解釋：「組蛋白能上、下移動DNA，一次暴露部分DNA，不過絕不會分離。」
Through genetic and biochemical experiments, Martienssen pinpointed the exact histones DDM1 displaces. Joshua-Tor used cryo-electron microscopy to capture detailed images of the enzyme interacting with DNA and associated packing proteins. They were able to see how DDM1 grabs onto particular histones to remodel packaged DNA. “An unexpected bond that ties DDM1 together turned out to correspond to the first mutation found all those years ago,” Joshua-Tor says.
透過遺傳及生化實驗，Martienssen精確找出了，DDM1遷移的確切組蛋白。Joshua-Tor使用了低溫電子顯微鏡，來捕捉與DNA及相關包裝蛋白質相互作用之酶的詳細圖像。他們能夠看到DDM1，如何抓住特定組蛋白，來重新塑造封包的DNA。Joshua-Tor宣稱：「一種將DDM1繫在一起之意想不到的束縛物，被證實相當於那些年前，發現的第一個突變體。」
圖3. 此卡通模型首度闡明了，在細胞分裂過程中，DDM1蛋白質(紫色)在何處及如何抓牢DNA(米色)。
This cartoon model illustrates, for the first time, where and how the DDM1 protein (purple) grips onto DNA (beige) during cell division.
The experiments also revealed how DDM1’s affinity for certain histones preserves epigenetic controls across generations. The team showed that a histone found only in pollen is resistant to DDM1 and acts as a placeholder during cell division. “It remembers where the histone was during plant development and retains that memory into the next generation,” Martienssen says.
此些實驗也揭露了，DDM1對某些組蛋白的親和性，如何在跨世代間，保存漸成論的遺傳控制能耐。該團隊證實了，僅在花粉中發現的組蛋白，對 DDM1具抵抗力，且在細胞分裂過程中，充當一種佔位物(可被特定物取代之物)。Martienssen宣稱：「它記住，在植物發育過程中，組蛋白的位置，並將那記憶保留到下一代。」
Plants may not be alone here. Humans also depend on DDM1-like proteins to maintain DNA methylation. The new discovery may help explain how those proteins keep our genomes functional and intact.
在這裡，植物可能並不孤單。人類也依賴類DDM1的蛋白質，來維持DNA 甲基化。此新發現可能有助於解釋，那些蛋白質如何保持咱們基因體的功能及完整性。
網址：https://www.cshl.edu/how-plants-pass-down-genetic-memories/
翻譯：許東榮
文章定位：
