一個基因的表達不僅取決于其位置也取決于其來源序列,。表觀遺傳學(xué)修飾,，或損傷DNA周圍的蛋白質(zhì)的改變,，也可以改變基因的表達模式,。表觀遺傳學(xué)改變可以從親本細胞向子細胞傳遞,，保證細胞系在幾代之間具有固定特性。轉(zhuǎn)座子或跳躍基因與其它基因不同,，因為它們在表觀遺傳學(xué)上幾乎無活性,。沉默的轉(zhuǎn)座子對保持基因組的完整性非常重要，因為這種移動的基因元件可以隨機插入基因組,，引起有害突變和基因沉默,。
科學(xué)家已經(jīng)得知一旦啟動，玉米植物將“記憶”并且保持轉(zhuǎn)座子世代沉默,，即使啟動機制消失,。來自麥吉爾大學(xué)和加州大學(xué)伯克利分校的研究者發(fā)現(xiàn)以上情況并非一成不變,。在基因組的某些區(qū)域,，啟動機制消失后轉(zhuǎn)座子將“再度覺醒”。該研究認為,，植物基因組的表觀遺傳學(xué)前景可能比先前想象的更微妙和有趣,，因為對表觀遺傳沉默的記憶取決于基因的位置。擦掉遺傳信息可能是表觀遺傳機制的重要組成部分,。 （生物谷Bioon.com）
生物谷推薦原始出處：
PLoS Genet 4(10): e1000216. doi:10.1371/journal.pgen.1000216
A Position Effect on the Heritability of Epigenetic Silencing
Jaswinder Singh1, Michael Freeling2, Damon Lisch2*
1 Plant Science Department, McGill University, Macdonald Campus, Ste. Anne de Bellevue, Quebec, Canada, 2 Department of Plant and Microbial Biology, University of California Berkeley, Berkeley, California, United States of America
In animals and yeast, position effects have been well documented. In animals, the best example of this process is Position Effect Variegation (PEV) in Drosophila melanogaster. In PEV, when genes are moved into close proximity to constitutive heterochromatin, their expression can become unstable, resulting in variegated patches of gene expression. This process is regulated by a variety of proteins implicated in both chromatin remodeling and RNAi-based silencing. A similar phenomenon is observed when transgenes are inserted into heterochromatic regions in fission yeast. In contrast, there are few examples of position effects in plants, and there are no documented examples in either plants or animals for positions that are associated with the reversal of previously established silenced states. MuDR transposons in maize can be heritably silenced by a naturally occurring rearranged version of MuDR. This element, Muk, produces a long hairpin RNA molecule that can trigger DNA methylation and heritable silencing of one or many MuDR elements. In most cases, MuDR elements remain inactive even after Muk segregates away. Thus, Muk-induced silencing involves a directed and heritable change in gene activity in the absence of changes in DNA sequence. Using classical genetic analysis, we have identified an exceptional position at which MuDR element silencing is unstable. Muk effectively silences the MuDR element at this position. However, after Muk is segregated away, element activity is restored. This restoration is accompanied by a reversal of DNA methylation. To our knowledge, this is the first documented example of a position effect that is associated with the reversal of epigenetic silencing. This observation suggests that there are cis-acting sequences that alter the propensity of an epigenetically silenced gene to remain inactive. This raises the interesting possibility that an important feature of local chromatin environments may be the capacity to erase previously established epigenetic marks.
