一个基因的表达不仅取决于其位置也取决于其来源序列。表观遗传学修饰，或损伤DNA周围的蛋白质的改变，也可以改变基因的表达模式。表观遗传学改变可以从亲本细胞向子细胞传递，保证细胞系在几代之间具有固定特性。转座子或跳跃基因与其它基因不同，因为它们在表观遗传学上几乎无活性。沉默的转座子对保持基因组的完整性非常重要，因为这种移动的基因元件可以随机插入基因组，引起有害突变和基因沉默。
科学家已经得知一旦启动，玉米植物将“记忆”并且保持转座子世代沉默，即使启动机制消失。来自麦吉尔大学和加州大学伯克利分校的研究者发现以上情况并非一成不变。在基因组的某些区域，启动机制消失后转座子将“再度觉醒”。该研究认为，植物基因组的表观遗传学前景可能比先前想象的更微妙和有趣，因为对表观遗传沉默的记忆取决于基因的位置。擦掉遗传信息可能是表观遗传机制的重要组成部分。 （生物谷Bioon.com）
生物谷推荐原始出处：
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.
