Abstract
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.
Highlights
Whether or not a gene is expressed can depend as much on its location within the genome as its primary DNA sequence
When MuDR and Mu killer (Muk) are combined genetically, MuDR elements become epigenetically silenced, and they generally remain so even after Muk is lost in subsequent generations
These results suggest that erasure of heritable information may be an important component of epigenetic regulation
Summary
Whether or not a gene is expressed can depend as much on its location within the genome as its primary DNA sequence. In PEV, when genes are moved into close proximity to constitutive heterochromatin, their activity can become unstable, resulting in variegated patches of gene expression This process is regulated by a variety of proteins implicated in both chromatin remodeling [3,4,5] and RNAi-based silencing [6]. The spread of heterochromatin can be blocked by insulating sites, such as those bound by Suppressor of Hairy-wing [7,8] and GAGA factor [9,10] These proteins are competent to alter the silenced state by actively remodeling chromatin. Some of the same proteins, such as GAGA factor, are involved in the epigenetic regulation of homeobox genes during Drosophila development These observations suggest that the process by which transposable elements are sequestered from the rest of the genome may have been recruited to regulate host gene expression as well
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