A Position Effect on the Heritability of Epigenetic Silencing

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

Loss of RNA–Dependent RNA Polymerase 2 (RDR2) Function Causes Widespread and Unexpected Changes in the Expression of Transposons, Genes, and 24-nt Small RNAs

Transposable elements (TEs) comprise a substantial portion of many eukaryotic genomes and are typically transcriptionally silenced. RNA–dependent RNA polymerase 2 (RDR2) is a component of the RNA–directed DNA methylation (RdDM) silencing pathway. In maize, loss of mediator of paramutation1 (mop1) encoded RDR2 function results in reactivation of transcriptionally silenced Mu transposons and a substantial reduction in the accumulation of 24 nt short-interfering RNAs (siRNAs) that recruit RNA silencing components. An RNA–seq experiment conducted on shoot apical meristems (SAMs) revealed that, as expected based on a model in which RDR2 generates 24 nt siRNAs that suppress expression, most differentially expressed DNA TEs (78%) were up-regulated in the mop1 mutant. In contrast, most differentially expressed retrotransposons (68%) were down-regulated. This striking difference suggests that distinct silencing mechanisms are applied to different silencing templates. In addition, >6,000 genes (24% of analyzed genes), including nearly 80% (286/361) of genes in chromatin modification pathways, were differentially expressed. Overall, two-thirds of differentially regulated genes were down-regulated in the mop1 mutant. This finding suggests that RDR2 plays a significant role in regulating the expression of not only transposons, but also of genes. A re-analysis of existing small RNA data identified both RDR2–sensitive and RDR2–resistant species of 24 nt siRNAs that we hypothesize may at least partially explain the complex changes in the expression of genes and transposons observed in the mop1 mutant

Ending hidden hunger

Producer: Mike DineenDirectors: Julian Chomet and Elizabeth JupeWriters: Mike Dineen, Julian Chomet and Elizabeth JupeNarrator: Peter UstinovContents: 1 videocassette (VHS) (NTSC) (20 min.); 1 script (sixth draft – October 22nd, 1992); 1 fax (changes to the script - November 17th, 1992); 1 fax (changes to the script - November 18th, 1992)French version available in IDRC Digital Library: Éradiquer la faim insoupconnéeKit came with VHS video which isn't yet converted to .mp4Looks at micronutrient malnutrition in developing countries, particularly its effects on children, and what can be done to overcome it

Endogenous transposable elements associated with virus infection in maize

Evidence that transposable DNA elements are ubiquitous in plants and that gross genetic instability may well provide the means for rapid evolutionary and even regulatory adaptation is rapidly accumulating. McClintock (1984) has suggested, for example, that stress in various forms might stimulate genome reorganization and the activation of transposable elements. Such a response would be analogous to, but much less specific than, the cascade of repair and recombination functions that is induced by DNA damage in Escherichia coli and is known as the SOS response. One possible source of such stress is systemic virus infection in plants. Although the symptoms of virus infection are generally not heritable, several studies have suggested that virally stressed plants may suffer unusual levels of genetic aherations. This notion is based mainly on genetic studies of virus-infected maize (Sprague et al. 1963; Sprague and McKinney 1966, 1971; Mottinger et al. 1984a) and earlier studies on..