Epigenetic regulation of transposable elements
in Arabidopsis

Epigenetic regulation of Transposable Elements (TE)

DNA methylation (5-methylcytosine) is a hallmark of TEs in many organisms, including plants in which it is established by RNA silencing pathways (RNA-directed DNA methylation) and associated with histone H3K9 dimethylation. DNA methylation negatively controls TE expression in a stable manner and typically is not involved in silencing protein coding genes. Inversely, in most model organisms, the highly conserved Polycomb Group (PcG) proteins, namely Polycomb Repressive Complex 2 (PRC2), which deposits histone H3 Lysine 27 trimethylation (H3K27me3), are a hallmark of transcriptional repression associated with protein- and miRNA-coding genes involved, in particular, in development. Thus, PcG and DNA methylation have been generally seen as mutually exclusive, specialized systems for the transcriptional silencing of genes and TEs respectively, in plants and mammals. Nevertheless, there is a growing body of evidence for a functional and mechanistic interplay between these pathways. In particular, many TE sequences, upon their loss of DNA methylation and/or in certain cell-types, display H3K27me3 marks and even, more rarely, in the presence of DNA methylation. This raises fundamental questions with regards to the differences and commonalities between PcG- and DNA methylation-mediated silencing and the role and mechanistic determinants of PcG targeting at TEs.