In multicellular eukaryotes, repression of gene transcription by Polycomb group (PcG) proteins is fundamental to cell fate determination and developmental transitions. Cis DNA motifs, called polycomb responsive elements (PREs), allow the recruitment of PcG proteins to their target genes, on which they deposit repressive epigenetic marks such as histone H3 lysine 27 trimethylation (H3K27me3). However, the understanding of the mechanisms controlling PcG recruitment is only fragmentary as certain genes do contain PREs in their promoter sequences, and yet are ubiquitously transcribed implying that they are not targeted by, or refractory to PcG-mediated silencing. This raises the interesting possibility that unknown mechanisms actively protect certain genes from PcG-mediated silencing.

The Plant Mobile Domain (PMD) is a protein domain that is widely spread in angiosperms, found associated with transposable elements (TEs) or corresponding to genes playing important roles in the model plant Arabidopsis thaliana. The PMD proteins MAINTENANCE OF MERISTEMS (MAIN) and MAIN-LIKE1 (MAIL1) act in a same complex that is required for proper transcription of many genes as well as the silencing of several TEs. Besides, their evolutionary conserved paralog MAIN-LIKE2 (MAIL2) also regulates the expression of several genes. Using complementary approaches combining forward genetics, epigenomics, biochemical and microscopic experiments, we have accumulating evidence showing that the PMD proteins secure the transcription of distinct sets of genes by antagonizing PcG-mediated silencing.