Plants coopt a transposon-derived protein domain for gene silencing

Transposable elements (TEs) are prevalent in most eukaryotes, and host genomes have devised silencing strategies to rein in TE activity. Transcriptional silencing is generally associated with DNA methylation and short interfering RNAs (siRNAs). Here, we show that the Arabidopsis genes MAIL1 and MAIN define an alternative silencing pathway independent of DNA methylation and siRNAs. Mutations in MAIL1 or MAIN lead to release of silencing and impaired condensation of pericentromeric heterochromatin. Phylogenetic analysis suggests not only that the MAIL1- and MAIN-encoded plant mobile domain (PMD) was domesticated from a retrotransposon, but also that host PMD proteins were in turn recaptured by DNA transposons. Our results reveal that a TE-derived domain has been co-opted early in plant evolution to enforce silencing, and that this domain can be reintegrated by TEs to serve again their function.