A Enhancer of zeste like methyltransferase catalyzes both H3 lysine 9 and 27 trimethylation and mediates transcriptional repression of transposable elements

The unicellular eukaryote Paramecium tetraurelia shares the highly conserved trimethylation of K9 and K27 on histone H3, with the peculiarity that a single histone methyltransferase Ezl1 is essential for the deposition of both marks during development. Here, we show that the Enhancer of zeste like protein Ezl1 is a dual methyltransferase that catalyzes trimethylation of histone H3 both on K27 and K9 in vitro. A catalytically dead mutant of Ezl1 allowed us to induce extensive and simultaneous loss of H3K9me3 and H3K27me3 in vivo. By combining transcriptional and chromatin profiling, we found that histone H3K27 and H3K9 trimethylation co-occur and repress multiple families of transposable elements. Our study suggests that, originally, the primary function of the ancient family of Enhancer of zeste proteins was to silence transposable elements.