The histone variant H2A.W cooperates with chromatin modifications and linker histone H1 to maintain transcriptional silencing of transposons in Arabidopsis

Abstract

Transposable elements (TEs) are marked by a complex array of chromatin modifications, but a central unifying mechanism for how they are silenced remains elusive. Histone H3 Lysine 9 methylation (H3K9me) is an important component of heterochromatin in most eukaryotes, including plants. In flowering plants, the specialized histone variant H2A.W occupies nucleosomes found at TE sequences. This variant is deposited by the chromatin remodeler DDM1 and confers specific biophysical properties to the nucleosomes. Here we use genetic and genomic strategies to evaluate the role of H2A.W in transposon silencing in Arabidopsis. Compared with mutants lacking either H2A.W or H3K9me, the combined loss of both H2A.W and H3K9me causes a dramatic increase in both the number of expressed TEs and their expression levels. Synergistic effects are also observed when H2A.W is lost in combination with histone H1 or CH methylation. Collectively, these TEs are also upregulated in mutants lacking DDM1, which are impaired in H2A.W deposition and lose heterochromatic marks. We conclude that H2A.W acts in combination with different elements of heterochromatin to maintain silencing across a large spectrum of TEs present primarily in pericentric heterochromatin in Arabidopsis. In mammals, the DDM1 ortholog LSH deposits macroH2A to heterochromatin and silences TEs. We thus propose that specialized H2A variants localized to heterochromatin interact with a complex array of histone modifications to silence TEs in eukaryotes.