SiRNA–Mediated Methylation of Arabidopsis Telomeres

Jan Vrbsky,Werner Aufsatz,Karel Riha,Lucia Daxinger,Thomas L Turner,Boris Vyskot,Svetlana Akimcheva,J Matthew Watson,Tetsuji Kakutani

doi:10.1371/journal.pgen.1000986

Abstract

Chromosome termini form a specialized type of heterochromatin that is important for chromosome stability. The recent discovery of telomeric RNA transcripts in yeast and vertebrates raised the question of whether RNA–based mechanisms are involved in the formation of telomeric heterochromatin. In this study, we performed detailed analysis of chromatin structure and RNA transcription at chromosome termini in Arabidopsis. Arabidopsis telomeres display features of intermediate heterochromatin that does not extensively spread to subtelomeric regions which encode transcriptionally active genes. We also found telomeric repeat–containing transcripts arising from telomeres and centromeric loci, a portion of which are processed into small interfering RNAs. These telomeric siRNAs contribute to the maintenance of telomeric chromatin through promoting methylation of asymmetric cytosines in telomeric (CCCTAAA)n repeats. The formation of telomeric siRNAs and methylation of telomeres relies on the RNA–dependent DNA methylation pathway. The loss of telomeric DNA methylation in rdr2 mutants is accompanied by only a modest effect on histone heterochromatic marks, indicating that maintenance of telomeric heterochromatin in Arabidopsis is reinforced by several independent mechanisms. In conclusion, this study provides evidence for an siRNA–directed mechanism of chromatin maintenance at telomeres in Arabidopsis.

Highlights

Telomeres safeguard the stability of eukaryotic chromosomes by protecting natural chromosome ends from triggering DNA damage responses
We have discovered that Arabidopsis telomeric nucleosomes contain a unique mixture of both active and inactive chromatin marks
Regulation of DNA methylation is achieved by telomeric repeat–containing small RNAs, which are derived from the processing of telomeric transcripts by the RNA– dependent DNA methylation pathway

Summary

Introduction

Telomeres safeguard the stability of eukaryotic chromosomes by protecting natural chromosome ends from triggering DNA damage responses. Chromosome termini consist of telomeric and subtelomeric repeats that are bound by a specific set of telomere binding proteins as well as nucleosomes that exhibit features of pericentric heterochromatin [1]. Deficiency in histone methyltransferases or the retinoblastoma tumor suppressor leads to disruption of telomeric heterochromatin and aberrant telomere elongation in mouse cells [4,5,6] Another important hallmark of heterochromatin in mammals is DNA methylation. Inactivation of DNA methyltransferases in mouse cells decreases 5-methylcytosine at subtelomeres and leads to increased telomeric recombination, without a concomitant change in histone modifications [7]. These data indicate a functional interaction between subtelomeric and telomeric chromatin

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Conclusion