siRNA–Mediated Methylation of Arabidopsis Telomeres

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.

Dysfunction of Chromatin Assembly Factor 1 Induces Shortening of Telomeres and Loss of 45S rDNA inArabidopsis thaliana

“…Recombination at telomeres is downregulated by their folding into a compact nucleoprotein structure (Fajkus et al, 1995; Fajkus and Trifonov, 2001;Schoeftner and Blasco, 2009;Vrbsky et al, 2010) and by controlled activity of the DNA repair factors (Denchi, 2009). Telomere rapid deletion is stochastic giving rise to heterogeneous telomere lengths (Watson and Shippen, 2007).…”

Transcription regulates telomere dynamics in human cancer cells

“…Telomeres, the heterochromatic structures located at the end of linear eukaryotic chromosomes, are transcribed by DNA-dependent RNA polymerase II (RNAPII) into telomeric repeat-containing RNA (TERRA) molecules (Solovei et al 1994;Azzalin et al 2007;Luke et al 2008;Schoeftner and Blasco 2008;Arora et al 2010;Vrbsky et al 2010). In human cells, the large majority of TERRA is transcribed from CpG dinucleotide-rich islands located on approximately half of the subtelomeres (Nergadze et al 2009).…”

Novel features of telomere biology revealed by the absence of telomeric DNA methylation

“…Mammalian DNA methylation is primarily found in the CG context (Ramsahoye et al 2000;Lister et al 2009). In contrast, plants have significant levels of DNA methylation in all sequence contexts (CG, CHG, and CHH, where H can be A, C, or T) (Law and Jacobsen 2010).Although subtelomeric DNA methylation has been reported in animals and plants, the presence of DNA methylation at telomeres remains an open question in both kingdoms (Blasco 2007;Vrbsky et al 2010;Vaquero-Sedas et al 2011;Ogrocká et al 2014). The methylation status of mammalian telomeres has not been investigated because, as mentioned above, mammals have low levels of non-CG methylation, which is the type of DNA methylation that should be associated with telomeric sequences (CCCTAA in mammals and CCCTAAA in plants).…”

“…Although subtelomeric DNA methylation has been reported in animals and plants, the presence of DNA methylation at telomeres remains an open question in both kingdoms (Blasco 2007;Vrbsky et al 2010;Vaquero-Sedas et al 2011;Ogrocká et al 2014). The methylation status of mammalian telomeres has not been investigated because, as mentioned above, mammals have low levels of non-CG methylation, which is the type of DNA methylation that should be associated with telomeric sequences (CCCTAA in mammals and CCCTAAA in plants).…”

“…The methylation status of mammalian telomeres has not been investigated because, as mentioned above, mammals have low levels of non-CG methylation, which is the type of DNA methylation that should be associated with telomeric sequences (CCCTAA in mammals and CCCTAAA in plants). In turn, although the methylation levels of plant telomeres have been studied by different groups, they remain controversial (Vrbsky et al 2010;Majerová et al 2011a,b;Vaquero-Sedas and Vega-Palas 2011a,b;Vaquero-Sedas et al 2011Ogrocká et al 2014). Therefore, it is important to settle the methylation status of telomeres.…”