Eva Majerová

Hypomethylating drugs efficiently decrease cytosine methylation in telomeric DNA and activate telomerase without affecting telomere lengths in tobacco cells

Telomere homeostasis is regulated at multiple levels, including the local chromatin structure of telomeres and subtelomeres. Recent reports demonstrated that a decrease in repressive chromatin marks, such as levels of cytosine methylation in subtelomeric regions, results in telomere elongation in mouse cells. Here we show that a considerable fraction of cytosines is methylated not only in subtelomeric, but also in telomeric DNA of tobacco BY-2 cells. Drug-induced hypomethylation (demonstrated at subtelomeric, telomeric, and global DNA levels) results in activation of telomerase. However, in contrast to mouse cells, the decrease in 5-methylcytosine levels and upregulation of telomerase do not result in any changes of telomere lengths. These results demonstrate the involvement of epigenetic mechanisms in the multilevel process of regulation of telomerase activity in plant cells and, at the same time, they indicate that changes in telomerase activity can be overridden by other factors governing telomere length stability.

Methylation of plant telomeric DNA: what do the results say?

Recently, we reported (Majerova et al. 2011) that hypomethylating drugs efficiently decrease methylation of telomeric DNA and activate telomerase without affecting telomere lengths in tobacco. The key issue of the Letter to Editor written by Vaquero-Sedas and Vega-Palas (2011) is, whether our results demonstrating methylation and druginduced hypomethylation of tobacco telomeric DNA repeats do correspond to the status of terminal telomeric sequences, or rather to internal telomeric sequences (ITSs) which might exist in tobacco genome as it is the case in Arabidopsis thaliana, the model organism which Vaquero- Sedas and Vega-Palas have been working with. They are referring to their recent article (Vaquero-Sedas et al. 2010) in which they conclude that ITSs in A. thaliana are heterochromatic, while telomeres are euchromatic. We are pleased that our paper has initiated this debate and take the opportunity to go through the key points of the Letter of Vaquero-Sedas and Vega-Palas