Telomere dynamics and telomerase-independent cell survival in Arabidopsis thaliana

Abstract

Telomeres are the nucleoprotein structures that protect the ends of eukaryotic chromosomes from being recognized as DNA double-strand breaks. Telomeres are recognized by the ribonucleoprotein telomerase, a reverse transcriptase that catalyzes addition of G-rich telomeric DNA repeats to the 3’ overhang of the telomere. The action of telomerase allows cells to overcome the end-replication problem defined by the inability of conventional DNA polymerases to fully replicate the end of the chromosome. Telomeric DNA tracts are maintained in a species-specific size range primarily through the competition between telomerase and the end-replication problem. In many organisms, recombinational activities can function at telomeres outside of the wild type range, in some cases resulting in telomerase-independent telomere maintenance. Telomere rapid deletion (TRD) can dramatically shorten elongated telomeres. Elongation of telomeres below the normal range in the absence of telomerase is known as alternative lengthening of telomeres (ALT). Here we demonstrate that telomeres in Arabidopsis thaliana are also subjected to these recombinational activities. Elongated telomeres in ku70 mutants are shortened by TRD. In contrast to other organisms, TRD functions on telomeres of wild type length. TRD produces extra-chromosomal telomeric circles, which can serve as substrates for ALT. In Arabidopsis, ALT may require the byproducts of TRD, as telomerase mutants with extremely short telomeres are unable to maintain telomeric repeats by recombination and instead secure their genome through an unknown mechanism. Finally, we follow the fate of cells with telomere-to-telomere fusions. Fusions are not propagated to viable progeny. We propose that a G1 checkpoint dependent upon the checkpoint protein ATM arrests cells following the break of a single telomere fusion. We design reporter constructs to follow the fate of individual cells with telomere fusions, and present initial characterization of their expression. We find no evidence for the propagation of telomere fusions in somatic cells, though later generation mutants will provide a better test of this hypothesis. This work begins the study of the fate of cells with telomere fusions in Arabidopsis. Furthermore, it sets the foundation for studying recombinational shortening and elongation of telomeres in Arabidopsis and the effects of these processes on telomere length regulation.