Telomerase Regulation in Arabidopsis thaliana

Telomeres form a nucleoprotein cap at the end of eukaryotic chromosomes. The telomere protein constituents repress the DNA damage response (DDR) and facilitate maintenance of terminal sequences by a specialized ribonucleoprotein complex called telomerase. In turn, factors involved in the DDR guarantee telomerase acts only in telomere homeostasis, and not at double-strand breaks (DSBs). Thus, the three pathways surrounding telomeres display incredible overlap and are immensely complex.

Here, I report a novel regulatory pathway that limits telomerase action during DNA damage. Duplication of the telomerase RNA subunit (TER) in Arabidopsis has given rise to a TER that is not required for telomere homeostasis. Indeed, this TER, termed TER2, is a competitive inhibitor of TER1 RNP complexes. Exposure to genotoxic agents results in TER2 upregulation and a subsequent inhibition of telomerase activity.

Using data from the 1,001 Arabidopsis genomes project, I determine that the TER duplication and inhibitory nature of TER2 is likely derived from a transposon-like element within TER2. This element is found throughout Brassicaceae, with at least 32 members in Arabidopsis lyrata. These findings highlight the complex and diverse mechanisms by which an organism will regulate telomerase action.

Here I characterize two members of the A. thaliana POT1 gene family. Contrary to POT1a, these proteins appear to have derived unique ways to perform their roles in chromosome-end protection. POT1b may protect telomeres as part of a TER2 telomerase RNP complex, as telomere defects only appear in the absence of both POT1b and TER2. POT1c is also appears to provide for chromosome end protection and appears to compete with POT1a to regulate telomerase access to the G-overhang. Together, these proteins represent part of a critical telomere capping complex distinct from CST.

Additionally, I describe a means for elucidating factors that regulate telomere addition at DSBs. This incredibly detrimental process, termed de novo telomere formation (DNTF), is toxic, and thus this work describes the first in depth characterization of DNTF in multicellular eukaryotes.

In summary, my work describes several novel regulatory and protective mechanisms for keeping telomeres and DSBs distinct.