Telomere Regulation in Arabidopsis thaliana by the CST Capping Complex and DNA Damage Response Proteins

Abstract

The ends of chormosomes are capped by telomeres, which distinguish the termini from damaged DNA. Paradoxically, DNA repair proteins are also required for telomere maintenance. How DNA repair pathways are regulated to maintain telomeres while remaining competent to repair DNA damage throughout the genome is unknown. In this dissertation, I used a genetic approach to investigate how critical components of telomerase and the telomere protein complex interact with the DNA damage response (DDR).

In the flowering plant, Arabidopsis thaliana telomeres are bound by the CST (CTC1/STN1/TEN1) heterotrimer. Loss of any CST component results in telomere shortening, telomere fusions, increased G-overhang length and telomere recombination.

To understand the phenotypes caused by CST deficiency, I examined telomeres from plants lacking CTC1 or STN1 and TERT or KU. My analysis showed that CST acts in a separate genetic pathway for telomere length regulation from both KU and TERT. Further, I found that KU and CST act in separate genetic pathways for regulation of G-overhang formation. These demonstrate that multiple pathways are used to maintain telomere length and architecture in plants.

My study of the interaction of telomere components with the DDR revealed ATR promotes genome stability and telomere length maintenance in the absence of CTC1, probably by activating programmed cell death of stem cells with high amounts of DNA damage. I also found that poly(ADP-ribosylation) is not required for maintenance of Arabidopsis telomeres, in contrast to human telomeres.

Finally, I found an unexpected connection between the DDR and telomerase. My research showed that ATR maintains telomerase activity levels. Further, induction of double- stranded DNA breaks in seedlings led to a rapid decrease in telomerase activity, which correlated with increased abundance of TER2, an alternate Arabidopsis telomerase RNA. I hypothesize that TER2 inhibits telomerase to prevent its inappropriate action at internal sites in chromosomes. These data reveal two ways that DDR pathways work in concert with telomerase to promote genome integrity.