Duplication and Diversification of Arabidopsis thaliana Telomerase RNP Components
Telomerase is a highly regulated ribonucleoprotein complex that stabilizes eukaryotic genomes by replenishing telomeric repeats on chromosome ends. Defects in telomerase RNP components involving the catalytic subunit TERT or the RNA template TER lead to stem cell-related diseases such as dyskeratosis congenita and idiopathic pulmonary fibrosis, while inappropriate telomerase expression is a rate-limiting step in carcinogenesis. In this study we report the discovery of a novel negative regulatory mechanism for telomerase that stems from duplication and diversification of key components of the telomerase RNP in the flowering plant Arabidopsis thaliana. We show that Arabidopsis encodes three distinct TERs: TER1, TER2 and a processed form of TER2 termed TER2S. Although all three RNAs can serve as templates for telomerase in vitro, in vivo they have different expression patterns, assemble into distinct RNPs with different protein binding partners, and play opposing roles in telomere maintenance. The TER1 RNP is analogous to the telomerase enzyme previously described in other eukaryotes, but the TER2 RNP is a negative regulator of telomerase activity and telomere maintenance in vivo. Furthermore, we demonstrate that the Protection Of Telomeres (POT1) paralogs in Arabidopsis (POT1a, POT1b and POT1c) are novel TER binding proteins. This finding is striking because in yeast and vertebrates, POT1 is an essential component of the telomere capping complex and functions to distinguish the chromosome terminus from a double-strand break. Thus, our data argue that Arabidopsis POT1 proteins have migrated off of the chromosome terminus and onto the telomerase RNP, indicating that duplication and diversification of Arabidopsis telomerase may be the end result of the co-evolution of the TER and POT1 RNP components. Additionally, given the dire consequences of misregulating telomerase in human cells, our discovery of a novel negative regulatory mechanism for telomerase in plants strongly suggests that additional modes of telomerase control remain to be elucidated in vertebrates.