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.

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Telomeres are the physical ends of linear chromosomes in eukaryotes. Telomeres not only protect chromosome ends from being recognized as double-strand breaks but also maintain the chromosome terminal sequences. These processes involve a number of telomere-related proteins. A major challenge in the field is to elucidate the full constitution of telomere-associated proteins and to understand how different protein complexes are regulated at chromosome termini. Here, I report the identification and characterization of STN1 (Suppressor of cdc thirteen, 1), CTC1 (Conserved Telomere maintenance Component 1) and TEN1 (Telomeric pathways in association with Stn1, 1) in Arabidopsis. CTC1/STN1/TEN1 (CST) forms a trimeric complex that specifically associates with telomeres. Loss of any component of the CST induces catastrophic telomere loss, disrupted telomere end architecture, and massive chromosome end-to-end fusions. Thus, CST plays an essential role in chromosome end protection. I also show that CST function at telomeres is independent of a previously characterized capping complex KU70/KU80, and that ATR is responsible for a checkpoint response in plants lacking CTC1/STN1. Additionally, I present data showing that Arabidopsis POT1a (Protection Of Telomere 1, a) has evolved as a telomerase recruitment factor. Unlike POT1 in other eukaryotes which binds and protects ss telomeric DNA, AtPOT1a interacts with telomerase RNA (TER). Based on an evolutionary analysis, we found that the POT1a lineage is under positive selection in the Brassicaceae family in which Arabidopsis belongs. Mutations of two positive selection sites significantly reduce POT1a?s activity in vivo. These data suggest POT1a is under pressure to evolve from a telomeric DNA binding protein to a TER binding protein. I also discovered that POT1a interacts with the novel telomere capping protein CTC1 in vitro and in vivo. Thus, I hypothesize that POT1a acts as a telomerase recruitment factor linking this enzyme to the chromosome termini via interacting with TER and CTC1. Finally, I dissected the functional domains of POT1a and demonstrated that both the N-terminus and the C-terminus of POT1a are required for its function in vivo. In summary, my work has uncovered several new and essential telomereassociated proteins that provide new insight into mechanisms of chromosome end protection and maintenance.

Die meisten Cyanobakterien und Chloroplasten von Rotalgen verfügen über Phycobilisomen, große lichtsammelnde Multiproteinkomplexe, die an der cytoplasmatischen Seite der Thylakoidmembran gebunden vorliegen. Unter stickstofflimitierten Bedingungen werden die Phycobilisomen proteolytisch abgebaut. Dieser Prozess schützt vor Fotoschäden unter den gegebenen Stressbedingungen und liefert gleichzeitig einen großen Vorrat an Stickstoff enthaltenden Substanzen. Das Gen nblA, welches in allen Phycobilisomen enthaltenden Organismen vorkommt, kodiert für ein Polypeptid von ungefähr 7 kDa, das eine Schlüsselrolle im Abbau der Phycobilisomen einnimmt. Die Wirkungsweise von NblA dabei wird jedoch bisher kaum verstanden. Ein Selenomethioninderivat von NblA des filamentösen Cyanobakteriums Anabaena sp. PCC 7120 wurde in Escherichia coli rekombinant hergestellt, gereinigt und kristallisiert. Die Röntgenkristallstruktur von NblA wurde mithilfe der single-wavelength anomalous dispersion-Methode bis zu einer Auflösung von 1,8 Angstrom bestimmt. Das finale Modell verfügt über einen kristallographischen R-Wert von 18,2% und einen freien R-Wert von 21,7%. Das kleine NblA-Protein von 65 Aminosäuren besteht aus zwei alpha-Helices, die in einem ca. 37°-Winkel in einer antiparallelen, V-förmigen Anordnung zueinander stehen. Zwei dieser Monomere bilden die grundlegende strukturelle Einheit von NblA, ein vier-Helix-Bündel, bei dem sich die Spitzen der "Vs" auf der gleichen Seite des Dimers überlagern und über eine nicht-kristallographische zweizählige Achse miteinander verknüpft sind. Auf der Grundlage von Bindungsstudien und der Kenntnis der NblA-Struktur konnte ein Modell für die Bindung von NblA an die Phycobilisomenstruktur postuliert werden. / Cyanobacterial light harvesting complexes, the phycobilisomes, are proteolytically degraded when the organisms are starved for combined nitrogen, a process referred to as chlorosis or bleaching. Gene nblA, present in all phycobilisome-containing organisms, encodes a protein of about 7 kDa that plays a key role in phycobilisome degradation. To gain deeper insights into the mode of action of NblA in this degradation process the crystal structure of NblA was determined and a model of its binding to phycobilisomes was proposed. For this purpose, NblA from Anabaena sp. PCC 7120 was produced as selenomethionine NblA derivative in Escherichia coli B834 (DE3), purified and crystalized. NblA crystals grew as long, but thin rods and belong to the monoclinic space group P2(1) with cell parameters of a = 43.2 A, b = 95.9 A, c = 104.8 A and Beta = 97.0°. They contain twelve NblA monomers in the asymmetric unit. The crystal structure with a resolution of 1.8 A was determined using the single-wavelength anomalous dispersion (SAD) technique and refined to final values for Rwork and Rfree of 0.182 and 0.217, respectively. The small NblA polypeptide of 65 amino acids consists of two alpha-helices which are assembled at a ~37° angle in an antiparallel, V-shaped arrangement. Two NblA monomers form the basic structural unit of NblA, a four-helix bundle with the tips of the V superimposing on the same side of a dimer. The dimer is formed by two molecules related by a non-crystallographic dyad axis. Based on the crystal structure presented here, pull-down experiments, and peptide scan data a model of binding of NblA to phycobilisomes was proposed. Considering the entire phycobilisome structure, NblA is predicted to bind via its amino acids Leu51 and Lys53 to the trimer-trimer interface of the phycobiliprotein hexamers.