Surprises from the Chromosome Front: Lessons from Arabidopsis on Telomeres and Telomerase

The study of plant telomeres dates back to Barbara McClintock’s pioneering experiments in maize in the late 1930s, in which she studied the fate of broken chromosomes. McClintock discovered that a repair process was at work on dicentric chromosomes after they broke apart during mitosis. She termed this process “chromosome healing,” because these chromosomes were safe from future fusion events (McClintock 1938). McClintock’s observation led to the recognition of the chromosome terminus as a critical mediator of genome integrity. Subsequently, it was discovered that the semiconservative mechanism of replicating linear, eukaryotic DNA results in a small, unreplicated segment of DNA at the chromosome terminus (Olovnikov 1971; Watson 1972). Terminal DNA attrition from the so-called “end-replication problem” would not be a concern in nongermline cells, but it had to be circumvented for the complete transfer of full-length chromosomes to offspring. In the 1980s, Elizabeth Blackburn and Carol Greider solved this riddle by identifying telomerase in the ciliated protozoan Tetrahymena thermophila (Greider and Blackburn 1985). Genetic and biochemical analysis followed, demonstrating that telomerase consists of a reverse transcriptase (TERT) and a template-providing RNA (TER) molecule (Greider and Blackburn 1989; Shippen-Lentz and Blackburn 1990). It has been demonstrated that telomerase-mediated maintenance of telomere tracts is highly conserved across eukaryotes and is crucial for cellular longevity. Indeed, altering the dynamics of telomere length maintenance or perturbing the specialized protein–DNA architecture that protects the chromosome terminus has profound consequences for the integrity of the entire genome (Jain and Cooper 2010; Wellinger 2010). Telomeres typically comprise long arrays of G-rich repeats. The extreme 3′ terminus of the chromosome is single stranded and is termed the G-overhang. The G-overhang is complementary to the template region within TER and is used to prime telomere repeat synthesis by TERT, thereby solving the end-replication problem. Because the chromosome end resembles a double-strand break (DSB), it must be differentiated from damaged DNA. The task is accomplished by the unusual t-loop architecture of terminal DNA and a suite of telomere-binding proteins that guard chromosome ends and regulate telomerase access (Palm and de Lange 2008). Two distinct telomere complexes have been defined: shelterin and CST (Fig. 1A,B) (Price et al. 2010). Figure 1 Telomere protein complexes in budding yeast, vertebrates, and Arabidopsis. (A) The primary telomere capping complex in budding yeast comprises the dsDNA-binding protein Rap1, which associates with two other proteins, Rif1 and Rif2. G-overhangs are bound ... Vertebrate shelterin is composed of six proteins (Palm and de Lange 2008). TRF1 and TRF2 bind directly to dsDNA, whereas RAP1 localizes at telomeres through its interaction with TRF2. TIN2 acts as a bridging component between TRF/RAP1 and the G-overhang–binding heterodimer POT1/TPP1. In most species, POT1 shows high specificity toward single-stranded G-rich DNA, and this interaction is heightened by TPP1. Loss of POT1 and/or TPP1 is lethal in vertebrates and results in increased G-overhang signals and activation of a DNA-damage response (Palm and de Lange 2008). In addition to protecting the G-overhang, POT1/TPP1 recruits telomerase to the telomere through direct interactions between TPP1 and TERT (Nandakumar et al. 2012; Zhong et al. 2012). The heterotrimer CST (Cdc13/CTC1, STN1, TEN1) is functionally conserved across all major eukaryotic lineages (Fig. 1A,B). In budding yeast, CST functions with the double-stranded binding protein Rap1 to stabilize chromosome ends (Giraud-Panis et al. 2010). Although yeast Rap1 is orthologous to vertebrate RAP1 (de Lange 2009), no other shelterin components have been uncovered in budding yeast. Loss of CST components results in extended G-overhangs, elongated telomeres, and activation of a DNA-damage response (Nugent et al. 1996; Grandin et al. 1997; Xu et al. 2009). In addition, CST facilitates telomerase recruitment through interactions with the telomerase accessory protein Est1 (Qi and Zakian 2000). In contrast to the situation in yeast, the vertebrate CST complex does not have a major role in telomere length regulation but is instead critical for proper G-overhang maintenance and telomere replication through interactions with DNA polymerase α primase (Miyake et al. 2009; Wang et al. 2012). Owing to the repetitive nature of telomeric DNA, it is a challenging substrate for replication machinery. CST helps to overcome this barrier by facilitating replication fork passage through the telomere region (Stewart et al. 2012). Recent data further suggest that human CST can inhibit telomerase by both sequestration of the G-overhang and physical interactions with POT1/TPP1 (Chen et al. 2012). Following replication and telomere extension, CST is necessary for C-strand fill-in along the elongated G-overhang (Wang et al. 2012). The requirement for adequate telomere maintenance and end protection is critical for mammals, where stem cell viability and germline development must be balanced with the limited proliferation program of somatic cells to avert cancer. However, all eukaryotes share a fundamental need to fully replicate their telomeres and thus maintain genomic integrity. Owing to its remarkable tolerance of telomere dysfunction, Arabidopsis thaliana has proven to be an outstanding model system to address fundamental questions in telomere biology. Here, we discuss recent advances in Arabidopsis telomere biology. Specifically, we review the discovery of the CST complex and multiple-telomerase RNA subunits in Arabidopsis. These contributions reveal the surprising and distinct ways in which plants use highly conserved telomere components to promote genome stability and organismal viability.