Telomerase Dependence of Telomere Lengthening in ku80 Mutant Arabidopsis

Abstract

We have identified a ku80 mutant of Arabidopsis and show that telomerase is needed to generate the longer telomeres observed in this mutant. Telomeres are specialized nucleoprotein structures at the ends of chromosomes that permit cells to distinguish chromosome ends from double-strand breaks, thus preventing chromosome fusion events. Ku80 deficiency results in the lengthening of telomeres, a phenotype also seen in an Arabidopsis ku70 mutant. Furthermore, homogeneous populations of ku80 mutant cells show a steady increase in the length of telomere tracts, which reach an equilibrium length and then stabilize. In contrast to that in mammals, Ku80 deficiency in Arabidopsis cells does not cause end-to-end fusion of chromosomes. This telomere lengthening is dependent on the presence of telomerase, although it is not attributable to a significant increase in telomerase activity per se. These results demonstrate the essential role of the Ku80 protein as a negative regulator of telomerase function in plant cells.

DNA Replication Factor C1 Mediates Genomic Stability and Transcriptional Gene Silencing inArabidopsis

“…Human RFC1 (RFC p140) can bind to the telomeric repeat and inhibit telomerase activity (Uchiumi et al, 1999). Mutations in DNA replication-related proteins, such as Pol a, RFC1, CTF18, and ELG1 in yeast, and KU70, KU80, and replication protein A 70a in Arabidopsis, increase telomere length (Gallego et al, 2003;Takashi et al, 2009). Here, we found that loss of function of ROS1 increased telomere length, while the impairing of RFC1 and other replication proteins decreased telomere length in Arabidopsis.…”

Suppression of RICE TELOMERE BINDING PROTEIN1 Results in Severe and Gradual Developmental Defects Accompanied by Genome Instability in Rice

“…Despite limited insight into the plant telomeres compared with mammalian systems, plant telomeres have recently attracted much interest, since it is becoming increasingly apparent that the function and architectural features of telomeres as well as the patterns of telomerase regulation are largely conserved in higher plants and animals (reviewed in Riha and Shippen, 2003a;McKnight and Shippen, 2004;Gallego and White, 2005;Lamb et al, 2007;Zellinger and Riha, 2007). As in human and other animal species, the stable maintenance of telomeres is essential for growth and development in Arabidopsis, and a number of nuclear proteins, such as RAD50, KU70/80, MRE11, Pot1/2, ATM, and ATR, have been shown to be involved in telomere regulation (Gallego and White, 2001;Riha and Shippen, 2003b;Gallego et al, 2003;Puizina et al, 2004;Shakirov et al, 2005;Vespa et al, 2005;Vannier et al, 2006). Disruption of the genes encoding these telomere-related proteins causes strikingly different developmental phenotypes accompanied by an abrupt onset of genome instability.…”

Protection of Arabidopsis Blunt-Ended Telomeres Is Mediated by a Physical Association with the Ku Heterodimer

“…A seedling growth assay on agar plates with bleomycin showed growth retardation of Arabidopsis At25 mutants comparable to Ku-null plants ( Figures 1D and 1E), indicating a deficiency in DNA repair. Inactivation of Ku in Arabidopsis leads to a 3-fold increase in telomere length and elevated telomeric recombination that results in the excision of extrachromosomal telomeric-circles (t-circles) (Riha et al, 2002;Gallego et al, 2003;Zellinger et al, 2007). While complementation of the KU80-null mutation with the wild-type KU80 prevented telomere elongation and t-circle formation, no suppression of these telomeric phenotypes was observed in plants complemented with the At25 construct ( Figures 1F and 1G).…”

“…Besides mediating protection of blunt-ended telomeres, Ku is also a negative regulator of telomerase in Arabidopsis (Bundock et al, 2002;Riha et al, 2002;Gallego et al, 2003;Riha and Shippen, 2003). It has been proposed that the dramatic elongation of Ku-depleted telomeres is caused by an increase in telomerase's access to unprotected chromosome termini (Kazda et al, 2012).…”

“…Inactivation of Ku in Arabidopsis thaliana leads to telomerasedependent telomere elongation, resection of blunt-ended telomeres by exonuclease 1, and increased telomeric recombination (Bundock et al, 2002;Riha et al, 2002;Gallego et al, 2003;Riha and Shippen, 2003;Zellinger et al, 2007;Kazda et al, 2012). Ku is also important for proper telomere function in many other eukaryotes, including mice, humans, yeast, fungi, and even Drosophila melanogaster, whose telomeres are not formed by arrays of canonical telomeric repeats, but rather by retrotransposons (Samper et al, 2000;d'Adda di Fagagna et al, 2001;Gravel et al, 1998;Maringele and Lydall, 2002;Celli et al, 2006;Wang et al, 2009;Chico et al, 2011;de Sena-Tomás et al, 2015;Melnikova et al, 2005).…”