Telomere Length Homeostasis Is Achieved via a Switch between Telomerase- Extendible and -Nonextendible States

Telomerase counteracts telomere erosion that stems from incomplete chromosome end replication and nucleolytic processing. A precise understanding of telomere length homeostasis has been hampered by the lack of assays that delineate the nonuniform telomere extension events of single chromosome molecules. Here, we measure telomere elongation at nucleotide resolution in Saccharomyces cerevisiae. The number of nucleotides added to a telomere in a single cell cycle varies between a few to more than 100 nucleotides and is independent of telomere length. Telomerase does not act on every telomere in each cell cycle, however. Instead, it exhibits an increasing preference for telomeres as their lengths decline. Deletion of the telomeric proteins Rif1 or Rif2 gives rise to longer telomeres by increasing the frequency of elongation events. Thus, by taking a molecular snapshot of a single round of telomere replication, we demonstrate that telomere length homeostasis is achieved via a switch between telomerase-extendible and -nonextendible states.

Telomere length homeostasis

Joachim Lingner

The physical ends of chromosomes, known as telomeres, protect chromosome ends from nucleolytic degradation and DNA repair activities. Conventional DNA replication enzymes lack the ability to fully replicate telomere ends. In addition, nucleolytic activities contribute to telomere erosion. Short telomeres trigger DNA damage checkpoints, which mediate cellular senescence. Telomere length homeostasis requires telomerase, a cellular reverse transcriptase, which uses an internal RNA moiety as a template for the synthesis of telomere repeats. Telomerase elongates the 3' ends of chromosomes, whereas the complementary strand is filled in by conventional DNA polymerases. In humans, telomerase is ubiquitously expressed only during the first weeks of embryogenesis, and is subsequently downregulated in most cell types. Correct telomere length setting is crucial for long-term survival. The telomere length reserve must be sufficient to avoid premature cellular senescence and the acceleration of age-related disease. On the other side, telomere shortening suppresses tumor formation through limiting the replicative potential of cells. In recent years, novel insight into the regulation of telomerase at chromosome ends has increased our understanding on how telomere length homeostasis in telomerase-positive cells is achieved. Factors that recruit telomerase to telomeres in a cell cycle-dependent manner have been identified in Saccharomyces cerevisiae. In humans, telomerase assembles with telomeres during S phase of the cell cycle. Presumably through mediating formation of alternative telomere structures, telomere-binding proteins regulate telomerase activity in cis to favor preferential elongation of the shortest telomeres. Phosphoinositide 3-kinase related kinases are also required for telomerase activation at chromosome ends, at least in budding and fission yeast. In vivo analysis of telomere elongation kinetics shows that telomerase does not act on every telomere in each cell cycle but that it exhibits an increasing preference for telomeres as their lengths decline. This suggests a model in which telomeres switch between extendible and nonextendible states in a length-dependent manner. In this review we expand this model to incorporate the finding that telomerase levels also limit telomere length and we propose a second switch between a non-telomerase-associated "extendible" and a telomerase-associated "extending" state.

Springer-Verlag2006

The Rat1p 5’ to 3’ exonuclease degrades telomeric repeat-containing RNA and promotes telomere elongation in Saccharomyces cerevisiae

Nahid Iglesias, Joachim Lingner, Andrea Bernardo Panza, Sophie Redon

Vertebrate telomeres are transcribed into telomeric repeat-containing RNA (TERRA) that associates with telomeres and may be important for telomere function. Here, we demonstrate that telomeres are also transcribed in Saccharomyces cerevisiae by RNA polymerase II (RNAPII). Yeast TERRA is polyadenylated and stabilized by Pap1p and regulated by the 5' to 3' exonuclease, Rat1p. rat1-1 mutant cells accumulate TERRA and harbor short telomeres because of defects in telomerase-mediated telomere elongation. Overexpression of RNaseH overcomes telomere elongation defects in rat1-1 cells, indicating that RNA/DNA hybrids inhibit telomerase function at chromosome ends in these mutants. Thus, telomeric transcription combined with Rat1p-dependent TERRA degradation is important for regulating telomerase in yeast. Telomere transcription is conserved in different kingdoms of the eukaryotic domain.

2008

Replication of Telomeres and the Regulation of Telomerase

Joachim Lingner, Verena Pfeiffer

Telomeres are the physical ends of eukaryotic chromosomes. They protect chromosome ends from DNA degradation, recombination, and DNA end fusions, and they are important for nuclear architecture. Telomeres provide a mechanism for their replication by semiconservative DNA replication and length maintenance by telomerase. Through telomerase repression and induced telomere shortening, telomeres provide the means to regulate cellular life span. In this review, we introduce the current knowledge on telomere composition and structure. We then discuss in depth the current understanding of how telomere components mediate their function during semiconservative DNA replication and how telomerase is regulated at the end of the chromosome. We focus our discussion on the telomeres from mammals and the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe.

Cold Spring Harbor Lab Press, Publications Dept2013

Telomere length homeostasis

Joachim Lingner

Springer-Verlag2006