The function of DNA polymerase alpha at telomeric G tails is important for telomere homeostasis

Maturation and shuttling of the yeast telomerase RNP: assembling something new using recycled parts

As the limiting component of the budding yeast telomerase, the Tlc1 RNA must undergo multiple consecutive modifications and rigorous quality checks throughout its lifecycle. These steps will ensure that only correctly processed and matured molecules are assembled into telomerase complexes that subsequently act at telomeres. The complex pathway of Tlc1 RNA maturation, involving 5'- and 3'-end processing, stabilisation and assembly with the protein subunits, requires at least one nucleo-cytoplasmic passage. Furthermore, it appears that the pathway is tightly coordinated with the association of various and changing proteins, including the export factor Xpo1, the Mex67/Mtr2 complex, the Kap122 importin, the Sm₇ ring and possibly the CBC and TREX-1 complexes. Although many of these maturation processes also affect other RNA species, the Tlc1 RNA exploits them in a new combination and, therefore, ultimately follows its own and unique pathway. In this review, we highlight recent new insights in maturation and subcellular shuttling of the budding yeast telomerase RNA and discuss how these events may be fine-tuned by the biochemical characteristics of the varying processing and transport factors as well as the final telomerase components. Finally, we indicate outstanding questions that we feel are important to be addressed for a complete understanding of the telomerase RNA lifecycle and that could have implications for the human telomerase as well.

Telomere Length and Oxidative Stress and Its Relation with Metabolic Syndrome Components in the Aging

Simple Summary

A link between telomere length and some age-related diseases has been identified, including metabolic syndrome. So far, there is no mechanism to explain the origin or cause of telomere shortening in this syndrome; however, oxidative stress is a constant factor. Therefore, we reviewed scientific evidence that supported the association between oxidative stress and telomere length dynamics, also examining how each of the metabolic syndrome components individually affects the length. In this regard, there is strong scientific evidence that an increase in the number of metabolic syndrome components is associated with a shorter telomere length, oxidative damage at the lipid and DNA level, and inflammation, as well as its other components, such as obesity, hyperglycemia, and hypertension, while for dyslipidemia, there is a little more discrepancy. The difficulty for the correct treatment of metabolic syndrome lies in its multifactorial nature. Hence, there is a need to carry out more studies on healthy lifestyles during aging to prevent and reduce oxidative damage and telomere wear during aging, and consequently the progression of chronic degenerative diseases, thus improving the living conditions of older people.

Abstract

A great amount of scientific evidence supports that Oxidative Stress (OxS) can contribute to telomeric attrition and also plays an important role in the development of certain age-related diseases, among them the metabolic syndrome (MetS), which is characterised by clinical and biochemical alterations such as obesity, dyslipidaemia, arterial hypertension, hyperglycaemia, and insulin resistance, all of which are considered as risk factors for type 2 diabetes mellitus (T2DM) and cardiovascular diseases, which are associated in turn with an increase of OxS. In this sense, we review scientific evidence that supports the association between OxS with telomere length (TL) dynamics and the relationship with MetS components in aging. It was analysed whether each MetS component affects the telomere length separately or if they all affect it together. Likewise, this review provides a summary of the structure and function of telomeres and telomerase, the mechanisms of telomeric DNA repair, how telomere length may influence the fate of cells or be linked to inflammation and the development of age-related diseases, and finally, how the lifestyles can affect telomere length.

At the Beginning of the End and in the Middle of the Beginning: Structure and Maintenance of Telomeric DNA Repeats and Interstitial Telomeric Sequences

Tandem DNA repeats derived from the ancestral (TTAGGG)n run were first detected at chromosome ends of the majority of living organisms, hence the name telomeric DNA repeats. Subsequently, it has become clear that telomeric motifs are also present within chromosomes, and they were suitably called interstitial telomeric sequences (ITSs). It is well known that telomeric DNA repeats play a key role in chromosome stability, preventing end-to-end fusions and precluding the recurrent DNA loss during replication. Recent data suggest that ITSs are also important genomic elements as they confer its karyotype plasticity. In fact, ITSs appeared to be among the most unstable microsatellite sequences as they are highly length polymorphic and can trigger chromosomal fragility and gross chromosomal rearrangements. Importantly, mechanisms responsible for their instability appear to be similar to the mechanisms that maintain the length of genuine telomeres. This review compares the mechanisms of maintenance and dynamic properties of telomeric repeats and ITSs and discusses the implications of these dynamics on genome stability.

The yeast telomerase module for telomere recruitment requires a specific RNA architecture

Telomerases are ribonucleoprotein (RNP) reverse transcriptases. While telomerases maintain genome stability, their composition varies significantly between species. Yeast telomerase RNPs contain an RNA that is comparatively large, and its overall folding shows long helical segments with distal functional parts. Here we investigated the essential stem IVc module of the budding yeast telomerase RNA, called Tlc1. The distal part of stem IVc includes a conserved sequence element CS2a and structurally conserved features for binding Pop1/Pop6/Pop7 proteins, which together function analogously to the P3 domains of the RNase P/MRP RNPs. A more proximal bulged stem with the CS2 element is thought to associate with Est1, a telomerase protein required for telomerase recruitment to telomeres. Previous work found that changes in CS2a cause a loss of all stem IVc proteins, not just the Pop proteins. Here we show that the association of Est1 with stem IVc indeed requires both the proximal bulged stem and the P3 domain with the associated Pop proteins. Separating the P3 domain from the Est1 binding site by inserting only 2 base pairs into the helical stem between the two sites causes a complete loss of Est1 from the RNP and hence a telomerase-negative phenotype in vivo. Still, the distal P3 domain with the associated Pop proteins remains intact. Moreover, the P3 domain ensures Est2 stability on the RNP independently of Est1 association. Therefore, the Tlc1 stem IVc recruitment module of the RNA requires a very tight architectural organization for telomerase function in vivo.

STN1-POLA2 interaction provides a basis for primase-pol α stimulation by human STN1

The CST (CTC1–STN1–TEN1) complex mediates critical functions in maintaining telomere DNA and overcoming genome-wide replication stress. A conserved biochemical function of the CST complex is its primase-Pol α (PP) stimulatory activity. In this report, we demonstrate the ability of purified human STN1 alone to promote PP activity in vitro. We show that this regulation is mediated primarily by the N-terminal OB fold of STN1, but does not require the DNA-binding activity of this domain. Rather, we observed a strong correlation between the PP-stimulatory activity of STN1 variants and their abilities to bind POLA2. Remarkably, the main binding target of STN1 in POLA2 is the latter's central OB fold domain. In the substrate-free structure of PP, this domain is positioned so as to block nucleic acid entry to the Pol α active site. Thus the STN1–POLA2 interaction may promote the necessary conformational change for nucleic acid delivery to Pol α and subsequent DNA synthesis. A disease-causing mutation in human STN1 engenders a selective defect in POLA2-binding and PP stimulation, indicating that these activities are critical for the in vivo function of STN1. Our findings have implications for the molecular mechanisms of PP, STN1 and STN1-related molecular pathology.

Association between clincopathological characteristics and hTERT expression as well as telomere length in ameloblastoma

OBJECTIVE

This study aimed to investigate the human telomerase reverse transcriptase (hTERT) expression and telomere length in ameloblastoma, and expolre the role of hTERT in the invasiveness and recurrence of ameloblastoma.

METHODS

hTERT expression was detected by immunohistochemistry and Western blotting and telomere length by fluorescence in situ hybridization (FISH) in human ameloblastoma, normal mucosa, and oral squamous cell carcinoma (OSCC). Association between clincopathological characteristics and hTERT expression as well as telomere length in ameloblastoma was analyzed.

RESULTS

hTERT expression in ameloblastoma was higher than that in normal oral mucosa, and the highest in OSCC (P<0.05). The hTERT positive rate and expression increased with the recurrence and malignant transformation. hTERT expression was significantly different among different types of ameloblastoma. The telomere was the shortest in OSCC and the longest in normal oral mucosa.

CONCLUSION

Ameloblastoma has shortened telomere, and is positive for hTERT expression which is related to the recurrence and malignancy of ameloblastoma. These findings indicate that telomerase is involved in the occurrence and development of ameloblastoma.

Regulating telomere length from the inside out: the replication fork model

In this Hypothesis, Greider describes a new model for telomere length regulation, which links DNA replication and telomere elongation.

Telomere length is regulated around an equilibrium set point. Telomeres shorten during replication and are lengthened by telomerase. Disruption of the length equilibrium leads to disease; thus, it is important to understand the mechanisms that regulate length at the molecular level. The prevailing protein-counting model for regulating telomerase access to elongate the telomere does not explain accumulating evidence of a role of DNA replication in telomere length regulation. Here I present an alternative model: the replication fork model that can explain how passage of a replication fork and regulation of origin firing affect telomere length.

Tying up the Ends: Plasticity in the Recognition of Single-Stranded DNA at Telomeres

Telomeres terminate nearly exclusively in single-stranded DNA (ssDNA) overhangs comprised of the G-rich 3' end. This overhang varies widely in length from species to species, ranging from just a few bases to several hundred nucleotides. These overhangs are not merely a remnant of DNA replication but rather are the result of complex further processing. Proper management of the telomeric overhang is required both to deter the action of the DNA damage machinery and to present the ends properly to the replicative enzyme telomerase. This Current Topic addresses the biochemical and structural features used by the proteins that manage these variable telomeric overhangs. The Pot1 protein tightly binds the single-stranded overhang, preventing DNA damage sensors from binding. Pot1 also orchestrates the access of telomerase to that same substrate. The remarkable plasticity of the binding interface exhibited by the Schizosaccharomyces pombe Pot1 provides mechanistic insight into how these roles may be accomplished, and disease-associated mutations clustered around the DNA-binding interface in the hPOT1 highlight the importance of this function. The budding yeast Cdc13-Stn1-Ten1, a telomeric RPA complex closely associated with telomere function, also interacts with ssDNA in a fashion that allows degenerate sequences to be recognized. A related human complex composed of hCTC1, hSTN1, and hTEN1 has recently emerged with links to both telomere maintenance and general DNA replication and also exhibits mutations associated with telomere pathologies. Overall, these sequence-specific ssDNA binders exhibit a range of recognition properties that allow them to perform their unique biological functions.

Getting it done at the ends: Pif1 family DNA helicases and telomeres

It is widely appreciated that the ends of linear DNA molecules cannot be fully replicated by the conventional replication apparatus. Less well known is that semi-conservative replication of telomeric DNA also presents problems for DNA replication. These problems likely arise from the atypical chromatin structure of telomeres, the GC-richness of telomeric DNA that makes it prone to forming DNA secondary structures, and from RNA-DNA hybrids, formed by transcripts of one or both DNA strands. Given the different aspects of telomeres that complicate their replication, it is not surprising that multiple DNA helicases promote replication of telomeric DNA. This review focuses on one such class of DNA helicases, the Pif1 family of 5'-3' DNA helicases. In budding and fission yeasts, Pif1 family helicases impact both telomerase-mediated and semi-conservative replication of telomeric DNA as well as recombination-mediated telomere lengthening.

Mte1 interacts with Mph1 and promotes crossover recombination and telomere maintenance

Silva et al. identify Mte1 as a novel interactor of the Mph1 helicase in Saccharomyces cerevisiae. Their data indicate that Mte1 plays a role in regulation of crossover recombination, response to replication stress, and telomere maintenance.

Mph1 is a member of the conserved FANCM family of DNA motor proteins that play key roles in genome maintenance processes underlying Fanconi anemia, a cancer predisposition syndrome in humans. Here, we identify Mte1 as a novel interactor of the Mph1 helicase in Saccharomyces cerevisiae. In vitro, Mte1 (Mph1-associated telomere maintenance protein 1) binds directly to DNA with a preference for branched molecules such as D loops and fork structures. In addition, Mte1 stimulates the helicase and fork regression activities of Mph1 while inhibiting the ability of Mph1 to dissociate recombination intermediates. Deletion of MTE1 reduces crossover recombination and suppresses the sensitivity of mph1Δ mutant cells to replication stress. Mph1 and Mte1 interdependently colocalize at DNA damage-induced foci and dysfunctional telomeres, and MTE1 deletion results in elongated telomeres. Taken together, our data indicate that Mte1 plays a role in regulation of crossover recombination, response to replication stress, and telomere maintenance.

A Single Templating RNA in Yeast Telomerase

The number of essential telomerase components in the active ribonucleoprotein (RNP) has important implications for its mechanism of action yet is by and large unknown. We report that two differentially tagged TLC1 RNAs endogenously expressed in a heterozygous diploid and simultaneously detected via multi-color fluorescence in situ hybridization (FISH) experiments do not co-localize. Probabilistic calculations combined with direct quantification of FISH signals demonstrate that the TLC1 RNA indeed occurs as a single molecule in these RNPs. In addition, two differentially tagged reverse-transcriptase subunits could not be co-immunoprecipitated. These results therefore show that, in yeast cells, telomerase is assembled and matured and occurs as a monomer when not on telomeres. Finally, combining these findings with previous evidence leads us to propose that the enzyme also acts as a monomer when elongating telomeres.

Role of swi7H4 mutant allele of DNA polymerase α in the DNA damage checkpoint response

Besides being a mediator of initiation of DNA replication, DNA polymerase α plays a key role in chromosome maintenance. Swi7H4, a novel temperature sensitive mutant of DNA polymerase α was shown to be defective in transcriptional silencing at the mating type centromere and telomere loci. It is also required for the establishment of chromatin state that can recruit the components of the heterochromatin machinery at these regions. Recently the role of DNA polymerase α in the S-phase alkylation damage response in S. pombe has also been studied. Here we investigate whether defects generated by swi7H4, a mutant allele of DNA polymerase α can activate a checkpoint response. We show that swi7H4 exhibit conditional synthetic lethality with chk1 null mutant and the double mutant of swi7H4 with chk1 deletion aggravate the chromosome segregation defects. More importantly swi7H4 mutant cells delay the mitotic progression at non permissive temperature that is mediated by checkpoint protein kinase Chk1. In addition we show that, in the swi7H4 mutant background, cells accumulate DNA damage at non permissive temperature activating the checkpoint kinase protein Chk1. Further, we observed synthetic lethality between swi7H4 and a number of genes involved in DNA repair pathway at semi permissive temperature. We summarize that defects in swi7H4 mutant results in DNA damage that delay mitosis in a Chk1 dependent manner that also require the damage repair pathway for proper recovery.

The CDC13-STN1-TEN1 complex stimulates Pol α activity by promoting RNA priming and primase-to-polymerase switch

Emerging evidence suggests that Cdc13-Stn1-Ten1 (CST), an RPA-like ssDNA-binding complex, may regulate primase-Pol α (PP) activity at telomeres constitutively, and at other genomic locations under conditions of replication stress. Here we examine the mechanisms of PP stimulation by CST using purified complexes derived from Candida glabrata. While CST does not enhance isolated DNA polymerase activity, it substantially augments both primase activity and primase-to-polymerase switching. CST also simultaneously shortens the RNA and lengthens the DNA in the chimeric products. Stn1, the most conserved subunit of CST, is alone capable of PP stimulation. Both the N-terminal OB fold and the C-terminal winged-helix domains of Stn1 can bind to the Pol12 subunit of the PP complex, and stimulate PP activity. Our findings provide mechanistic insights on a well-conserved pathway of PP regulation that is critical for genome stability.

Elucidation of the DNA end-replication problem in Saccharomyces cerevisiae

The model for telomere shortening at each replication cycle is currently incomplete, and the exact contribution of the telomeric 3' overhang to the shortening rate remains unclear. Here, we demonstrate key steps of the mechanism of telomere replication in Saccharomyces cerevisiae. By following the dynamics of telomeres during replication at near-nucleotide resolution, we find that the leading-strand synthesis generates blunt-end intermediates before being 5'-resected and filled in. Importantly, the shortening rate is set by positioning the last Okazaki fragments at the very ends of the chromosome. Thus, telomeres shorten in direct proportion to the 3' overhang lengths of 5-10 nucleotides that are present in parental templates. Furthermore, the telomeric protein Cdc13 coordinates leading- and lagging-strand syntheses. Taken together, our data unravel a precise choreography of telomere replication elucidating the DNA end-replication problem and provide a framework to understand the control of the cell proliferation potential.

SUMOylation regulates telomere length by targeting the shelterin subunit Tpz1(Tpp1) to modulate shelterin-Stn1 interaction in fission yeast

Telomeres protect DNA ends of linear eukaryotic chromosomes from degradation and fusion, and ensure complete replication of the terminal DNA through recruitment of telomerase. The regulation of telomerase is a critical area of telomere research and includes cis regulation by the shelterin complex in mammals and fission yeast. We have identified a key component of this regulatory pathway as the SUMOylation [the covalent attachment of a small ubiquitin-like modifier (SUMO) to target proteins] of a shelterin subunit in fission yeast. SUMOylation is known to be involved in the negative regulation of telomere extension by telomerase; however, how SUMOylation limits the action of telomerase was unknown until now. We show that SUMOylation of the shelterin subunit TPP1 homolog in Schizosaccharomyces pombe (Tpz1) on lysine 242 is important for telomere length homeostasis. Furthermore, we establish that Tpz1 SUMOylation prevents telomerase accumulation at telomeres by promoting recruitment of Stn1-Ten1 to telomeres. Our findings provide major mechanistic insights into how the SUMOylation pathway collaborates with shelterin and Stn1-Ten1 complexes to regulate telomere length.

Fission yeast shelterin regulates DNA polymerases and Rad3(ATR) kinase to limit telomere extension

Studies in fission yeast have previously identified evolutionarily conserved shelterin and Stn1-Ten1 complexes, and established Rad3^ATR/Tel1^ATM-dependent phosphorylation of the shelterin subunit Ccq1 at Thr93 as the critical post-translational modification for telomerase recruitment to telomeres. Furthermore, shelterin subunits Poz1, Rap1 and Taz1 have been identified as negative regulators of Thr93 phosphorylation and telomerase recruitment. However, it remained unclear how telomere maintenance is dynamically regulated during the cell cycle. Thus, we investigated how loss of Poz1, Rap1 and Taz1 affects cell cycle regulation of Ccq1 Thr93 phosphorylation and telomere association of telomerase (Trt1^TERT), DNA polymerases, Replication Protein A (RPA) complex, Rad3^ATR-Rad26^ATRIP checkpoint kinase complex, Tel1^ATM kinase, shelterin subunits (Tpz1, Ccq1 and Poz1) and Stn1. We further investigated how telomere shortening, caused by trt1Δ or catalytically dead Trt1-D743A, affects cell cycle-regulated telomere association of telomerase and DNA polymerases. These analyses established that fission yeast shelterin maintains telomere length homeostasis by coordinating the differential arrival of leading (Polε) and lagging (Polα) strand DNA polymerases at telomeres to modulate Rad3^ATR association, Ccq1 Thr93 phosphorylation and telomerase recruitment.

Stable maintenance of telomeres is critical to maintain a stable genome and to prevent accumulation of undesired mutations that may lead to formation of tumors. Telomere dysfunction can also lead to premature aging due to depletion of the stem cell population, highlighting the importance of understanding the regulatory mechanisms that ensure stable telomere maintenance. Based on careful analysis of cell cycle-regulated changes in telomere association of telomerase, DNA polymerases, Replication Protein A, checkpoint kinases, telomere protection complex shelterin, and Stn1-Ten1 complex, we will provide here a new and dynamic model of telomere length regulation in fission yeast, which suggests that shelterin-dependent regulation of differential arrival of leading and lagging strand DNA polymerase at telomeres is responsible for modulating Rad3^ATR checkpoint kinase accumulation and Rad3^ATR-dependent phosphorylation of shelterin subunit Ccq1 to control telomerase recruitment to telomeres.

Telomere-end processing: mechanisms and regulation

Telomeres are specialized nucleoprotein complexes that provide protection to the ends of eukaryotic chromosomes. Telomeric DNA consists of tandemly repeated G-rich sequences that terminate with a 3' single-stranded overhang, which is important for telomere extension by the telomerase enzyme. This structure, as well as most of the proteins that specifically bind double and single-stranded telomeric DNA, are conserved from yeast to humans, suggesting that the mechanisms underlying telomere identity are based on common principles. The telomeric 3' overhang is generated by different events depending on whether the newly synthesized strand is the product of leading- or lagging-strand synthesis. Here, we review the mechanisms that regulate these processes at Saccharomyces cerevisiae and mammalian telomeres.

Dna2 is involved in CA strand resection and nascent lagging strand completion at native yeast telomeres

Post-replicational telomere end processing involves both extension by telomerase and resection to produce 3'-GT-overhangs that extend beyond the complementary 5'-CA-rich strand. Resection must be carefully controlled to maintain telomere length. At short de novo telomeres generated artificially by HO endonuclease in the G2 phase, we show that dna2-defective strains are impaired in both telomere elongation and sequential 5'-CA resection. At native telomeres in dna2 mutants, GT-overhangs do clearly elongate during late S phase but are shorter than in wild type, suggesting a role for Dna2 in 5'-CA resection but also indicating significant redundancy with other nucleases. Surprisingly, elimination of Mre11 nuclease or Exo1, which are complementary to Dna2 in resection of internal double strand breaks, does not lead to further shortening of GT-overhangs in dna2 mutants. A second step in end processing involves filling in of the CA-strand to maintain appropriate telomere length. We show that Dna2 is required for normal telomeric CA-strand fill-in. Yeast dna2 mutants, like mutants in DNA ligase 1 (cdc9), accumulate low molecular weight, nascent lagging strand DNA replication intermediates at telomeres. Based on this and other results, we propose that FEN1 is not sufficient and that either Dna2 or Exo1 is required to supplement FEN1 in maturing lagging strands at telomeres. Telomeres may be among the subset of genomic locations where Dna2 helicase/nuclease is essential for the two-nuclease pathway of primer processing on lagging strands.

Everything you ever wanted to know about Saccharomyces cerevisiae telomeres: beginning to end

The mechanisms that maintain the stability of chromosome ends have broad impact on genome integrity in all eukaryotes. Budding yeast is a premier organism for telomere studies. Many fundamental concepts of telomere and telomerase function were first established in yeast and then extended to other organisms. We present a comprehensive review of yeast telomere biology that covers capping, replication, recombination, and transcription. We think of it as yeast telomeres—soup to nuts.

Telomere replication: poised but puzzling

Faithful replication of chromosomes is essential for maintaining genome stability. Telomeres, the chromosomal termini, pose quite a challenge to replication machinery due to the complexity in their structures and sequences. Efficient and complete replication of chromosomes is critical to prevent aberrant telomeres as well as to avoid unnecessary loss of telomere DNA. Compelling evidence supports the emerging picture of synergistic actions between DNA replication proteins and telomere protective components in telomere synthesis. This review discusses the actions of various replication and telomere-specific binding proteins that ensure accurate telomere replication and their roles in telomere maintenance and protection.

Mapping the FEN1 interaction domain with hTERT

The activity of telomerase in cancer cells is tightly regulated by numerous proteins including DNA replication factors. However, it is unclear how replication proteins regulate telomerase action in higher eukaryotic cells. Previously we have demonstrated that the multifunctional DNA replication and repair protein flap endonuclease 1 (FEN1) is in complex with telomerase and may regulate telomerase activity in mammalian cells. In this study, we further analyzed the nature of this association. Our results show that FEN1 and telomerase association occurs throughout the S phase, with the maximum association in the mid S phase. We further mapped the physical domains in FEN1 required for this association and found that the C-terminus and the nuclease domain of FEN1 are involved in this interaction, whereas the PCNA binding ability of FEN1 is dispensable for the interaction. These results provide insights into the nature of possible protein-protein associations that telomerase participates in for maintaining functional telomeres.

When the caps fall off: responses to telomere uncapping in yeast

Telomeres protect the ends of linear chromosomes from activities that cause sequence losses or challenge chromosome integrity. Furthermore, these ends must be hidden from detection by the DNA damage recognition and response pathways. In particular, they must not fuse with each other. These fundamental and very first functions attributed to telomeres are also summarized with the term 'chromosome capping'. However, telomeres can become uncapped and the foremost cellular responses to such events aim to restore genome stability in the most conservative fashion possible. I will provide an outline of cellular responses to uncapping in budding yeast and briefly discuss the reverse, namely avoidance mechanisms that prevent telomere formation at inappropriate places.

RPA-like mammalian Ctc1-Stn1-Ten1 complex binds to single-stranded DNA and protects telomeres independently of the Pot1 pathway

Budding yeast Cdc13, Stn1, and Ten1 form the CST complex to protect telomeres from lethal DNA degradation. It remains unknown whether similar complexes are conserved in higher eukaryotes or not. Here we isolated mammalian STN1 and TEN1 homologs and CTC1 (conserved telomere maintenance component 1). The three proteins contain putative OB-fold domains and form a complex called CST, which binds to single-stranded DNA with high affinity in a sequence-independent manner. CST associates with a fraction of telomeres consistently during the cell cycle, in quiescent cells and Pot1-knockdown cells. It does not colocalize with replication foci in S phase. Significant increases in the abundance of single-stranded G-strand telomeric DNA were observed in Stn1-knockdown cells. We propose that CST is a replication protein A (RPA)-like complex that is not directly involved in conventional DNA replication at forks but plays a role in DNA metabolism frequently required by telomeres.

TEN1 is essential for CDC13-mediated telomere capping

Telomere binding proteins protect chromosome ends from degradation and mask chromosome termini from checkpoint surveillance. In Saccharomyces cerevisiae, Cdc13 binds single-stranded G-rich telomere repeats, maintaining telomere integrity and length. Two additional proteins, Ten1 and Stn1, interact with Cdc13 but their contributions to telomere integrity are not well defined. Ten1 is known to prevent accumulation of aberrant single-stranded telomere DNA; whether this results from defective end protection or defective telomere replication is unclear. Here we report our analysis of a new group of ten1 temperature-sensitive (ts) mutants. At permissive temperatures, ten1-ts strains display greatly elongated telomeres. After shift to nonpermissive conditions, however, ten1-ts mutants accumulate extensive telomeric single-stranded DNA. Cdk1 activity is required to generate these single-stranded regions, and deleting the EXO1 nuclease partially suppresses ten1-ts growth defects. This is similar to cdc13-1 mutants, suggesting ten1-ts strains are defective for end protection. Moreover, like Cdc13, our analysis reveals Ten1 promotes de novo telomere addition. Interestingly, in ten1-ts strains at high temperatures, telomeric single-stranded DNA and Rad52-YFP repair foci are strongly induced despite Cdc13 remaining associated with telomeres, revealing Cdc13 telomere binding is not sufficient for end protection. Finally, unlike cdc13-1 mutants, ten1-ts strains display strong synthetic interactions with mutations in the POLalpha complex. These results emphasize that Cdc13 relies on Ten1 to execute its essential function, but leave open the possibility that Ten1 has a Cdc13-independent role in DNA replication.

Aberrant DNA polymerase alpha is excluded from the nucleus by defective import and degradation in the nucleus

DNA polymerase alpha is essential for the onset of eukaryotic DNA replication. Its correct folding and assembly within the nuclear replication pre-initiation complex is crucial for normal cell cycle progression and genome maintenance. Due to a single point mutation in the largest DNA polymerase alpha subunit, p180, the temperature-sensitive mouse cell line tsFT20 exhibits heat-labile DNA polymerase alpha activity and S phase arrest at restrictive temperature. In this study, we show that an aberrant form of endogenous p180 in tsFT20 cells (p180(tsFT20)) is strictly localized in the cytoplasm while its wild-type counterpart enters the nucleus. Time-lapse fluorescence microscopy with enhanced green fluorescent protein-tagged or photoactivatable green fluorescent protein-tagged p180(tsFT20) variants and inhibitor analysis revealed that the exclusion of aberrant p180(tsFT20) from the nucleus is due to two distinct mechanisms: first, the inability of newly synthesized (cytoplasmic) p180(tsFT20) to enter the nucleus and second, proteasome-dependent degradation of nuclear-localized protein. The nuclear import defect seems to result from an impaired association of aberrant de novo synthesized p180(tsFT20) with the second subunit of DNA polymerase alpha, p68. In accordance, we show that RNA interference of p68 results in a decrease of the overall p180 protein level and in a specific increase of cytoplasmic localized p180 in NIH3T3 cells. Taken together, our data suggest two mechanisms that prevent the nuclear expression of aberrant DNA polymerase alpha.

Telomere extension occurs at most chromosome ends and is uncoupled from fill-in in human cancer cells

Telomeres are thought to be maintained by the preferential recruitment of telomerase to the shortest telomeres. The extension of the G-rich telomeric strand by telomerase is also believed to be coordinated with the complementary synthesis of the C strand by the conventional replication machinery. However, we show that under telomere length-maintenance conditions in cancer cells, human telomerase extends most chromosome ends during each S phase and is not preferentially recruited to the shortest telomeres. Telomerase rapidly extends the G-rich strand following telomere replication but fill-in of the C strand is delayed into late S phase. This late C-strand fill-in is not executed by conventional Okazaki fragment synthesis but by a mechanism using a series of small incremental steps. These findings highlight differences between telomerase actions during steady state versus nonequilibrium conditions and reveal steps in the human telomere maintenance pathway that may provide additional targets for the development of anti-telomerase therapeutics.

The DNA damage response at eroded telomeres and tethering to the nuclear pore complex

The ends of linear eukaryotic chromosomes are protected by telomeres, which serve to ensure proper chromosome replication and to prevent spurious recombination at chromosome ends. In this study, we show by single cell analysis that in the absence of telomerase, a single short telomere is sufficient to induce the recruitment of checkpoint and recombination proteins. Notably, a DNA damage response at eroded telomeres starts many generations before senescence and is characterized by the recruitment of Cdc13 (cell division cycle 13), replication protein A, DNA damage checkpoint proteins and the DNA repair protein Rad52 into a single focus. Moreover, we show that eroded telomeres, although remaining at the nuclear periphery, move to the nuclear pore complex. Our results link the DNA damage response at eroded telomeres to changes in subnuclear localization and suggest the existence of collapsed replication forks at eroded telomeres.

Distinct roles for yeast Stn1 in telomere capping and telomerase inhibition

The budding yeast Cdc13, Stn1 and Ten1 (CST) proteins are proposed to function as an RPA-like complex at telomeres that protects ('caps') chromosome ends and regulates their elongation by telomerase. We show that Stn1 has a critical function in both processes through the deployment of two separable domains. The N terminus of Stn1 interacts with Ten1 and carries out its essential capping function. The C terminus of Stn1 binds both Cdc13 and Pol12, and we present genetic data indicating that the Stn1-Cdc13 interaction is required to limit continuous telomerase action. Stn1 telomere association, similar to that of Cdc13, peaks during S phase. Significantly, the magnitude of Stn1 telomere binding is independent of telomere TG tract length, suggesting that the negative effect of Stn1 on telomerase action might be regulated by a modification of CST activity or structure in cis at individual telomeres. Genetic analysis suggests that the Tell kinase exerts an effect in parallel with the Stn1 C terminus to counteract its inhibition of telomerase. These data provide new insights into the coordination of telomere capping and telomerase regulation.

Differential arrival of leading and lagging strand DNA polymerases at fission yeast telomeres

To maintain genomic integrity, telomeres must undergo switches from a protected state to an accessible state that allows telomerase recruitment. To better understand how telomere accessibility is regulated in fission yeast, we analysed cell cycle-dependent recruitment of telomere-specific proteins (telomerase Trt1, Taz1, Rap1, Pot1 and Stn1), DNA replication proteins (DNA polymerases, MCM, RPA), checkpoint protein Rad26 and DNA repair protein Nbs1 to telomeres. Quantitative chromatin immunoprecipitation studies revealed that MCM, Nbs1 and Stn1 could be recruited to telomeres in the absence of telomere replication in S-phase. In contrast, Trt1, Pot1, RPA and Rad26 failed to efficiently associate with telomeres unless telomeres are actively replicated. Unexpectedly, the leading strand DNA polymerase epsilon (Polepsilon) arrived at telomeres earlier than the lagging strand DNA polymerases alpha (Polalpha) and delta (Poldelta). Recruitment of RPA and Rad26 to telomeres matched arrival of DNA Polepsilon, whereas S-phase specific recruitment of Trt1, Pot1 and Stn1 matched arrival of DNA Polalpha. Thus, the conversion of telomere states involves an unanticipated intermediate step where lagging strand synthesis is delayed until telomerase is recruited.

Human flap endonuclease I is in complex with telomerase and is required for telomerase-mediated telomere maintenance

Studies from budding yeast and ciliates have suggested that telomerase extension of telomeres requires the conventional DNA replication machinery, yet little is known about how DNA replication proteins regulate telomerase action in higher eukaryotic cells. Here we investigate the role of one of the DNA replication factors, flap endonuclease I (FEN1), in regulating telomerase activity in mammalian cells. FEN1 is a nuclease that plays an important role in DNA replication, repair, and recombination. We show that FEN1 is in complex with telomerase in vivo via telomeric DNA. We further demonstrate that FEN1 deficiency in mouse embryonic fibroblasts leads to an increase in telomere end-to-end fusions. In cancer cells, FEN1 deficiency induces gradual shortening of telomeres but does not alter the single-stranded G-overhangs. This is, to our knowledge, the first evidence that FEN1 and telomerase physically co-exist as a complex and that FEN1 can regulate telomerase activity at telomeres in mammalian cells.

Hypermethylation of yeast telomerase RNA by the snRNA and snoRNA methyltransferase Tgs1

Telomerase in Saccharomyces cerevisiae consists of three protein subunits and the RNA moiety TLC1, which together ensure the complete replication of chromosome ends. TLC1 shares several features with snRNA, among them the presence of a trimethylguanosine (m(3)G) cap structure at the 5' end of the RNA. Here, we report that the yeast snRNA and snoRNA methyltransferase Tgs1 is responsible for TLC1 m(3)G cap formation. The absence of Tgs1 caused changes in telomere length and structure, improved telomeric silencing and stabilized telomeric recombination. Genetic analyses implicated a role for the TLC1 m(3)G cap in the coordination between telomerase and DNA polymerase for end replication. Furthermore, tgs1Delta cells displayed a shortened replicative lifespan, suggesting that the loss of the m(3)G cap of TLC1 causes premature aging.

Hyper-phosphorylated retinoblastoma protein suppresses telomere elongation

Immortalized cell lines maintain telomeres by the expression of telomerase or by a mechanism designated alternative lengthening of telomeres (ALT). Although DNA polymerase alpha (pol-alpha) is reported to be required for telomere maintenance, the critical role of pol-alpha in telomere maintenance has not been firmly determined. We examined the role of retinoblastoma protein (pRb) and pol-alpha in the regulation of telomere length, using telomere-fiber FISH. Telomere length varied dependent on the intracellular abundance of pol-alpha or pRb in HeLa cells. A proportion of hyper-phosphorylated pRb (ppRb) molecules localized to sites of telomeric DNA replication in HeLa cells. Pol-alpha might thus contribute to telomere maintenance, and might be regulated by ppRb.

How telomeres are replicated

The replication of the ends of linear chromosomes, or telomeres, poses unique problems, which must be solved to maintain genome integrity and to allow cell division to occur. Here, we describe and compare the timing and specific mechanisms that are required to initiate, control and coordinate synthesis of the leading and lagging strands at telomeres in yeasts, ciliates and mammals. Overall, it emerges that telomere replication relies on a strong synergy between the conventional replication machinery, telomere protection systems, DNA-damage-response pathways and chromosomal organization.

The role of Stn1p in Saccharomyces cerevisiae telomere capping can be separated from its interaction with Cdc13p

The function of telomeres is twofold: to facilitate complete chromosome replication and to protect chromosome ends against fusions and illegitimate recombination. In the budding yeast Saccharomyces cerevisiae, interactions among Cdc13p, Stn1p, and Ten1p are thought to be critical for promoting these processes. We have identified distinct Stn1p domains that mediate interaction with either Ten1p or Cdc13p, allowing analysis of whether the interaction between Cdc13p and Stn1p is indeed essential for telomere capping or length regulation. Consistent with the model that the Stn1p essential function is to promote telomere end protection through Cdc13p, stn1 alleles that truncate the C-terminal 123 residues fail to interact with Cdc13p and do not support viability when expressed at endogenous levels. Remarkably, more extensive deletions that remove an additional 185 C-terminal residues from Stn1p now allow cell growth at endogenous expression levels. The viability of these stn1-t alleles improves with increasing expression level, indicating that increased stn1-t dosage can compensate for the loss of Cdc13p-Stn1p interaction. However, telomere length is misregulated at all expression levels. Thus, an amino-terminal region of Stn1p is sufficient for its essential function, while a central region of Stn1p either negatively regulates the STN1 essential function or destabilizes the mutant Stn1 protein.

Mrc1 protects uncapped budding yeast telomeres from exonuclease EXO1

Mrc1 (Mediator of Replication Checkpoint 1) is a component of the DNA replication fork machinery and is necessary for checkpoint activation after replication stress. In this study, we addressed the role of Mrc1 at uncapped telomeres. Our experiments show that Mrc1 contributes to the vitality of both cdc13-1 and yku70Δ telomere capping mutants. Cells with telomere capping defects containing MRC1 or mrc1^AQ, a checkpoint defective allele, exhibit similar growth, suggesting growth defects of cdc13-1 mrc1Δ are not due to checkpoint defects. This is in accordance with Mrc1-independent Rad53 activation after telomere uncapping. Poor growth of cdc13-1 mutants in the absence of Mrc1 is a result of enhanced single stranded DNA accumulation at uncapped telomeres. Consistent with this, deletion of EXO1, encoding a nuclease that contributes to single stranded DNA accumulation after telomere uncapping, improves growth of cdc13-1 mrc1Δ strains and decreases ssDNA production. Our observations show that Mrc1, a core component of the replication fork, plays an important role in telomere capping, protecting from nucleases and checkpoint pathways.

The DNA damage machinery and homologous recombination pathway act consecutively to protect human telomeres

Telomeres protect chromosome ends from being detected as lesions and from triggering DNA damage checkpoints. Paradoxically, telomere function depends on checkpoint proteins such as ATM and ATR, but a molecular model explaining this seemingly contradictory relationship has been missing so far. Here we show that the DNA damage machinery acts on telomeres in at least two independent steps. First, the ATR-dependent machinery is recruited to telomeres before telomere replication is completed, likely in response to single-stranded DNA resulting from replication fork stalling. Second, after replication, telomeres attract ATM and the homologous recombination (HR) machinery. In vivo and in vitro results suggest that the HR machinery is required for formation of a telomere-specific structure at chromosome ends after replication. Our results suggest that telomere ends need to be recognized as DNA damage to complete end replication and to acquire a structure that is essential for function.

The telomerase-recruitment domain of the telomere binding protein Cdc13 is regulated by Mec1p/Tel1p-dependent phosphorylation

The DNA damage-responsive protein kinases ATM and ATR phosphorylate SQ/TQ motifs that lie in clusters in most of their in vivo targets. Budding yeast Cdc13p contains two clusters of SQ/TQ motifs, suggesting that it might be a target of Mec1p/Tel1p (yeast ATR/ATM). Here we demonstrated that the telomerase recruitment domain of Cdc13p is phosphorylated by Mec1p and Tel1p. Gel analysis showed that Cdc13p contains a Mec1/Tel1-dependent post-translational modification. Using an immunoprecipitate (IP)-kinase assay, we showed that Mec1p phosphorylates Cdc13p on serine 225, 249, 255 and 306, and Tel1p phosphorylates Cdc13p on serine 225, 249 and 255 in vitro. Phenotypic analysis in vivo revealed that the mutations in the Cdc13p SQ motifs phosphorylated by Mec1p and Tel1p caused multiple telomere and growth defects. In addition, normal telomere length and growth could be restored by expressing a Cdc13-Est1p hybrid protein. These results demonstrate the telomerase recruitment domain of Cdc13p as an important new telomere-specific target of Mec1p/Tel1p.

Expression of human telomerase reverse transcriptase (hTERT) protein is significantly associated with the progression, recurrence and prognosis of oral squamous cell carcinoma in Taiwan

This study used an immunohistochemical technique to examine the expression of human telomerase reverse transcriptase (hTERT) protein in 82 specimens of OSCC, 116 specimens of oral epithelial dysplasia (OED), and 21 specimens of normal oral mucosa (NOM). The cytoplasmic and nuclear hTERT staining intensity (SI; 0, no staining; 1, weak; 2, moderate; 3, strong), labeling indices (LIs, defined as the percentage of positive cells in total cells), and labeling scores (LSs, defined as LI x SI) in OSCC, OED, and NOM samples were calculated and compared among groups. The correlation between the cytoplasmic or nuclear hTERT LS in OSCCs and clinicopathological parameters or survival of OSCC patients was analyzed statistically. The mean cytoplasmic hTERT LSs increased significantly from NOM (87+/-17%) through OED (95+/-18%) to OSCC samples (114+/-33%, p=0.000). The mean nuclear hTERT LSs also increased from NOM (80+/-14%) to OED (91+/-20%) and then decreased to OSCC samples (86+/-35%) with no statistically significant difference among the 3 groups. A significant correlation was found between the higher mean cytoplasmic hTERT LSs and OSCCs occurring in male patients (p=0.023), with larger tumor sizes (T3 and T4, p=0.048), with more advanced clinical stages (stages 3 and 4, p=0.033), or from patients with areca quid chewing (p= 0.029), cigarette smoking (p=0.027), or alcohol drinking habit (p=0.025). In addition, OSCC patients with nuclear hTERT LSs greater than 100% were prone to have a higher recurrence rate (p=0.044) and a lower 5-year survival rate (p=0.011). Our results indicate that the increased expression of hTERT protein is an early event in oral carcinogenesis and hTERT may be a biomarker for OSCCs. Measuring the amount of cytoplasmic or nuclear expression of hTERT in OSCC samples may predict the oral cancer progression, recurrence, and prognosis in Taiwan.

Alterations of DNA and chromatin structures at telomeres and genetic instability in mouse cells defective in DNA polymerase alpha

Telomere length is controlled by a homeostatic mechanism that involves telomerase, telomere-associated proteins, and conventional replication machinery. Specifically, the coordinated actions of the lagging strand synthesis and telomerase have been argued. Although DNA polymerase alpha, an enzyme important for the lagging strand synthesis, has been indicated to function in telomere metabolism in yeasts and ciliates, it has not been characterized in higher eukaryotes. Here, we investigated the impact of compromised polymerase alpha activity on telomeres, using tsFT20 mouse mutant cells harboring a temperature-sensitive polymerase alpha mutant allele. When polymerase alpha was temperature-inducibly inactivated, we observed sequential events that included an initial extension of the G-tail followed by a marked increase in the overall telomere length occurring in telomerase-independent and -dependent manners, respectively. These alterations of telomeric DNA were accompanied by alterations of telomeric chromatin structures as revealed by quantitative chromatin immunoprecipitation and immunofluorescence analyses of TRF1 and POT1. Unexpectedly, polymerase alpha inhibition resulted in a significantly high incidence of Robertsonian chromosome fusions without noticeable increases in other types of chromosomal aberrations. These results indicate that although DNA polymerase alpha is essential for genome-wide DNA replication, hypomorphic activity leads to a rather specific spectrum of chromosomal abnormality.

Telomere length homeostasis requires that telomerase levels are limiting

Stabilization of telomere length in germline and highly proliferative human cells is required for long-term survival and for the immortal phenotype of cancer-derived cells. This is achieved through expression of telomerase reverse transcriptase (TERT), which synthesizes telomeric repeats through reverse transcription of its tightly associated RNA template (TR). The telomeric repeat binding factor TRF1 inhibits telomerase at telomeres in cis in a length-dependent manner to achieve telomere length homeostasis. Here we manipulate telomerase activity over a wide range in cancer and primary cells. Concomitant overexpression of TERT and TR was necessary and sufficient to substantially increase telomerase activity. Upon overexpression, more telomerase associated with telomeres and telomeres elongated at a constant rate (up to 0.8 kb/population doubling (PD)) in a length-independent manner. Thus, in less than 50 PDs, the length of telomeres increased 3-8-fold beyond physiological size, while telomere-bound TRF1 and TRF2 increased proportionally to telomere length. Thus, long telomeres do not permanently adopt a structural state that is non-extendible. A low cellular concentration of telomerase is critical to achieve preferential elongation of short telomeres and telomere length homeostasis.

Chromosome healing byde novotelomere addition inSaccharomyces cerevisiae

The repair of spontaneous or induced DNA damage by homologous recombination (HR) in Saccharomyces cerevisiae will suppress chromosome rearrangements. Alternative chromosome healing pathways can result in chromosomal instability. One of these pathways is de novo telomere addition where the end of a broken chromosome is stabilized by telomerase-dependent addition of telomeres at non-telomeric sites. De novo telomere addition requires the recruitment of telomerase to chromosomal targets. Subsequently, annealing of the telomerase reverse transcriptase RNA-template (guide RNA) at short regions of homology is followed by extension of the nascent 3'-end of the broken chromosome to copy a short region of the telomerase guide RNA; multiple cycles of this process yield the new telomere. Proteins including Pif1 helicase, the single-stranded DNA-binding protein Cdc13 and the Ku heterocomplex are known to participate in native telomere functions and also regulate the de novo telomere addition reaction. Studies of the sequences added at de novo telomeres have lead to a detailed description of the annealing-extension-dissociation cycles that copy the telomerase guide RNA, which can explain the heterogeneity of telomeric repeats at de novo and native telomeres in S. cerevisiae.

Limited TTP supply affects telomere length regulation in a telomerase-independent fashion

An adequate supply of nucleotides is essential for DNA replication and DNA repair. Moreover, inhibition of TTP synthesis can cause cell death by a poorly characterized mechanism called thymine-less death. In the yeast Saccharomyces cerevisiae, the genes encoding thymidylate synthetase (CDC21) and thymidylate kinase (CDC8) are both essential for de novo TTP synthesis. The effects of temperature-sensitive mutations in these genes have been characterized and, curiously, the phenotypes displayed by cells harboring them include shortened telomeric repeat tracts. This finding raised the possibility that the enzyme telomerase is very sensitive to TTP-pools. We tested this possibility in vivo by assessing telomerase-dependent extension in situations of lowered TTP supply. The results show that the above-mentioned short telomere phenotype is not a consequence of an inability of telomerase to elongate telomeres when TTP synthesis is impaired. Moreover, this telomere shortening was abolished in cells harboring a mutation in DNA polymerase α. Previously, this same mutation was shown to affect the coordination between conventional replication and telomerase-mediated extension. These results thus re-emphasize the importance of the interplay between conventional replication and telomerase-mediated addition of telomeric repeats in telomere replication.

The telomerase cycle: normal and pathological aspects

Telomeres are nucleoprotein complexes that cap the end of eukaryotic chromosomes and are essential for their function and stability. Telomerase, a reverse transcriptase that extends the single-stranded G-rich 3' protruding ends of chromosomes, stabilizes telomere length in germ line cells and regenerative tissues as well as in tumor cells. In the absence of telomerase telomeres shorten with cell division, a process able to trigger cell growth arrest. When telomerase is present in the cell, its activity is tightly regulated at its site of action by factors specifically bound to the telomeric DNA. Recent data indicate that telomeres reorganize during the cell cycle. This review summarizes our current knowledge on how telomeres are dynamically organized and remodeled during cell cycle and stress response, pointing out the conservation and the difference between yeast and human. We then focus on the cellular consequences of telomere modifications in normal and cancer cells. This leads to a discussion of the different roles, seemingly contradictory, of telomeres and telomerase during the initiation and the progression of a cancer.

Mcl1p is a polymerase alpha replication accessory factor important for S-phase DNA damage survival

Mcl1p is an essential fission yeast chromatin-binding protein that belongs to a family of highly conserved eukaryotic proteins important for sister chromatid cohesion. The essential function is believed to result from its role as a Pol1p (polymerase alpha) accessory protein, a conclusion based primarily on analogy to Ctf4p's interaction with Pol1p. In this study, we show that Mcl1p also binds to Pol1p with high affinity for the N terminus of Pol1p during S phase and DNA damage. Characterization of an inducible allele of mcl1+, (nmt41)mcl1-MH, shows that altered expression levels of Mcl1p lead to sensitivity to DNA-damaging agents and synthetic lethality with the replication checkpoint mutations rad3Delta, rqh1Delta, and hsk1-1312. Further, we find that the overexpression of the S-phase checkpoint kinase, Cds1, or the loss of Hsk1 kinase activity can disrupt Mcl1p's interaction with chromatin and Pol1p during replication arrest with hydroxyurea. We take these data to mean that Mcl1p is a dynamic component of the polymerase alpha complex during replication and is important for the replication stress response in fission yeast.

Effects of single-stranded DNA binding proteins on primer extension by telomerase

We present a biochemical analysis of the effects of three single-stranded DNA binding proteins on extension of oligonucleotide primers by the Tetrahymena telomerase. One of them, a human protein designated translin, which was shown to specifically bind the G-rich Tetrahymena and human telomeric repeats, slightly stimulated the primer extension reactions at molar ratios of translin/primer of <1:2. At higher molar ratios, it inhibited the reactions by up to 80%. The inhibition was caused by binding of translin to the primers, rather than by a direct interaction of this protein with telomerase. A second protein, the general human single-stranded DNA binding protein Replication Protein A (RPA), similarly affected the primer extension by telomerase, even though its mode of binding to DNA differs from that of translin. A third protein, the E. coli single-stranded DNA binding protein (SSB), whose binding to DNA is highly cooperative, caused more substantial stimulation and inhibition at the lower and the higher molar ratios of SSB/primer, respectively. Both telomere-specific and general single-stranded DNA binding proteins are found in living cells in telomeric complexes. Based on our data, we propose that these proteins may exert either stimulatory or inhibitory effects on intracellular telomerases, depending on their local concentrations.

Fission yeast Dna2 is required for generation of the telomeric single-strand overhang

It has been suggested that the Schizosaccharomyces pombe Rad50 (Rad50-Rad32-Nbs1) complex is required for the resection of the C-rich strand at telomere ends in taz1-d cells. However, the nuclease-deficient Rad32-D25A mutant can still resect the C-rich strand, suggesting the existence of a nuclease that resects the C-rich strand. Here, we demonstrate that a taz1-d dna2-2C double mutant lost the G-rich overhang at a semipermissive temperature. The amount of G-rich overhang in S phase in the dna2-C2 mutant was lower than that in wild-type cells at the semipermissive temperature. Dna2 bound to telomere DNA in a chromatin immunoprecipitation assay. Moreover, telomere length decreased with each generation after shift of the dna2-2C mutant to the semipermissive temperature. These results suggest that Dna2 is involved in the generation of G-rich overhangs in both wild-type cells and taz1-d cells. The dna2-C2 mutant was not gamma ray sensitive at the semipermissive temperature, suggesting that the ability to process double-strand break (DSB) ends was not affected in the dna2-C2 mutant. Our results reveal that DSB ends and telomere ends are processed by different mechanisms.