Uncapping and deregulation of telomeres lead to detrimental cellular consequences in yeast

Leishmania major telomerase RNA knockout: From altered cell proliferation to decreased parasite infectivity

This study focuses on the biological impacts of deleting the telomerase RNA from Leishmania major (LeishTER), a parasite responsible for causing leishmaniases, for which no effective treatment or prevention is available. TER is a critical player in the telomerase ribonucleoprotein complex, containing the template sequence copied by the reverse transcriptase component during telomere elongation. The success of knocking out both LeishTER alleles was confirmed, and no off-targets were detected. LmTER-/- cells share similar characteristics with other TER-depleted eukaryotes, such as altered growth patterns and partial G0/G1 cell cycle arrest in early passages, telomere shortening, and elevated TERRA expression. They also exhibit increased γH2A phosphorylation, suggesting that the loss of LeishTER induces DNA damage signaling. Moreover, pro-survival autophagic signals and mitochondrion alterations were shown without any detectable plasma membrane modifications. LmTER-/- retained the ability to transform into metacyclics, but their infectivity capacity was compromised. Furthermore, the overexpression of LeishTER was also deleterious, inducing a dominant negative effect that led to telomere shortening and growth impairments. These findings highlight TER's vital role in parasite homeostasis, opening discussions about its potential as a drug target candidate against Leishmania.

CTC1 increases the radioresistance of human melanoma cells by inhibiting telomere shortening and apoptosis

Melanoma has traditionally been viewed as a radioresistant cancer. However, recent studies suggest that under certain clinical circumstances, radiotherapy may play a significant role in the treatment of melanoma. Previous studies have demonstrated that telomere length is a hallmark of radiosensitivity. The newly discovered mammalian CTC1-STN1-TEN1 (CST) complex has been demonstrated to be an important telomere maintenance factor. In this study, by establishing a radiosensitive/radioresistant human melanoma cell model, MDA-MB-435/MDA-MB-435R, we aimed to investigate the association of CTC1 expression with radiosensitivity in human melanoma cell lines, and to elucidate the possible underlying mechanisms. We found that CTC1 mRNA and protein levels were markedly increased in the MDA-MB-435R cells compared with the MDA-MB-435 cells. Moreover, the downregulation of CTC1 enhanced radiosensitivity, induced DNA damage and promoted telomere shortening and apoptosis in both cell lines. Taken together, our findings suggest that CTC1 increases the radioresistance of human melanoma cells by inhibiting telomere shortening and apoptosis. Thus, CTC1 may be an attractive target gene for the treatment of human melanoma.

Targeting telomeric G-quadruplexes with the ruthenium(II) complexes [Ru(bpy)(2)(ptpn)](2+) and [Ru(phen)(2)(ptpn)](2+)

Two ruthenium(II) polypyridyl complexes, [Ru(bpy)(2)(ptpn)](2+) (1) (bpy = 2,2'-bipyridine, ptpn = 3-(1,10-phenanthroline-2-yl)-as-triazino[5,6-f]1,10-phenanthroline) and [Ru(phen)(2)(ptpn)](2+) (2) (phen = 1,10-phenanthroline), were synthesized and characterized. Crystal structure analysis shows that complex 1 has a large planar aromatic area and possesses the potential to fit the geometric structure of G-quadruplex. The interaction of the G-quadruplex DNA with Ru(ii) complexes was explored by means of circular dichroism (CD), fluorescence resonance energy transfer (FRET) melting assay, competitive FRET assay and polymerase chain reaction (PCR) stop assay. The results indicated that complexes 1 and 2 both have the ability to promote the formation and stabilization of the human telomeric d[(TTAGGG)(n)] (HTG22) quadruplex and exhibit high G-quadruplex DNA selectivity over duplex DNA. The telomere repeat amplification protocol (TRAP) assay and long-term proliferation experiments further demonstrate that the Ru(II) complexes are potent telomerase inhibitors and HeLa cell proliferation inhibitors.

"Poisoning" yeast telomeres distinguishes between redundant telomere capping pathways

In most eukaryotes, telomeres are composed of tandem arrays of species-specific DNA repeats ending with a G-rich 3' overhang. In budding yeast, Cdc13 binds this overhang and recruits Ten1-Stn1 and the telomerase protein Est1 to protect (cap) and elongate the telomeres, respectively. To dissect and study the various pathways employed to cap and maintain the telomere end, we engineered telomerase to incorporate Tetrahymena telomeric repeats (G₄T₂) onto the telomeres of the budding yeast Kluyveromyces lactis. These heterologous repeats caused telomere-telomere fusions, cell cycle arrest at G2/M, and severely reduced viability--the hallmarks of telomere uncapping. Fusing Cdc13 or Est1 to universal minicircle sequence binding protein (UMSBP), a small protein that binds the single-stranded G₄T₂ repeats, rescued the cell viability and restored telomere capping, but not telomerase-mediated telomere maintenance. Surprisingly, Cdc13-UMSBP-mediated telomere capping was dependent on the homologous recombination factor Rad52, while Est1-UMSBP was not. Thus, our results distinguish between two, redundant, telomere capping pathways.

TGF-beta induces telomerase-dependent pancreatic tumor cell cycle arrest

Recent studies suggest that transforming growth factor beta (TGF-beta) inhibits telomerase activity by repression of the telomerase reverse transcriptase (TERT) gene. In this report, we show that TGF-beta induces TERT repression-dependent apoptosis in pancreatic tumor, vascular smooth muscle, and cervical cancer cell cultures. TGF-beta activates Smad3 signaling, induces TERT gene repression and results in G1/S phase cell cycle arrest and apoptosis. TERT over-expression stimulates the G1/S phase transition and alienates TGF-beta-induced cell cycle arrest and apoptosis. Our data suggest that telomere maintenance is a limiting factor of the transition of the cell cycle. TGF-beta triggers cell cycle arrest and death by a mechanism involving telomerase deregulation of telomere maintenance.

Interaction of [Ru(bpy)2(dppz)]2+ with human telomeric DNA: preferential binding to G-quadruplexes over i-motif

Inspired by the enormous importance attributed to the structure and function of human telomeric DNA, we focus our attention on the interaction of [Ru(bpy)(2)(dppz)](2+) with the guanine-rich single-strand oligomer 5'-AGGGTTAGGGTTAGGGTTAGGG-3' (22AG) and the complementary cytosine-rich strand (22CT). In Na(+) buffer, 22AG may adopt an antiparallel basket quadruplex, whereas, it favours a mixed parallel/antiparallel structure in K(+) buffer. 22CT may self-associate at acidic pH into an i-motif. In this paper, the interaction between [Ru(bpy)(2)(dppz)](2+) and each unusual DNA was evaluated. It was interesting that [Ru(bpy)(2)(dppz)](2+) could promote the human telomeric repeat 22AG to fold into intramolecular antiparallel G-quadruplex without any other cations. What's more, [Ru(bpy)(2)(dppz)](2+) was found to have a strong preference for binding to G-quadruplexes that were induced through either Na(+) or K(+), while weak binding to i-motif was observed. The results also indicated that [Ru(bpy)(2)(dppz)](2+) could serve as a prominent molecular "light switch" for both G-quadruplexes, revealing a potential application of the title complex in luminescent signaling of G-quadruplex DNA.

The molecular basis of melanomagenesis and the metastatic phenotype

The last two decades have seen spectacular advances in our understanding of the biology of melanoma and, in particular, have elucidated the mechanisms operative in disease initiation and progression. With respect to the former, the genetics of melanoma and in particular the impact of genetic defects on dysregulation of the cell cycle are key issues in malignant transformation and are a major focus of this review. With respect to the latter, consideration also is given to the acquisition of growth factor autonomy and the capacity for invasion and metastasis from the standpoint of cell adhesion, motility, and matrix digestion. These events have specific morphologic correlates that will be briefly addressed. Where relevant, we will address certain of the modern pharmacogenetic strategies that flow from these novel observations concerning melanoma biology.

Telomerase and its potential for therapeutic intervention

Telomerase and telomeres are attractive targets for anticancer therapy. This is supported by the fact that the majority of human cancers express the enzyme telomerase which is essential to maintain their telomere length and thus, to ensure indefinite cell proliferation--a hallmark of cancer. Tumours have relatively shorter telomeres compared to normal cell types, opening the possibility that human cancers may be considerably more susceptible to killing by agents that inhibit telomere replication than normal cells. Advances in the understanding of the regulation of telomerase activity and the telomere structure, as well as the identification of telomerase and telomere associated binding proteins have opened new avenues for therapeutic intervention. Here, we review telomere and telomerase biology and the various approaches which have been developed to inhibit the telomere/telomerase complex over the past decade. They include inhibitors of the enzyme catalytic subunit and RNA component, agents that target telomeres, telomerase vaccines and drugs targeting binding proteins. The emerging role of telomerase in cancer stem cells and the implications for cancer therapy are also discussed.

Mechanism of acridine-based telomerase inhibition and telomere shortening

The trisubstituted acridine compound BRACO-19 has been developed as a ligand for stabilising G-quadruplex structures. It is shown here that BRACO-19 produces short- and long-term growth arrest in cancer cell lines, and is significantly less potent in a normal cell line. BRACO-19 reduces telomerase activity and long-term telomere length attrition is observed. It is also shown that BRACO-19 binds to telomeric single-stranded overhang DNA, consistent with quadruplex formation, and the single-stranded protein hPOT1 has been shown to be displaced from the overhang in vitro and in cellular experiments. It is concluded that the cellular activity of BRACO-19 can be ascribed both to the uncapping of 3' telomere ends and to telomere shortening that may preferentially affect cells with short telomeres.

Telomere uncapping by the G-quadruplex ligand RHPS4 inhibits clonogenic tumour cell growth in vitro and in vivo consistent with a cancer stem cell targeting mechanism

The pentacyclic acridinium methosulfate salt RHPS4 induces the 3'single-stranded guanine-rich telomeric overhang to fold into a G-quadruplex structure. Stabilisation of the latter is incompatible with an attachment of telomerase to the telomere and thus G-quadruplex ligands can effectively inhibit both the catalytic and capping functions of telomerase. In this study, we examined mechanisms underlying telomere uncapping by RHPS4 in uterus carcinoma cells (UXF1138L) with short telomeres and compared the susceptibility of bulk and clonogenic cancer cells to the G-quadruplex ligand. We show that treatment of UXF1138L cells with RHPS4 leads to the displacement of the telomerase catalytic subunit (hTERT) from the nucleus, induction of telomere-initiated DNA-damage signalling and chromosome fusions. We further report that RHPS4 is more potent against cancer cells that grow as colonies in soft agar than cells growing as monolayers. Human cord blood and HEK293T embryonic kidney cell colony forming units, however, were more resistant to RHPS4. RHPS4-treated UXF1138L xenografts had a decreased clonogenicity, showed loss of nuclear hTERT expression and an induction of mitotic abnormalities compared with controls. Although single-agent RHPS4 had limited in vivo efficacy, a combination of RHPS4 with the mitotic spindle poison Taxol caused tumour remissions and further enhancement of telomere dysfunction.

Identification and characterization of bovine regulator of telomere length elongation helicase gene (RTEL): molecular cloning, expression distribution, splice variants and DNA methylation profile

Background

The genetic basis of telomere length heterogeneity among mammalian species is still not well understood. Recently, a gene named regulator of telomere length elongation helicase (RTEL) was identified and predicted to be an essential participant in species-specific telomere length regulation in two murine species. To obtain broader insights into its structure and biological functions and to ascertain whether RTEL is also a candidate gene in the regulation of telomere length diversity in other mammalian species, data from other mammals may be helpful.

Results

Here we report the cDNA cloning, genomic structure, chromosomal location, alternative splicing pattern, expression distribution and DNA methylation profile of the bovine homolog of RTEL. The longest transcript of bovine RTEL is 4440 nt, encompassing 24.8 kb of genomic sequence that was mapped to chromosome 13q2.2. It encodes a conserved helicase-like protein containing seven characterized helicase motifs in the first 750 aa and a PIP box in the C-terminus. Four splice variants were identified within the transcripts in both the coding and 5'-untranslated regions; Western blot revealed that the most abundant splice variant SV-1 was translated to a truncated isoform of RTEL. The different 5'UTRs imply alternative transcription start sites in the promoter; Bovine RTEL was transcribed at the blastocyst stage, and expression levels were highest in adult testis, liver and ovary. DNA methylation analysis of tissues that differed significantly in expression level indicated that relatively low DNA methylation is associated with higher expression.

Conclusion

In this study, we have identified and characterized a bovine RTEL homolog and obtained basic information about it, including gene structure, expression distribution, splice variants and profile of DNA methylation around two putative transcription start sites. These data may be helpful for further comparative and functional analysis of RTEL in mammals.

Mutated telomeres sensitize tumor cells to anticancer drugs independently of telomere shortening and mechanisms of telomere maintenance

Telomerase is a ribonucleoprotein complex that maintains the stability of chromosome ends and regulates replicative potential. Telomerase is upregulated in over 85% of human tumors, but not in adjacent normal tissues and represents a promising target for anticancer therapy. Most telomerase-based therapies rely on the inhibition of telomerase activity and require extensive telomere shortening before inducing any antiproliferative effect. Disturbances of telomere structure rather than length may be more effective in inducing cell death. Telomerase RNA subunits (hTRs) with mutations in the template region reconstitute active holoenzymes that incorporate mutated telomeric sequences. Here, we analysed the feasibility of an anticancer approach based on the combination of telomere destabilization and conventional chemotherapeutic drugs. We show that a mutant template hTR dictates the synthesis of mutated telomeric repeats in telomerase-positive cancer cells, without significantly affecting their viability and proliferative ability. Nevertheless, the mutant hTR increased sensitivity to anticancer drugs in cells with different initial telomere lengths and mechanisms of telomere maintenance and without requiring overall telomere shortening. This report is the first to show that interfering with telomere structure maintenance in a telomerase-dependent manner may be used to increase the susceptibility of tumor cells to anticancer drugs and may lead to the development of a general therapy for the treatment of human cancers.

A mutation in the STN1 gene triggers an alternative lengthening of telomere-like runaway recombinational telomere elongation and rapid deletion in yeast

Some human cancer cells achieve immortalization by using a recombinational mechanism termed ALT (alternative lengthening of telomeres). A characteristic feature of ALT cells is the presence of extremely long and heterogeneous telomeres. The molecular mechanism triggering and maintaining this pathway is currently unknown. In Kluyveromyces lactis, we have identified a novel allele of the STN1 gene that produces a runaway ALT-like telomeric phenotype by recombination despite the presence of an active telomerase pathway. Additionally, stn1-M1 cells are synthetically lethal in combination with rad52 and display chronic growth and telomere capping defects including extensive 3' single-stranded telomere DNA and highly elevated subtelomere gene conversion. Strikingly, stn1-M1 cells undergo a very high rate of telomere rapid deletion (TRD) upon reintroduction of STN1. Our results suggest that the protein encoded by STN1, which protects the terminal 3' telomere DNA, can regulate both ALT and TRD.

Chromosome ends: different sequences may provide conserved functions

The structures of specific chromosome regions, centromeres and telomeres, present a number of puzzles. As functions performed by these regions are ubiquitous and essential, their DNA, proteins and chromatin structure are expected to be conserved. Recent studies of centromeric DNA from human, Drosophila and plant species have demonstrated that a hidden universal centromere-specific sequence is highly unlikely. The DNA of telomeres is more conserved consisting of a tandemly repeated 6-8 bp Arabidopsis-like sequence in a majority of organisms as diverse as protozoan, fungi, mammals and plants. However, there are alternatives to short DNA repeats at the ends of chromosomes and for telomere elongation by telomerase. Here we focus on the similarities and diversity that exist among the structural elements, DNA sequences and proteins, that make up terminal domains (telomeres and subtelomeres), and how organisms use these in different ways to fulfil the functions of end-replication and end-protection.

The G-Quadruplex-Interactive Molecule BRACO-19 Inhibits Tumor Growth, Consistent with Telomere Targeting and Interference with Telomerase Function

Interference with telomerase and telomere maintenance is emerging as an attractive target for anticancer therapies. Ligand-induced stabilization of G-quadruplex formation by the telomeric DNA single-stranded 3' overhang inhibits telomerase from catalyzing telomeric DNA synthesis and from capping telomeric ends. We report here the effects of a 3,6,9-trisubstituted acridine compound, BRACO-19, on telomerase function in vitro and in vivo. The biological activity of BRACO-19 was evaluated in the human uterus carcinoma cell line UXF1138L, which has very short telomeres (2.7 kb). In vitro, nuclear human telomerase reverse transcriptase (hTERT) expression was drastically decreased after 24 hours, induction of cellular senescence and complete cessation of growth was seen after 15 days, paralleled by telomere shortening of ca. 0.4 kb. In vivo, BRACO-19 was highly active as a single agent against early-stage (68 mm(3)) tumors in a s.c. growing xenograft model established from UXF1138L cells, if given chronically at 2 mg per kg per day i.p. BRACO-19 produced growth inhibition of 96% compared with controls accompanied by partial regressions (P < 0.018). Immunostaining of xenograft tissues showed that this response was paralleled by loss of nuclear hTERT protein expression and an increase in atypical mitoses indicative of telomere dysfunction. Cytoplasmic hTERT expression and its colocalization with ubiquitin was observed suggesting that hTERT is bound to ubiquitin and targeted for enhanced degradation upon BRACO-19 treatment. This is in accord with a model of induced displacement of telomerase from the telomere. The in vitro and in vivo data presented here is consistent with the G-quadruplex binding ligand BRACO-19 producing an anticancer effect by inhibiting the capping and catalytic functions of telomerase.

Genetic dissection of the Kluyveromyces lactis telomere and evidence for telomere capping defects in TER1 mutants with long telomeres

In the yeast Kluyveromyces lactis, the telomeres are composed of perfect 25-bp repeats copied from a 30-nucleotide RNA template defined by 5-nucleotide terminal repeats. A genetic dissection of the K. lactis telomere was performed by using mutant telomerase RNA (TER1) alleles to incorporate mutated telomeric repeats. This analysis has shown that each telomeric repeat contains several functional regions, some of which may physically overlap. Mutations in the terminal repeats of the template RNA typically lead to telomere shortening, as do mutations in the right side of the Rap1p binding site. Mutations in the left half of the Rap1p binding site, however, lead to the immediate formation of long telomeres. When mutated, the region immediately 3' of the Rap1p binding site on the TG-rich strand of the telomere leads to telomeres that are initially short but eventually undergo extreme telomere elongation. Mutations between this region and the 3' terminal repeat cause elevated recombination despite the presence of telomeres of nearly wild-type length. Mutants with highly elongated telomeres were further characterized and exhibit signs of telomere capping defects, including elevated levels of subtelomeric recombination and the formation of extrachromosomal and single-stranded telomeric DNA. Lengthening caused by some Rap1 binding site mutations can be suppressed by high-copy-number RAP1. Mutated telomeric repeats from a delayed elongation mutant are shown to be defective at regulating telomere length in cells with wild-type telomerase, indicating that the telomeric repeats are defective at telomere length regulation.

Rapid inhibition of cancer cell growth induced by lentiviral delivery and expression of mutant-template telomerase RNA and anti-telomerase short-interfering RNA

In human cancers, telomeres are commonly maintained by elevated levels of the ribonucleoprotein enzyme telomerase, which contains an intrinsic templating RNA moiety (human telomerase RNA; hTER) and the core protein (human telomerase reverse transcriptase). We developed a lentiviral system for efficient overexpression of mutant-template human telomerase RNA (MT-hTer) to add mutant DNA to telomeres in cancer cells. We show that such MT-hTer overexpression rapidly inhibits cell growth and induces apoptosis in telomerase-positive precancerous or cancer cells but not in telomerase-negative cells. These rapid effects occurred independent of wild-type p53 and telomere length. Tumor growth and progression were significantly decreased in xenografts of human tumor cells overexpressing MT-hTers. Expression of a hairpin short-interfering RNA that specifically targeted the endogenous wild-type hTER template region, but spared the MT-hTers, also caused p53-independent cell growth inhibition and apoptosis, and when coexpressed with MT-hTer, synergistically killed cancer cells. Hence, anti-wild-type-hTER short-interfering RNA and MT-hTers may act through distinct pathways and, particularly in combination, represent a promising approach to anticancer therapies.

Pol12, the B subunit of DNA polymerase alpha, functions in both telomere capping and length regulation

The regulation of telomerase action, and its coordination with conventional DNA replication and chromosome end "capping," are still poorly understood. Here we describe a genetic screen in yeast for mutants with relaxed telomere length regulation, and the identification of Pol12, the B subunit of the DNA polymerase alpha (Pol1)-primase complex, as a new factor involved in this process. Unlike many POL1 and POL12 mutations, which also cause telomere elongation, the pol12-216 mutation described here does not lead to either reduced Pol1 function, increased telomeric single-stranded DNA, or a reduction in telomeric gene silencing. Instead, and again unlike mutations affecting POL1, pol12-216 is lethal in combination with a mutation in the telomere end-binding and capping protein Stn1. Significantly, Pol12 and Stn1 interact in both two-hybrid and biochemical assays, and their synthetic-lethal interaction appears to be caused, at least in part, by a loss of telomere capping. These data reveal a novel function for Pol12 and a new connection between DNA polymerase alpha and Stn1. We propose that Pol12, together with Stn1, plays a key role in linking telomerase action with the completion of lagging strand synthesis, and in a regulatory step required for telomere capping.

Telomeres and telomerase

Telomeres are the protective DNA-protein complexes found at the ends of eukaryotic chromosomes. Telomeric DNA consists of tandem repeats of a simple, often G-rich, sequence specified by the action of telomerase, and complete replication of telomeric DNA requires telomerase. Telomerase is a specialized cellular ribonucleoprotein reverse transcriptase. By copying a short template sequence within its intrinsic RNA moiety, telomerase synthesizes the telomeric DNA strand running 5' to 3' towards the distal end of the chromosome, thus extending it. Fusion of a telomere, either with another telomere or with a broken DNA end, generally constitutes a catastrophic event for genomic stability. Telomerase acts to prevent such fusions. The molecular consequences of telomere failure, and the molecular contributors to telomere function, with an emphasis on telomerase, are discussed here.

Mutant telomere sequences lead to impaired chromosome separation and a unique checkpoint response

Mutation of the template region in the RNA component of telomerase can cause incorporation of mutant DNA sequences at telomeres. We made all 63 mutant sequence combinations at template positions 474-476 of the yeast telomerase RNA, TLC1. Mutants contained faithfully incorporated template mutations, as well as misincorporated sequences in telomeres, a phenotype not previously reported for Saccharomyces cerevisiae telomerase template mutants. Although growth rates and telomere profiles varied widely among the tlc1 mutants, chromosome separation and segregation were always aberrant. The mutants showed defects in sister chromatid separation at centromeres as well as telomeres, suggesting activation of a cell cycle checkpoint. Deletion of the DNA damage response genes DDC1, MEC3, or DDC2/SML1 failed to restore chromosome separation in the tlc1 template mutants. These results suggest that mutant telomere sequences elicit a checkpoint that is genetically distinct from those activated by deletion of telomerase or DNA damage.

Molecular biology of hematopoietic stem cells

Human CD34+ hematopoietic stem and progenitor cells are capable of maintaining a life-long supply of the entire spectrum of blood cells dependent on systemic needs. Recent studies suggest that hematopoietic stem cells are, beyond their hematopoietic potential, able to differentiate into nonhematopoietic cell types, which could open novel avenues in the field of cellular therapy. Here, we concentrate on the molecular biology underlying basic features of hematopoietic stem cells. Immunofluorescence analyses, culture assays, and transplantation models permit an extensive immunological as well as functional characterization of human hematopoietic stem and progenitor cells. New methods such as cDNA array technology have demonstrated that distinct gene expression patterns of transcription factors and cell cycle genes molecularly control self-renewal, differentiation, and proliferation. Furthermore, several adhesion molecules have been shown to play an important role in the regulation of hematopoiesis and stem cell trafficking. Progress has also been made in elucidating molecular mechanisms of stem cell aging that limit replicative potential. Finally, more recent data provide the first molecular basis for a better understanding of transdifferentiation and developmental plasticity of hematopoietic stem cells. These findings could be helpful for non-hematopoietic cell therapeutic approaches.

A novel pseudoknot element is essential for the action of a yeast telomerase

Telomerase contains an essential RNA, which includes the template sequence copied by the reverse transcription action of telomerase into telomeric DNA. Using phylogenetic comparison, we identified seven conserved sequences in telomerase RNAs from Kluyveromyces budding yeasts. We show that two of these sequences, CS3 and CS4, are essential for normal telomerase function and can base-pair to form a putative long-range pseudoknot. Disrupting this base-pairing was deleterious to cell growth, telomere maintenance, and telomerase activity. Restoration of the base-pairing potential alleviated these phenotypes. Mutating this pseudoknot caused a novel mode of shifting of the boundaries of the RNA template sequence copied by telomerase. A phylogenetically derived model of yeast TER structure indicates that these RNAs can form two alternative predicted core conformations of similar stability: one brings the CS3/CS4 pseudoknot spatially close to the template; in the other, CS3 and CS4 move apart and the conformation of the template is altered. We propose that such disruption of the pseudoknot, and potentially the predicted telomerase RNA conformation, affects polymerization to cause the observed shifts in template usage.

Biochemical aspects of telomerase function

Arthur Kornberg "never met a dull enzyme" (For the Love of Enzymes: The Odyssey of a Biochemist, Harvard University Press, 1989) and telomerase is no exception. Telomerase is a remarkable polymerase that uses an internal RNA template to reverse-transcribe telomere DNA, one nucleotide at a time, onto telomeric, G-rich single-stranded DNA. In the 17 years since its discovery, the characterization of telomerase enzyme components has uncovered a highly conserved family of telomerase reverse transcriptases that, together with the telomerase RNA, appear to comprise the enzymatic core of telomerase. While not as comprehensively understood as yet, some telomerase-associated proteins also serve crucial roles in telomerase function in vivo, such as telomerase ribonudeoprotein (RNP) assembly, recruitment to the telomere, and the coordination of DNA replication at the telomere. A selected overview of the biochemical properties of this unique enzyme, in vitro and in vivo, will be presented.

Loss of cap structure causes mitotic defect in Tetrahymena thermophila telomerase mutants

Mutation of the telomeric repeat sequence has severe cellular consequences in a variety of systems. A Tetrahymena thermophila telomerase template mutant, ter1-43AA, displays an acute mitotic chromosome segregation defect. In the study described here we investigated the molecular basis for this lethality. Although cloned ter1-43AA macronuclear telomeres had long tracts of wild-type G4T2 repeats, they were capped by a mixture of G4T3 repeats, shown previously to be non-lethal, and G4T4 repeats, the telomeric sequence normally found in hypotrichous ciliates such as Oxytricha. To test further the functionality of the G4T4 repeat sequence in T. thermophila, we devised a new template mutation, ter1-44+AA, that resulted in more uniform synthesis of this sequence at telomere caps in vivo. The ter1-44+AA mutant displayed the most severe mitotic defect reported to date, with up to 85% of the population having micronuclei in anaphase, providing firm evidence that the hypotrich repeat sequence is not functional in Tetrahymena. Surprisingly, in spite of the telomeric sequence mutation, neither the ter1-43AA nor ter1-44+AA mutant displayed any significant loss of telomere length regulation. These results demonstrate that loss of telomere cap integrity, rather than length regulation, leads to the anaphase defect.

Yeast telomerase is specialized for C/A-rich RNA templates

Telomeres, the protective caps of eukaryotic chromosomes, are maintained by the enzyme telomerase. This telomere-specific reverse transcriptase (RT) uses a small region of its RNA subunit as template to synthesize telomeric DNA, which is generally G/T rich in the strand that contains the 3' end. To further our understanding of why telomeres are usually G/T rich, we screened Saccharomyces cerevisiae telomerase RNA (TLC1) libraries with randomized template sequences for complementation of a tlc1 deletion and decapping of existing telomeres. Surprisingly, the vast majority of the 60 000 different mutant telomerase templates tested showed no activity in vivo. This deficiency was not due to impaired assembly with the catalytic subunit (Est2p) nor could it be alleviated by enforced telomerase recruitment to the telomeres. Rather, the mutant templates reduced the nucleotide addition processivity of telomerase. The functional RNA template sequences recovered in our screens preferentially contained two or more consecutive rC nucleotides, reminiscent of the wild-type template. Thus, in contrast to retroviral RTs that can reverse transcribe any RNA sequence into DNA, the budding yeast telomerase RT is specialized for its C-rich RNA template.

Mutant telomeres inhibit transcriptional silencing at native telomeres of the yeast Kluyveromyces lactis

We report the identification and characterization of transcriptional silencing at native telomeres in the budding yeast Kluyveromyces lactis. We show that K. lactis telomeres are able to repress the transcription of a gene located at the junction between the telomeric repeat tract and the subtelomeric domain. As in Saccharomyces cerevisiae, switching between the repressed and derepressed transcriptional states occurs. C-terminal truncation of the telomere binding protein Rap1p, which leads to a regulated alteration in telomere length, reduces telomeric silencing. In addition, telomeric silencing is reduced dramatically in telomerase RNA mutants in which telomere length control has been lost. This is consistent with the possibility that the structure of the entire telomere affects the silencing functions exhibited by its internal domain.

Telomeric protein distributions and remodeling through the cell cycle in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, telomeric DNA is protected by a nonnucleosomal protein complex, tethered by the protein Rap1. Rif and Sir proteins, which interact with Rap1p, are thought to have further interactions with conventional nucleosomic chromatin to create a repressive structure that protects the chromosome end. We showed by microarray analysis that Rif1p association with the chromosome ends extends to subtelomeric regions many kilobases internal to the terminal telomeric repeats and correlates strongly with the previously determined genomic footprints of Rap1p and the Sir2-4 proteins in these regions. Although the end-protection function of telomeres is essential for genomic stability, telomeric DNA must also be copied by the conventional DNA replication machinery and replenished by telomerase, suggesting that transient remodeling of the telomeric chromatin might result in distinct protein complexes at different stages of the cell cycle. Using chromatin immunoprecipitation, we monitored the association of Rap1p, Rif1p, Rif2p, and the protein component of telomerase, Est2p, with telomeric DNA through the cell cycle. We provide evidence for dynamic remodeling of these components at telomeres.

Telomeres, X-inactivation ratios, and hematopoietic stem cell transplantation in humans: a review

The marrow repopulating hematopoietic stem cells (HSCs) in an auto- or allograft represent a small fraction of the normal complement of HSCs, yet are required to reconstitute hematopoiesis and sustain it for the lifetime of the recipient. Such a burden imposes a "replicative stress" upon hematopoietic stem/progenitor cells. The finding of accelerated telomere shortening in hematopoietic stem cell transplant (HSCT) recipients raised the specter of accelerated hematopoietic aging. Here, we review the HSCT telomere literature and other studies of surrogate markers of HSC behavior conducted in human HSCT recipients. We present a paradigm for posttransplant hematopoietic reconstitution and speculate on the fate of HSCs in the human transplant setting.

MEC3, MEC1, and DDC2 are essential components of a telomere checkpoint pathway required for cell cycle arrest during senescence in Saccharomyces cerevisiae

When telomerase is absent and/or telomeres become critically short, cells undergo a progressive decline in viability termed senescence. The telomere checkpoint model predicts that cells will respond to a damaged or critically short telomere by transiently arresting and activating repair of the telomere. We examined the senescence of telomerase-deficient Saccharomyces cerevisiae at the cellular level to ask if the loss of telomerase activity triggers a checkpoint response. As telomerase-deficient mutants were serially subcultured, cells exhibited a progressive decline in average growth rate and an increase in the number of cells delayed in the G2/M stage of the cell cycle. MEC3, MEC1, and DDC2, genes important for the DNA damage checkpoint response, were required for the cell cycle delay in telomerase-deficient cells. In contrast, TEL1, RAD9, and RAD53, genes also required for the DNA damage checkpoint response, were not required for the G2/M delay in telomerase-deficient cells. We propose that the telomere checkpoint is distinct from the DNA damage checkpoint and requires a specific set of gene products to delay the cell cycle and presumably to activate telomerase and/or other telomere repair activities.

A quantitative assay for telomere protection in Saccharomyces cerevisiae

Telomeres are the protective ends of linear chromosomes. Telomeric components have been identified and described by their abilities to bind telomeric DNA, affect telomere repeat length, participate in telomeric DNA replication, or modulate transcriptional silencing of telomere-adjacent genes; however, their roles in chromosome end protection are not as well defined. We have developed a genetic, quantitative assay in Saccharomyces cerevisiae to measure whether various telomeric components protect chromosome ends from homologous recombination. This "chromosomal cap" assay has revealed that the telomeric end-binding proteins, Cdc13p and Ku, both protect the chromosome end from homologous recombination, as does the ATM-related kinase, Tel1p. We propose that Cdc13p and Ku structurally inhibit recombination at telomeres and that Tel1p regulates the chromosomal cap, acting through Cdc13p. Analysis with recombination mutants indicated that telomeric homologous recombination events proceeded by different mechanisms, depending on which capping component was compromised. Furthermore, we found that neither telomere repeat length nor telomeric silencing correlated with chromosomal capping efficiency. This capping assay provides a sensitive in vivo approach for identifying the components of chromosome ends and the mechanisms by which they are protected.

New ways not to make ends meet: telomerase, DNA damage proteins and heterochromatin

Telomeres are stabilized, and telomeric DNA is replenished, by the action of the ribonucleoprotein reverse transcriptase telomerase. Telomere capping functions include the ability of telomeres to protect chromosome ends from cellular DNA-damage responses such as cell cycle arrest or apoptosis. This property of telomeres is especially important for cancer cells, which continue proliferating despite chromosome aberrations. Telomere capping is influenced by multiple, mutually reinforcing factors including telomere length, although telomere length is only one of several determinants of telomere functionality. For example, many cancer cells express high levels of telomerase yet maintain relatively short telomeres. We consider three aspects of telomere capping that have emerged relatively recently: (1) a new role for telomerase in telomere capping independent of its function in telomere elongation. Support for this novel function comes from experiments showing an increase in replicative potential with the reactivation of telomerase, without net telomere lengthening; (2) the role at telomeres of DNA damage proteins. We propose a model in which two factors specifically target telomeres for the action of telomerase, as opposed to recombination or non-homologous end-joining: binding by telomeric proteins that limits DNA damage responses at telomeres, and the affinity of the telomerase RNP for telomeric proteins and DNA; and (3) we discuss a potential protective role of amplified subtelomeric DNAs, which may aid capping of telomeres maintained by non-telomerase based mechanisms through the formation of heterochromatin.

A Trypanosoma brucei protein complex that binds G-overhangs and co-purifies with telomerase activity

The chromosomal ends of Trypanosoma brucei, like those of most eukaryotes, contain conserved 5'-TTAGGG-3' repeated sequences and are maintained by the action of telomerase. Fractionated T. brucei cell extracts with telomerase activity were used as a source of potential regulatory factors or telomerase-associated components that might interact with T. brucei telomeres. Electrophoretic mobility shift assays and UV cross-linking were used to detect possible single-stranded telomeric protein.DNA complexes and to estimate the approximate size of the protein constituents. Three single-stranded telomeric protein.DNA complexes were observed. Complex C3 was highly specific for the G-strand telomeric repeat sequence and shares biochemical characteristics with G-rich, single-stranded telomeric binding proteins and with components of the telomerase holoenzyme described in yeast, ciliates, and humans. Susceptibility to RNase A or chemical nuclease (hydroxyl radical) pre-treatment showed that complex C3 was tightly associated with an RNA component. Matrix-assisted laser desorption/ionization-time of flight mass spectrometry was used to estimate the molecular mass of the peptides obtained by in-gel Lys-C digestion of low abundance C3-associated proteins. The molecular masses of the peptides showed no homologies with other proteins from trypanosomes or with any protein in the data bases screened.

Dynamics of telomeric DNA turnover in yeast

Telomerase adds telomeric DNA repeats to telomeric termini using a sequence within its RNA subunit as a template. We characterized two mutations in the Kluyveromyces lactis telomerase RNA gene (TER1) template. Each initially produced normally regulated telomeres. One mutation, ter1-AA, had a cryptic defect in length regulation that was apparent only if the mutant gene was transformed into a TER1 deletion strain to permit extensive replacement of basal wild-type repeats with mutant repeats. This mutant differs from previously studied delayed elongation mutants in a number of properties. The second mutation, TER1-Bcl, which generates a BclI restriction site in newly synthesized telomeric repeats, was indistinguishable from wild type in all phenotypes assayed: cell growth, telomere length, and in vivo telomerase fidelity. TER1-Bcl cells demonstrated that the outer halves of the telomeric repeat tracts turn over within a few hundred cell divisions, while the innermost few repeats typically resisted turnover for at least 3000 cell divisions. Similarly deep but incomplete turnover was also observed in two other TER1 template mutants with highly elongated telomeres. These results indicate that most DNA turnover in functionally normal telomeres is due to gradual replicative sequence loss and additions by telomerase but that there are other processes that also contribute to turnover.

Telomere formation by rap1p binding site arrays reveals end-specific length regulation requirements and active telomeric recombination

Rap1p, the major telomere repeat binding protein in yeast, has been implicated in both de novo telomere formation and telomere length regulation. To characterize the role of Rap1p in these processes in more detail, we studied the generation of telomeres in vivo from linear DNA substrates containing defined arrays of Rap1p binding sites. Consistent with previous work, our results indicate that synthetic Rap1p binding sites within the internal half of a telomeric array are recognized as an integral part of the telomere complex in an orientation-independent manner that is largely insensitive to the precise spacing between adjacent sites. By extending the lengths of these constructs, we found that several different Rap1p site arrays could never be found at the very distal end of a telomere, even when correctly oriented. Instead, these synthetic arrays were always followed by a short ( approximately 100-bp) "cap" of genuine TG repeat sequence, indicating a remarkably strict sequence requirement for an end-specific function(s) of the telomere. Despite this fact, even misoriented Rap1p site arrays promote telomere formation when they are placed at the distal end of a telomere-healing substrate, provided that at least a single correctly oriented site is present within the array. Surprisingly, these heterogeneous arrays of Rap1p binding sites generate telomeres through a RAD52-dependent fusion resolution reaction that results in an inversion of the original array. Our results provide new insights into the nature of telomere end capping and reveal one way by which recombination can resolve a defect in this process.

Molecular basis for telomere repeat divergence in budding yeast

Telomerase is a ribonucleoprotein enzyme that adds repetitive sequences to the ends of linear chromosomes, thereby counteracting nucleotide loss due to incomplete replication. A short region of the telomerase RNA subunit serves as template for nucleotide addition onto the telomere 3' end. Although Saccharomyces cerevisiae contains only one telomerase RNA gene, telomere repeat sequences are degenerate in this organism. Based on a detailed analysis of the telomere sequences specified by wild-type and mutant RNA templates in vivo, we show that the divergence of telomere repeats is due to abortive reverse transcription in the 3' and 5' regions of the template and due to the alignment of telomeres in multiple registers within the RNA template. Through the interpretation of wild-type telomere sequences, we identify nucleotides in the template that are not accessible for base pairing during substrate annealing. Rather, these positions become available as templates for reverse transcription only after alignment with adjacent nucleotides has occurred, indicating that a conformational change takes place upon substrate binding. We also infer that the central part of the template region is reverse transcribed processively. The inaccessibility of certain template positions for alignment and the processive polymerization of the central template portion may serve to reduce the possible repeat diversification and enhance the incorporation of binding sites for Rap1p, the telomere binding protein of budding yeast.

Intrachromatid excision of telomeric DNA as a mechanism for telomere size control in Saccharomyces cerevisiae

We have previously identified a process in the yeast Saccharomyces cerevisiae that results in the contraction of elongated telomeres to wild-type length within a few generations. We have termed this process telomeric rapid deletion (TRD). In this study, we use a combination of physical and genetic assays to investigate the mechanism of TRD. First, to distinguish among several recombinational and nucleolytic pathways, we developed a novel physical assay in which HaeIII restriction sites are positioned within the telomeric tract. Specific telomeres were subsequently tested for HaeIII site movement between telomeres and for HaeIII site retention during TRD. Second, genetic analyses have demonstrated that mutations in RAD50 and MRE11 inhibit TRD. TRD, however, is independent of the Rap1p C-terminal domain, a central regulator of telomere size control. Our results provide evidence that TRD is an intrachromatid deletion process in which sequences near the extreme terminus invade end-distal sequences and excise the intervening sequences. We propose that the Mre11p-Rad50p-Xrs2p complex prepares the invading telomeric overhang for strand invasion, possibly through end processing or through alterations in chromatin structure.

Role for telomere cap structure in meiosis

Telomeres, the natural ends of eukaryotic chromosomes, are essential for the protection of chromosomes from end-to-end fusions, recombination, and shortening. Here we explore their role in the process of meiotic division in the budding yeast, Kluyveromyces lactis. Telomerase RNA mutants that cause unusually long telomeres with deregulated structure led to severely defective meiosis. The severity of the meiotic phenotype of two mutants correlated with the degree of loss of binding of the telomere binding protein Rap1p. We show that telomere size and the extent of potential Rap1p binding to the entire telomere are irrelevant to the process of meiosis. Moreover, we demonstrate that extreme difference in telomere size between two homologous chromosomes is compatible with the normal function of telomeres during meiosis. In contrast, the structure of the most terminal telomeric repeats is critical for normal meiosis. Our results demonstrate that telomeres play a critical role during meiotic division and that their terminal cap structure is essential for this role.

Switching and signaling at the telomere

This review describes the structure of telomeres, the protective DNA-protein complexes at eukaryotic chromosomal ends, and several molecular mechanisms involved in telomere functions. Also discussed are cellular responses to compromising the functions of telomeres and of telomerase, which synthesizes telomeric DNA.

Telomerase can inhibit the recombination-based pathway of telomere maintenance in human cells

Telomere length can be maintained by telomerase or by a recombination-based pathway. Because individual telomeres in cells using the recombination-based pathway of telomere maintenance appear to periodically become extremely short, cells using this pathway to maintain telomeres may be faced with a continuous state of crisis. We expressed telomerase in a human cell line that uses the recombination-based pathway of telomere maintenance to test whether telomerase would prevent telomeres from becoming critically short and examine the effects that this might have on the recombination-based pathway of telomere maintenance. In these cells, telomerase maintains the length of the shortest telomeres. In some cases, the long heterogeneous telomeres are completely lost, and the cells now permanently contain short telomeres after only 40 population doublings. This corresponds to a telomere reduction rate of 500 base pairs/population doubling, a rate that is much faster than expected for normal telomere shortening but is consistent with the rapid telomere deletion events observed in cells using the recombination-based pathway of telomere maintenance (Murnane, J. P., Sabatier, L., Marder, B. A., and Morgan, W. F. (1994) EMBO J. 13, 4953-4962). We also observed reductions in the fraction of cells containing alternative lengthening of telomere-associated promyelocytic leukemia bodies and extrachromosomal telomere repeats; however, no alterations in the rate of sister chromatid exchange were observed. Our results demonstrate that human cells using the recombination-based pathway of telomere maintenance retain factors required for telomerase to maintain telomeres and that once the telomerase-based pathway of telomere length regulation is engaged, recombination-based elongation of telomeres can be functionally inhibited.

Altering telomere structure allows telomerase to act in yeast lacking ATM kinases

BACKGROUND

Telomerase is a ribonucleoprotein that copies a short RNA template into telomeric DNA, maintaining eukaryotic chromosome ends and preventing replicative senescence. Telomeres differentiate chromosome ends from DNA double-stranded breaks. Nevertheless, the DNA damage-responsive ATM kinases Tel1p and Mec1p are required for normal telomere maintenance in Saccharomyces cerevisiae. We tested whether the ATM kinases are required for telomerase enzyme activity or whether it is their action on the telomere that allows telomeric DNA synthesis.

RESULTS

Cells lacking Tel1p and Mec1p had wild-type levels of telomerase activity in vitro. Furthermore, altering telomere structure in three different ways showed that telomerase can function in ATM kinase-deleted cells: tel1 mec1 cells senesced more slowly than tel1 mec1 cells that also lacked TLC1, which encodes telomerase RNA, suggesting that tel1 mec1 cells have residual telomerase function; deleting the telomere-associated proteins Rif1p and Rif2p in tel1 mec1 cells prevented senescence; we isolated a point mutation in the telomerase RNA template domain (tlc1-476A) that altered telomeric DNA sequences, causing uncontrolled telomeric DNA elongation and increasing single strandedness. In tel1 mec1 cells, tlc1-476A telomerase was also capable of uncontrolled synthesis, but only after telomeres had shortened for >30 generations.

CONCLUSION

Our results show that, without Tel1p and Mec1p, telomerase is still active and can act in vivo when the telomere structure is disrupted by various means. Hence, a primary function of the ATM-family kinases in telomere maintenance is to act on the substrate of telomerase, the telomere, rather than to activate the enzymatic activity of telomerase.

A low threshold level of expression of mutant-template telomerase RNA inhibits human tumor cell proliferation

The ribonucleoprotein telomerase synthesizes telomeric DNA by copying an intrinsic RNA template. In most cancer cells, telomerase is highly activated. Here we report a telomerase-based antitumor strategy: expression of mutant-template telomerase RNAs in human cancer cells. We expressed mutant-template human telomerase RNAs in prostate (LNCaP) and breast (MCF-7) cancer cell lines. Even a low threshold level of expression of telomerase RNA gene constructs containing various mutant templates, but not the control wild-type template, decreased cellular viability and increased apoptosis. This occurred despite the retention of normal levels of the endogenous wild-type telomerase RNA and endogenous wild-type telomerase activity and unaltered stable telomere lengths. In vivo tumor xenografts of a breast cancer cell line expressing a mutant-template telomerase RNA also had decreased growth rates. Therefore, mutant-template telomerase RNAs exert a strongly dominant-negative effect on cell proliferation and tumor growth. These results support the potential use of mutant-template telomerase RNA expression as an antineoplastic strategy.

Telomeric chromatin: replicating and wrapping up chromosome ends

Recent advances in our understanding of the specialized chromatin structure at telomeres, the ends of eukaryotic chromosomes, have focused on three separate areas: replication of telomeres through the coordinated action of conventional DNA polymerases and the telomerase enzyme, protection of the chromosome end from DNA damage checkpoint sensors and DNA-repair processes, and the discovery of a novel deacetylase enzyme (Sir2p) required for the establishment and maintenance of telomeric heterochromatin. Although the number of proteins and the complexity of their interactions at telomeres continues to grow, a picture of at least some of the major players and mechanisms underlying telomere replication, end 'capping' and chromatin assembly is beginning to emerge.

Telomeres and their control

Telomeres are DNA and protein structures that form complexes protecting the ends of chromosomes. Understanding of the mechanisms maintaining telomeres and insights into their function have advanced considerably in recent years. This review summarizes the currently known components of the telomere/telomerase functional complex, and focuses on how they act in the control of processes occurring at telomeres. These include processes acting on the telomeric DNA and on telomeric proteins. Key among them are DNA replication and elongation of one telomeric DNA strand by telomerase. In some situations, homologous recombination of telomeric and subtelomeric DNA is induced. All these processes act to replenish or restore telomeres. Conversely, degradative processes that shorten telomeric DNA, and nonhomologous end-joining of telomeric DNA, can lead to loss of telomere function and genomic instability. Hence they too must normally be tightly controlled.

Expression of mutant telomerase in immortal telomerase-negative human cells results in cell cycle deregulation, nuclear and chromosomal abnormalities and rapid loss of viability

We have reconstituted wild type or mutant telomerase activity in two human cell lines that lack constitutive expression of both core subunits of the enzyme and maintain telomeres by a telomerase-independent mechanism (ALT cells). Wild type telomerase RNA and four telomerase RNAs with single point mutations in their template domain were used to express enzymes specifying different telomeric DNA sequences. Expression of wild type telomerase for up to 32 days had no detectable effect on cell growth or viability. In contrast, cells expressing mutant telomerases had slower growth rate, abnormal cell cycle and reduced viability. Dramatically aberrant nuclei, typical of cells undergoing mitotic catastrophe, and large numbers of fused chromosomes were also characteristic of these populations. Notably, all phenotypes were apparent within the first few cell divisions after expression of the enzymes. Unlike wild type, mutant telomerase activity was progressively selected against with cell culturing, and this correlated with the disappearance of cells with aberrant phenotypes. Our results suggest that even very limited synthesis of mutated sequences can affect telomere structure in human cells, and that the toxicity of mutant telomerases is due to telomere malfunction.

Telomere fusions caused by mutating the terminal region of telomeric DNA

Mutations in the template region of a telomerase RNA gene can lead to the corresponding sequence alterations appearing in newly synthesized telomeric repeats. We analyzed a set of mutations in the template region of the telomerase RNA gene (TER1) of the budding yeast Kluyveromyces lactis that were predicted to lead to synthesis of mutant telomeric repeats with disrupted binding of the telomeric protein Rap1p. We showed previously that mutating the left side of the 12-bp consensus Rap1p binding site led to immediate and severe telomere elongation. Here, we show that, in contrast, mutating either the right side of the site or both sides together leads initially to telomere shortening. On additional passaging, certain mutants of both categories exhibit telomere-telomere fusions. Often, six new Bal-31-resistant, telomere repeat-containing bands appeared, and we infer that each of the six K. lactis chromosomes became circularized. These fusions were not stable, appearing occasionally to resolve and then reform. We demonstrate directly that a linear minichromosome introduced into one of the fusion mutant strains circularized by means of end-to-end fusions of the mutant repeat tracts. In contrast to the chromosomal circularization reported previously in Schizosaccharomyces pombe mutants defective in telomere maintenance, the K. lactis telomere fusions retained their telomeric DNA repeat sequences.

Genetics and molecular physiology of the yeast Kluyveromyces lactis

With the recent development of powerful molecular genetic tools, Kluyveromyces lactis has become an excellent alternative yeast model organism for studying the relationships between genetics and physiology. In particular, comparative yeast research has been providing insights into the strikingly different physiological strategies that are reflected by dominance of respiration over fermentation in K. lactis versus Saccharomyces cerevisiae. Other than S. cerevisiae, whose physiology is exceptionally affected by the so-called glucose effect, K. lactis is adapted to aerobiosis and its respiratory system does not underlie glucose repression. As a consequence, K. lactis has been successfully established in biomass-directed industrial applications and large-scale expression of biotechnically relevant gene products. In addition, K. lactis maintains species-specific phenomena such as the "DNA-killer system, " analyses of which are promising to extend our knowledge about microbial competition and the fundamentals of plasmid biology.

Telomerase RNA template mutations reveal sequence-specific requirements for the activation and repression of telomerase action at telomeres

Telomeric DNA is maintained within a length range characteristic of an organism or cell type. Significant deviations outside this range are associated with altered telomere function. The yeast telomere-binding protein Rap1p negatively regulates telomere length. Telomere elongation is responsive to both the number of Rap1p molecules bound to a telomere and the Rap1p-centered DNA-protein complex at the extreme telomeric end. Previously, we showed that a specific trinucleotide substitution in the Saccharomyces cerevisiae telomerase gene (TLC1) RNA template abolished the enzymatic activity of telomerase, causing the same cell senescence and telomere shortening phenotypes as a complete tlc1 deletion. Here we analyze effects of six single- and double-base changes within these same three positions. All six mutant telomerases had in vitro enzymatic activity levels similar to the wild-type levels. The base changes predicted from the mutations all disrupted Rap1p binding in vitro to the corresponding duplex DNAs. However, they caused two classes of effects on telomere homeostasis: (i) rapid, RAD52-independent telomere lengthening and poor length regulation, whose severity correlated with the decrease in in vitro Rap1p binding affinity (this is consistent with loss of negative regulation of telomerase action at these telomeres; and (ii) telomere shortening that, depending on the template mutation, either established a new short telomere set length with normal cell growth or was progressive and led to cellular senescence. Hence, disrupting Rap1p binding at the telomeric terminus is not sufficient to deregulate telomere elongation. This provides further evidence that both positive and negative cis-acting regulators of telomerase act at telomeres.