A functional telomerase RNA swap in vivo reveals the importance of nontemplate RNA domains

Engineered domain swapping indicates context dependent functional role of RNA G-quadruplexes

RNA domain swapping typically demonstrates conservation of the native function of the domain in a non-native context. In contrast, we employed RNA engineering to demonstrate deviation of G-quadruplex (GQ) function that is contingent upon its context dependent location, which is opposite to their native functional role. Known translation repressing RNA GQs were engineered into human VEGF IRES A replacing the endogenous GQ domain essential for translation. Alternatively, the translation inhibitory GQ motif within the 5'-UTR of MT3-MMP mRNA was replaced with two known GQ motifs that are essential for translation. The results indicate that the engineered GQ domains can adopt GQ structures in a foreign environment with a functional role reversal to accommodate the need of the endogenous swapped motifs. The observations establish the functionality and context dependent modularity of RNA GQ structures.

The MRT-1 nuclease is required for DNA crosslink repair and telomerase activity in vivo in Caenorhabditis elegans

The telomerase reverse transcriptase adds de novo DNA repeats to chromosome termini. Here we define Caenorhabditis elegans MRT-1 as a novel factor required for telomerase-mediated telomere replication and the DNA-damage response. MRT-1 is composed of an N-terminal domain homologous to the second OB-fold of POT1 telomere-binding proteins and a C-terminal SNM1 family nuclease domain, which confer single-strand DNA-binding and processive 3'-to-5' exonuclease activity, respectively. Furthermore, telomerase activity in vivo depends on a functional MRT-1 OB-fold. We show that MRT-1 acts in the same telomere replication pathway as telomerase and the 9-1-1 DNA-damage response complex. MRT-1 is dispensable for DNA double-strand break repair, but functions with the 9-1-1 complex to promote DNA interstrand cross-link (ICL) repair. Our data reveal MRT-1 as a dual-domain protein required for telomerase function and ICL repair, which raises the possibility that telomeres and ICL lesions may share a common feature that plays a critical role in de novo telomere repeat addition.

The structure and function of telomerase reverse transcriptase

The structure and integrity of telomeres are essential for genome stability. Telomere dysregulation can lead to cell death, cell senescence, or abnormal cell proliferation. The maintenance of telomere repeats in most eukaryotic organisms requires telomerase, which consists of a reverse transcriptase (RT) and an RNA template that dictates the synthesis of the G-rich strand of telomere terminal repeats. Structurally, telomerase reverse transcriptase (TERT) contains unique and variable N- and C-terminal extensions that flank a central RT-like domain. The enzymology of telomerase includes features that are both similar to and distinct from those characteristic of other RTs. Two distinguishing features of TERT are its stable association with the telomerase RNA and its ability to repetitively reverse transcribe the template segment of RNA. Here we discuss TERT structure and function; its regulation by RNA-DNA, TERT-DNA, TERT-RNA, TERT-TERT interactions, and TERT-associated proteins; and the relationship between telomerase enzymology and telomere maintenance.

A human-Tetrahymena pseudoknot chimeric telomerase RNA reconstitutes a nonprocessive enzyme in vitro that is defective in telomere elongation

The phylogenetically-derived secondary structures of telomerase RNAs (TR) from ciliates, yeasts and vertebrates are surprisingly conserved and contain a pseudoknot domain at a similar location downstream of the template. As the pseudoknot domains of Tetrahymena TR (tTR) and human TR (hTR) mediate certain similar functions, we hypothesized that they might be functionally interchangeable. We constructed a chimeric TR (htTR) by exchanging the hTR pseudoknot sequences for the tTR pseudoknot region. The chimeric RNA reconstituted human telomerase activity when coexpressed with hTERT in vitro, but exhibited defects in repeat addition processivity and levels of DNA synthesis compared to hTR. Activity was dependent on tTR sequences within the chimeric RNA. htTR interacted with hTERT in vitro and dimerized predominantly via a region of its hTR backbone, the J7b/8a loop. Introduction of htTR in telomerase-negative cells stably expressing hTERT did not reconstitute an active enzyme able to elongate telomeres. Thus, our results indicate that the chimeric RNA reconstituted a weakly active nonprocessive human telomerase enzyme in vitro that was defective in telomere elongation in vivo. This suggests that there may be species-specific requirements for pseudoknot functions.

Processive utilization of the human telomerase template: lack of a requirement for template switching

The ribonucleoprotein telomerase is a specialized reverse transcriptase minimally composed of an RNA, TER, and a protein catalytic subunit, TERT. The TER and TERT subunits of telomerase associate to form a dimeric enzyme in several organisms, including human. A small portion of TER, the template domain, is used by telomerase for the synthesis of tandem repeats of telomeric DNA. We studied some of the requirements for processive template usage by human telomerase. A blunt-ended duplex DNA primer was not utilized by telomerase. With a duplex telomeric DNA primer, a single-stranded 3' overhang with a minimum length of approximately 6 bases was required for efficient priming activity. Large substitutions in the human TER templating domain did not abolish enzymatic activity, although insertion of two residues into this sequence reduced processivity, as did a template mutation that results in a mismatch between the template region used for copying DNA and the region used for alignment of the substrate primer. Finally, by using a complementary pair of catalytically inactive telomerase RNA pseudoknot mutants in combination with a marked template, we demonstrated that processive synthesis by an obligatory dimer of human telomerase does not require template switching. These results indicate that processive template usage by human telomerase, like that of Tetrahymena telomerase, is strongly dependent on the base identities in the template domain and that a dimeric human telomerase can processively utilize a single template.

A human telomerase-associated nuclease

Ciliate and yeast telomerase possess a nucleolytic activity capable of removing DNA from the 3' end of a single-stranded oligonucleotide substrate. The nuclease activity is thought to assist in enzyme proofreading and/or processivity. Herein, we report a previously uncharacterized human telomerase-associated nuclease activity that shares several properties with ciliate and yeast telomerases. Partially purified human telomerase, either from cell extracts or recombinantly produced, demonstrated an ability to remove 3' nontelomeric nucleotides from a substrate containing 5' telomeric DNA, followed by extension of the newly exposed telomeric sequence. This cleavage/extension activity was apparent at more than one position within the telomeric DNA and was influenced by sequences 5' to the telomeric/nontelomeric boundary and by substitution with a methylphosphonate moiety at the telomeric/nontelomeric DNA boundary. Our data suggest that human telomerase is associated with an evolutionarily conserved nucleolytic activity and support a model in which telomerase-substrate interactions can occur distal from the 3' primer end.

Human telomerase catalyzes nucleolytic primer cleavage

Telomerase is a reverse transcriptase that uses an integral RNA molecule to add de novo G-rich repeats onto telomeric DNA, or onto nontelomeric DNA generated during chromosome fragmentation and breakage events. A telomerase-mediated DNA substrate cleavage activity has been reported in ciliates and yeasts. Nucleolytic cleavage may serve a proofreading function, enhance processivity or ensure that nontemplate telomerase RNA sequences are not copied into DNA. We identified and characterized a human telomerase-mediated nucleolytic cleavage activity using enzyme reconstituted in a rabbit reticulocyte lysate in vitro transcription/translation system and native enzyme extracted from cells. We found that telomerase catalyzed the removal of nucleotides from DNA substrates including those that can form a mismatch with the RNA template or that contain nontelomeric sequences located 3' to a telomeric sequence. Unlike Tetrahymena telomerase, human telomerase catalyzed the removal of more than one nucleotide (up to 13) from telomeric primers. DNA substrates predicted to align at the 3'-end of the RNA template were not cleaved, consistent with cleavage being dictated by the template 5'-end. We also found some differences in the nuclease activity between RRL-reconstituted human telomerase and native enzyme.

Protein binding to expanded telomere repeats in Tetrahymena thermophila

The ends of eukaryotic chromosomes are protected by DNA-protein structures called telomeres. Telomeric DNA is highly conserved, usually consisting of long tracts of a repeating G-rich sequence. Tetrahymena thermophila telomeric DNA consists of alternating blocks of GGGG and TT sequences (i.e. a G4T2 repeat sequence). We examined the relative importance of the guanine and thymine elements of the repeat sequence in promoting in vitro binding by T. thermophila proteins. We identified single- and, for the first time, double-stranded telomere binding activities from a crude T. thermophila protein extract and tested the binding of these activities to altered telomere repeat sequences. All deletions or substitutions made to the guanine element virtually abolished binding, indicating that four G's are essential for recognition by the binding activity. However, G's alone are not sufficient for efficient binding, as elimination of the thymine element dramatically reduced binding. By contrast, substantial expansion of the thymine element was well tolerated, even though one such change, G4T4, is lethal in vivo. We tested up to a four-fold expansion of the thymine element and found that highly efficient binding was still achieved. These results suggest a minimal recognition sequence for T. thermophila proteins, with the T element providing an important spacer between essential G elements.

Roles for RNA in telomerase nucleotide and repeat addition processivity

Telomerase is a ribonucleoprotein reverse transcriptase with two subunits critical for catalytic activity, the protein telomerase reverse transcriptase (TERT) and telomerase RNA. In this study, we establish additional roles of the telomerase RNA subunit by demonstrating that RNA motifs stimulate the processivity of nucleotide and repeat addition. These functions are both functionally and physically separable from the roles of other RNA motifs in establishing a properly defined template. Binding of Tetrahymena telomerase RNA stem IV to TERT enhances nucleotide addition processivity, while a cooperation of the RNA pseudoknot and stem IV promotes repeat addition processivity. The low processivity of DNA synthesis by telomerase ribonucleoproteins lacking the pseudoknot and/or stem IV can be rescued by addition of the deleted region in trans. These findings demonstrate RNA elements with roles in telomerase elongation processivity that are distinct from RNA elements that specify the internal template.

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.

Determinants in mammalian telomerase RNA that mediate enzyme processivity and cross-species incompatibility

Telomerase contains two essential components: an RNA molecule that templates telomeric repeat synthesis and a catalytic protein component. Human telomerase is processive, while the mouse enzyme has much lower processivity. We have identified nucleotide determinants in the telomerase RNA that are responsible for this difference in processivity. Mutations adjacent to the template region of human and mouse telomerase RNA significantly altered telomerase processivity both in vitro and in vivo. We also identified functionally important nucleotides in the pseudoknot domain of telomerase RNA that potentially mediate the incompatibility between human TERT and mouse telomerase RNA. These experiments identify essential residues of the telomerase RNA that regulate telomerase activity and processivity.

Telomerase recognizes its template by using an adjacent RNA motif

Telomerase adds telomeric repeats to chromosome 3' ends, forestalling the cellular senescence, apoptosis, and genomic instability that result from telomere loss caused by incomplete DNA replication. The telomerase ribonucleoprotein is dedicated to synthesis of tandem, simple-sequence repeats by virtue of its specialization for copying only a specific template region within the integral RNA. Here, using circularly permuted variants of Tetrahymena thermophila telomerase RNA, we identify the features that allow recognition of the template region within the RNA. We engineered a template-less telomerase ribonucleoprotein that can position and reverse transcribe an exchangeable RNA oligonucleotide template accurately. Only a short "template-recognition" element sequence tag is required to direct efficient use of adjacent 5' residues as a template for telomeric repeat synthesis. Our findings reveal molecular requirements for template selection by telomerase and physically resolve templating from other RNA functions in catalysis.

Telomerase: biological function and potential role in cancer management

Telomeres are the specialized ends of eukaryotic chromosomes, thought to have many functions, most importantly serving as a clock signaling entry into cellular senescence. These structures are maintained by the reverse transcriptase telomerase, a peculiar enzyme in both structure, since it contains its own template RNA and function, since it is inactivated in most normal tissues but activated in the vast majority of malignant tumors. These features have made telomerase a subject of intense investigation, both to understand its cellular role and regulation and to exploit its activation in cancer to develop drugs or diagnostic methods based on telomerase. This work gathers all the information currently available in the biological and clinical fields of telomerase research.

Functional analysis of conserved residues in the putative "finger" domain of telomerase reverse transcriptase

Telomerase is a ribonucleoprotein reverse transcriptase (RT) responsible for the maintenance of one strand of telomere terminal repeats. The catalytic protein subunit of telomerase, known generically as telomerase reverse transcriptase (TERT), exhibits significant homology to RTs encoded by retroviruses and retroelements. The polymerization mechanisms of telomerase may therefore be similar to those of the "conventional" RTs. In this study, we explored the extent of mechanistic conservation by analyzing mutations of conserved residues within the putative "finger" domain of TERT. Previous analysis has implicated this domain of retroviral RTs in nucleotide and RNA binding and in processivity control. Our results demonstrate that residues conserved between TERT and human immunodeficiency virus-1 RT are more likely than TERT-specific residues to be required for enzyme activity. In addition, residues presumed to make direct contact with either the RNA or nucleotide substrate appear to be functionally more important. Furthermore, distinct biochemical defects can be observed for alterations in the putative RNA- and nucleotide-binding TERT residues in a manner that can be rationalized by their postulated mechanisms of action. This study thus supports a high degree of mechanistic conservation between telomerase and retroviral RTs and underscores the roles of distinct aspects of telomerase biochemistry in telomere length maintenance.

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.

Functional regions of human telomerase reverse transcriptase and human telomerase RNA required for telomerase activity and RNA-protein interactions

Telomerase is a specialized reverse transcriptase (RT) that is minimally composed of a protein catalytic subunit and an RNA component. The RNA subunit contains a short template sequence that directs the synthesis of DNA repeats at the ends of chromosomes. Human telomerase activity can be reconstituted in vitro by the expression of the human telomerase protein catalytic subunit (hTERT) in the presence of recombinant human telomerase RNA (hTR) in a rabbit reticulocyte lysate (RRL) system. We analyzed telomerase activity and binding of hTR to hTERT in RRL by expressing different hTERT and hTR variants. hTRs containing nucleotide substitutions that are predicted to disrupt base pairing in the P3 helix of the pseudoknot weakly reconstituted human telomerase activity yet retained their ability to bind hTERT. Our results also identified two distinct regions of hTR that can independently bind hTERT in vitro. Furthermore, sequences or structures between nucleotides 208 and 330 of hTR (which include the conserved CR4-CR5 domain) were found to be important for hTERT-hTR interactions and for telomerase activity reconstitution. Human TERT carboxy-terminal amino acid deletions extending to motif E or the deletion of the first 280 amino acids abolished human telomerase activity without affecting the ability of hTERT to associate with hTR, suggesting that the RT and RNA binding functions of hTERT are separable. These results indicate that the reconstitution of human telomerase activity in vitro requires regions of hTERT that (i) are distinct from the conserved RT motifs and (ii) bind nucleotides distal to the hTR template sequence.

Telomere maintenance mechanisms as a target for drug development

The shortening of the telomeric DNA sequences at the ends of chromosomes is thought to play a critical role in regulating the lifespan of human cells. Since all dividing cells are subject to the loss of telomeric sequences, cells with long proliferative lifespans need mechanisms to maintain telomere integrity. It appears that the activation of the enzyme telomerase is the major mechanism by which these cells maintain their telomeres. The proposal that a critical step in the process of the malignant transformation of cells is the upregulation of expression of telomerase has made this enzyme a potentially useful prognostic and diagnostic marker for cancer, as well as a new target for therapeutic intervention for the treatment of patients with cancer. It is now clear that simply inhibiting telomerase may not result in the anticancer effects that were originally hypothesized. While telomerase may not be the universal target for cancer therapy, we certainly believe that targeting the telomere maintenance mechanisms will be important in future research aimed toward a successful strategy for curing cancer.

Polymerization defects within human telomerase are distinct from telomerase RNA and TEP1 binding

The minimal, active core of human telomerase is postulated to contain two components, the telomerase RNA hTER and the telomerase reverse transcriptase hTERT. The reconstitution of human telomerase activity in vitro has facilitated the identification of sequences within the telomerase RNA and the RT motifs of hTERT that are essential for telomerase activity. However, the precise role of residues outside the RT domain of hTERT is unknown. Here we have delineated several regions within hTERT that are important for telomerase catalysis, primer use, and interaction with the telomerase RNA and the telomerase-associated protein TEP1. In particular, certain deletions of the amino and carboxy terminus of hTERT that retained an interaction with telomerase RNA and TEP1 were nonetheless completely inactive in vitro and in vivo. Furthermore, hTERT truncations lacking the amino terminus that were competent to bind the telomerase RNA were severely compromised for the ability to elongate telomeric and nontelomeric primers. These results suggest that the interaction of telomerase RNA with hTERT can be functionally uncoupled from polymerization, and that there are regions outside the RT domain of hTERT that are critical for telomerase activity and primer use. These results establish that the human telomerase RT possesses unique polymerization determinants that distinguish it from other RTs.

Ciliate telomerase biochemistry

Telomerase is a cellular reverse transcriptase specialized for use of a template carried within the RNA component of the enzyme ribonucleoprotein complex. Substrates for telomerase are single-stranded oligonucleotides in vitro and chromosome ends in vivo. In vitro, a bound substrate is extended by an initial round of DNA synthesis on the internal RNA template and in some cases by multiple rounds of template copying before product dissociation. In vivo, de novo synthesis of one strand of a telomeric repeat sequence by telomerase balances the sequence loss resulting from incomplete replication of linear chromosome ends by RNA primer-requiring DNA polymerases. Telomerase biochemistry has been studied extensively by using partially purified cell extracts. Telomerase components are being identified and beginning to be produced in recombinant form. This review focuses on the enzyme mechanism of telomerases from ciliate species, thus far the most intensively studied systems.

Three telomerases with completely non-telomeric template replacements are catalytically active

Telomerase is a reverse transcriptase minimally composed of a reverse transcriptase protein subunit and an internal RNA component that contains the templating region. Point mutations of template RNA bases can cause loss of enzymatic activity, reduced processivity and misincorporation in vitro. Here we report the first complete replacement of the nine base TETRAHYMENA: thermophila telomerase templating region in vivo with non-telomeric sequences. Rather than ablating telomerase activity, three such replaced telomerases (U9, AUN and AU4) were effective in polymerization in vitro. In vivo, the AU4 and AUN genes caused telomere shortening. We demonstrated the fidelity of the AUN and U9 telomerases in vitro and utilized AUN telomerase to demonstrate that 5' end primer recognition by telomerase is independent of template base pairing. However, the mutant AUN template telomerase catalyzed an abnormal DNA cleavage reaction. For these U-only and AU- substituted templates, we conclude that base-specific interactions between the telomerase template and protein (or distant parts of the RNA) are not absolutely required for the minimal core telomerase functions of nucleotide addition and base discrimination.

Interference footprinting analysis of telomerase elongation complexes

Telomerase is a reverse transcriptase that adds single-stranded telomeric repeats to the ends of linear eukaryotic chromosomes. It consists of an RNA molecule including a template sequence, a protein subunit containing reverse transcriptase motifs, and auxiliary proteins. We have carried out an interference footprinting analysis of the Tetrahymena telomerase elongation complexes. In this study, single-stranded oligonucleotide primers containing telomeric sequences were modified with base-specific chemical reagents and extended with the telomerase by a single (32)P-labeled dGMP or dTMP. Base modifications that interfered with the primer extension reactions were mapped by footprinting. Major functional interactions were detected between the telomerase and the six or seven 3'-terminal residues of the primers. These interactions occurred not only with the RNA template region, but also with another region in the enzyme ribonucleoprotein complex designated the telomerase DNA interacting surface (TDIS). This was indicated by footprints generated with dimethyl sulfate (that did not affect Watson-Crick hydrogen bonding) and by footprinting assays performed with mutant primers. In primers aligned at a distance of 2 nucleotides along the RNA template region, the footprints of the six or seven 3'-terminal residues were shifted by 2 nucleotides. This shift indicated that during the elongation reaction, TDIS moved in concert with the 3' ends of the primers relative to the template region. Weak interactions occurred between the telomerase and residues located upstream of the seventh nucleotide. These interactions were stronger in primers that were impaired in the ability to align with the template.

Inhibition of telomerase activity by a hammerhead ribozyme targeting the RNA component of telomerase in human melanoma cells

Reactivation of telomerase, an RNA-dependent DNA polymerase that synthesizes new telomeric repeats at the end of chromosomes, is a very common feature in human cancers. Telomerase is thought to be essential in maintaining the proliferative capacity of tumor cells and, as a consequence, it could represent an attractive target for new anti-cancer therapies. In this study, we generated a hammerhead ribozyme composed of a catalytic domain with flanking sequences complementary to the RNA component of human telomerase and designed to cleave specifically a site located at the end of the telomerase template sequence. In vitro the ribozyme induced cleavage of a synthetic RNA substrate obtained by cloning a portion of the RNA component of human telomerase. The extent of cleavage was dependent on the ribozyme/substrate ratio as well as the Mg2+ concentration. Moreover, when added to cell extracts from two human melanoma cell lines (JR8 and M14), or three melanoma surgical specimens, the ribozyme inhibited telomerase activity in a concentration- and time-dependent manner. When the ribozyme was delivered to growing JR8 melanoma cells by (N-(1-(2,3 dioleoxyloxy)propil)-N,N,N trimethylammonium methylsulfate-mediated transfer, a marked inhibition of telomerase activity was observed. Next, the ribozyme sequence was cloned in an expression vector and JR8 cells were transfected with it. The cell clones obtained showed a reduced telomerase activity and telomerase RNA levels and expressed the ribozyme. Moreover, ribozyme transfectants had significantly longer doubling times than control cells and showed a dendritic appearance in monolayer culture. No telomere shortening, however, was observed in these clones. Overall, our results indicate that the hammerhead ribozyme is a potentially useful tool for the inactivation of telomerase in human tumors.

Telomeres, telomerase, and myc. An update

Normal human somatic cells have a finite life span in vivo as well as in vitro and retire into senescence after a predictable time. Cellular senescence is triggered by the activation of two interdependent mechanisms. One induces irreversible cell cycle exit involving activation of two tumorsuppressor genes, p53 and pRb, and the proper time point is indicated by a critical shortening of chromosomal ends due to the end-replication problem of DNA synthesis. The development of a malignant cancer cell is only possible when both mechanisms are circumvented. The majority of human cancers and tumor cell lines produce telomerase, a ribonucleoprotein with two components required for core enzyme activity: telomerase RNA (TR) and a telomerase reverse transcriptase protein (TERT). Telomerase adds hexameric DNA repeats (TTAGGG) to telomeric ends and thus compensates the progressive loss of telomeric sequences inherent to DNA replication. While TR of telomerase is present in almost all human cells, human TERT (hTERT) was found rate limiting for telomerase activity. Ectopic expression of hTERT in otherwise mortal human cells induced efficient elongation of telomeres and permanent cell growth. While hTERT-mediated immortalization seems to have no effect on growth potential and cell cycle check points, it bestows an increased susceptibility to experimental transformation. One oncogene that might activate TERT in the natural context is c-myc. Myc genes are frequently deregulated in human tumors and myc overexpression may cause telomerase reactivation and telomere stabilization which, in turn, would allow permanent proliferation. Is this a general strategy of incipient cancer cells to escape senescence? Several recent observations indicate that other scenarios may be conceived as well.

Studies of the molecular mechanisms in the regulation of telomerase activity

Telomerase, a specialized RNA-directed DNA polymerase that extends telomeres of eukaryotic chromosomes, is repressed in normal human somatic cells but is activated during development and upon neoplasia. Whereas activation is involved in immortalization of neoplastic cells, repression of telomerase permits consecutive shortening of telomeres in a chromosome replication-dependent fashion. This cell cycle-dependent, unidirectional catabolism of telomeres constitutes a mechanism for cells to record the extent of DNA loss and cell division number; when telomeres become critically short, the cells terminate chromosome replication and enter cellular senescence. Although neither the telomere signaling mechanisms nor the mechanisms whereby telomerase is repressed in normal cells and activated in neoplastic cells have been established, inhibition of telomerase has been shown to compromise the growth of cancer cells in culture; conversely, forced expression of the enzyme in senescent human cells extends their life span to one typical of young cells. Thus, to switch telomerase on and off has potentially important implications in anti-aging and anti-cancer therapy. There is abundant evidence that the regulation of telomerase is multifactorial in mammalian cells, involving telomerase gene expression, post-translational protein-protein interactions, and protein phosphorylation. Several proto-oncogenes and tumor suppressor genes have been implicated in the regulation of telomerase activity, both directly and indirectly; these include c-Myc, Bcl-2, p21(WAF1), Rb, p53, PKC, Akt/PKB, and protein phosphatase 2A. These findings are evidence for the complexity of telomerase control mechanisms and constitute a point of departure for piecing together an integrated picture of telomerase structure, function, and regulation in aging and tumor development-Liu, J.-P. Studies of the molecular mechanisms in the regulation of telomerase activity.

Identification of an essential proximal sequence element in the promoter of the telomerase RNA gene of Tetrahymena thermophila

Telomerase is a ribonucleoprotein reverse transcriptase that synthesizes and maintains telomeric DNA. Studies of telomeres and telomerase are facilitated by the large number of linear DNA molecules found in ciliated protozoa, such as Tetrahymena thermophila. To examine the expression of telomerase, we investigated the transcription of the RNA polymerase III-directed gene encoding the RNA subunit (TER1) of this enzyme. A chimeric gene containing the Glaucoma chattoni TER1 transcribed region flanked by 5' and 3' Tetrahymena regions was used to identify promoter elements following transformation of Tetrahymena cells. Disruption of a conserved proximal sequence element (PSE) located at -55 in the Tetrahymena TER1 5' flanking region eliminated expression of the chimeric gene. In addition, mutation of an A/T-rich element at -25 decreased expression markedly. A gel mobility shift assay and protein-DNA cross-linking identified a PSE-binding polypeptide of 50-60 kDa in Tetrahymena extracts. Gel filtration analysis revealed a native molecular mass of approximately 160 kDa for this binding activity. Our results point to a similar architecture between ciliate telomerase RNA and metazoan U6 small nuclear RNA promoters.

Telomerase RNA function in recombinant Tetrahymena telomerase

Telomerase is a ribonucleoprotein reverse transcriptase specialized for use of a sequence within its integral RNA component as the template for DNA synthesis. Telomerase adds telomeric simple sequence repeats to single-stranded primers in vitro or chromosome ends in vivo. We have investigated the sequences and structures of recombinant Tetrahymena thermophila telomerase RNA necessary for physical association and activity with the catalytic protein subunit expressed in rabbit reticulocyte lysate. In contrast with previous results using another reconstitution method, we find that phylogenetically conserved primary sequences and a phylogenetically nonconserved secondary structure are essential for telomerase RNA function. Telomerase RNA binding to the catalytic protein subunit requires sequences 5' of the template and is highly sequence specific. Other telomerase RNA sequences are required for enzyme activity and proper template use but not for protein interaction affinity. In addition, we demonstrate that the production of active recombinant telomerase requires a factor in rabbit reticulocyte lysate that promotes ribonucleoprotein assembly. These studies demonstrate multiple functions for the telomerase RNA and indicate that recombinant telomerase activity requires more than the catalytic protein and RNA components of the enzyme that have been identified to date.

Telomerase in kinetoplastid parasitic protozoa

We have identified telomerase activity in extracts of three evolutionarily diverse kinetoplastid species: Trypanosoma brucei, Leishmania major, and Leishmania tarentolae. Telomerase activity was initially detected in extracts from insect form cells of all three kinetoplastid species by using a modification of the one-tube telomere repeat amplification protocol [Kim, N., et al. (1994) Science 266, 2011-2015], although better results were subsequently achieved with the two-tube telomere repeat amplification protocol [Autexier, C., Pruzan, R., Funk, W. & Greider, C. (1996) EMBO J. 15, 5928-5935]. The activity in T. brucei extracts was sufficiently robust to enable its detection in a direct assay of telomerase; enzyme processivity was found to be relatively low. The in vitro properties of telomerase suggest a possible templating domain sequence for the telomerase RNA of T. brucei. Telomerase activity is likely to contribute to telomere maintenance in these parasitic organisms and provides a new target for chemotherapeutic intervention.

Telomerases

Telomerases are RNA-dependent polymerases that catalyse the synthesis of the telomeric DNA at the tips of eukaryotic chromosomes. The recent identification of the catalytic subunit of telomerases from several different species suggests that the core of the telomerase is conserved. The proposed sequence and structural homology between the telomerase catalytic subunit and reverse transcriptases, together with a wealth of genetic and biochemical information, has led to significant advances in our understanding of the mechanism by which telomerases synthesise telomeric DNA.

Specific telomerase RNA residues distant from the template are essential for telomerase function

The reverse transcriptase telomerase is a ribonucleoprotein complex that adds telomeric repeats to chromosome ends, using a sequence within its endogenous RNA component as a template. Although templating domains of telomerase RNA have been studied in detail, little is known about the roles of the remaining residues, particularly in yeast. We examined the functions of nontemplate telomerase residues in the telomerase RNA of budding yeast Kluyveromyces lactis. Although approximately half of the RNA residues were dispensable for function, four specific regions were essential for telomerase action in vivo. We analyzed the effects of mutating these regions on in vivo function, in vitro telomerase activity, and telomerase RNP assembly. Deletion of two regions resulted in synthesis of stable RNAs that appeared unable to assemble into a stable RNP. Mutating a region near the 5' end of the RNA allowed RNP assembly but abolished enzymatic activity. Mutations in another specific small region of the RNA led to an inactive telomerase RNP with an altered RNA conformation.

RNA-dependent activation of primer RNA production by influenza virus polymerase: different regions of the same protein subunit constitute the two required RNA-binding sites

The capped RNA primers required for the initiation of influenza virus mRNA synthesis are produced by the viral polymerase itself, which consists of three proteins PB1, PB2 and PA. Production of primers is activated only when the 5'- and 3'-terminal sequences of virion RNA (vRNA) bind sequentially to the polymerase, indicating that vRNA molecules function not only as templates for mRNA synthesis but also as essential cofactors which activate catalytic functions. Using thio U-substituted RNA and UV crosslinking, we demonstrate that the 5' and 3' sequences of vRNA bind to different amino acid sequences in the same protein subunit, the PB1 protein. Mutagenesis experiments proved that these two amino acid sequences constitute the functional RNA-binding sites. The 5' sequence of vRNA binds to an amino acid sequence centered around two arginine residues at positions 571 and 572, causing an allosteric alteration which activates two new functions of the polymerase complex. In addition to the PB2 protein subunit acquiring the ability to bind 5'-capped ends of RNAs, the PB1 protein itself acquires the ability to bind the 3' sequence of vRNA, via a ribonucleoprotein 1 (RNP1)-like motif, amino acids 249-256, which contains two phenylalanine residues required for binding. Binding to this site induces a second allosteric alteration which results in the activation of the endonuclease that produces the capped RNA primers needed for mRNA synthesis. Hence, the PB1 protein plays a central role in the catalytic activity of the viral polymerase, not only in the catalysis of RNA-chain elongation but also in the activation of the enzyme activities that produce capped RNA primers.

Expression of mouse telomerase catalytic subunit in embryos and adult tissues

Telomerase is a ribonucleoprotein complex that elongates telomeres, allowing the stable maintenance of chromosomes during multiple cell divisions. Here, we describe the isolation and characterization of the catalytic subunit of mouse telomerase, mTERT (mouse telomerase reverse transcriptase), an essential protein component of the telomerase complex. During embryonic development, mTERT mRNA is abundantly expressed in the whole embryo, especially in regions of intense proliferation. We found that the mTERT mRNA expression in both embryonic and adult tissues is independent of the essential RNA component of telomerase, mTR, and therefore, of the formation of active telomerase complexes. mTERT protein is present exclusively in tissues with telomerase activity, such as testis, spleen, and thymus. mTERT protein is barely detectable in the thymus of mTR-/- mice, suggesting that mTERT protein stability in this tissue may depend on the actual assembly of active telomerase complexes. Finally, we found that mouse and human telomerase catalytic subunit is located in the cell nucleus, and its localization is not regulated during cell cycle progression.

The telomere and telomerase: how do they interact?

The tandemly repeated DNA sequence of telomeres is typically specified by the ribonucleoprotein enzyme telomerase. Telomerase copies part of its intrinsic RNA moiety to make one strand of the telomeric repeat DNA. Recent work has led to the concept of a telomere homeostasis system. We have been studying two key physical components of this system: the telomere itself and telomerase. Mutating the template sequence of telomerase RNA caused various phenotypes: (1) mutating specific residues in the ciliate Tetrahymena and two yeasts showed that they are required for critical aspects of telomerase action; (2) certain mutated telomeric sequences caused a previously unreported phenotype, i.e. a strong anaphase block in Tetrahymena micronuclei; and (3) certain template mutations in the telomerase RNA gene of the yeast Kluyveromyces lactis led to unregulated telomere elongation, which in some cases was directly related to loss of binding to K. lactis Rap1p. Using K. lactis carrying alterations in the genes for Rap1p and other silencing components, we proposed a general model for telomere length homeostasis: namely, that the structure and DNA length of the DNA-protein complex that comprises the telomere are key determinants of telomerase access, and hence the frequency of action of telomerase, at the telomere.

Flexible positioning of the telomerase-associated nuclease leads to preferential elimination of nontelomeric DNA

In addition to a reverse transcriptase activity, telomerase is associated with a DNA endonuclease that removes nucleotides from a primer 3' terminus prior to telomere repeat addition. Here we examine the DNA specificity of the primer cleavage-elongation reaction carried out by the Euplotes crassus telomerase. We show that the primer cleavage activity copurified with the E. crassus telomerase polymerase, indicating that it either is an intrinsic property of telomerase or is catalyzed by a tightly associated factor. Using chimeric primers containing stretches of telomeric DNA that could be precisely positioned on the RNA template, we found that the cleavage site is more flexible than originally proposed. Primers harboring mismatches in dT tracts that aligned opposite nucleotides 37 to 40 in the RNA template were cleaved to eliminate the mismatched residues along with the adjacent 3' sequence. The cleaved product was then elongated to generate perfect telomeric repeats. Mismatches in dG tracts were not removed, implying that the nuclease does not track coordinately with the polymerase active site. Our data indicate that the telomerase-associated nuclease could provide a rudimentary proofreading function in telomere synthesis by eliminating mismatches between the DNA primer and the 5' region of the telomerase RNA template.

Mutational analysis of the Tetrahymena telomerase RNA: identification of residues affecting telomerase activity in vitro

Telomere-specific repeat sequences are essential for chromosome end stability. Telomerase maintains telomere length by adding sequences de novo onto chromosome ends. The template domain of the telomerase RNA component dictates synthesis of species-specific telomeric repeats and other regions of the RNA have been suggested to be important for enzyme structure and/or catalysis. Using enzyme reconstituted in vitro with RNAs containing deletions or substitutions we identified nucleotides in the RNA component that are important for telomerase activity. Although many changes to conserved features in the RNA secondary structure did not abolish enzyme activity, levels of activity were often greatly reduced, suggesting that regions other than the template play a role in telomerase function. The template boundary was only altered by changes in stem II that affected the conserved region upstream of the template, not by changes in other regions, such as stems I, III and IV, consistent with a role of the conserved region in defining the 5' boundary of the template. Surprisingly, telomerase RNAs with substitutions or deletion of residues potentially abolishing the conserved pseudoknot structure had wild-type levels of telomerase activity. This suggests that this base pairing interaction may not be required for telomerase activity per se but may be conserved as a regulatory site for the enzyme in vivo.