Molecular cloning and characterization of AtTERT, a telomerase reverse transcriptase homolog in Arabidopsis thaliana

Characterisation of the Arabidopsis thaliana telomerase TERT-TR complex

Most eukaryotic organisms employ a telomerase complex for the maintenance of chromosome ends. The core of this complex is composed of telomerase reverse transcriptase (TERT) and telomerase RNA (TR) subunits. The TERT reverse transcriptase (RT) domain synthesises telomeric DNA using the TR template sequence. The other TERT domains contribute to this process in different ways. In particular, the TERT RNA-binding domain (TRBD) interacts with specific TR motif(s). Using a yeast 3-hybrid system, we show the critical role of Arabidopsis thaliana (At) TRBD and embryophyta-conserved KRxR motif in the unstructured linker preceding the TRBD domain for binding to the recently identified AtTR subunit. We also show the essential role of the predicted P4 stem and pseudoknot AtTR structures and provide evidence for the binding of AtTRBD to pseudoknot and KRxR motif stabilising interaction with the P4 stem structure. Our results thus provide the first insight into the core part of the plant telomerase complex.

Analysis of MsTERT Gene Expression Profile in Alfalfa (Medicago sativa) Indicates Their Response to Abiotic Stress and Seed Aging

Seed aging is always taken as a crucial factor for vigor loss due to delayed seed germination and seedling growth, which limits hay production. Many studies have found that telomeres are closely related to abiotic stress and seed vigor. However, the molecular mechanism of telomeres' response to abiotic stress, seed vigor, and the maintenance mechanism of plant telomere homeostasis still remain unclear. Alfalfa (Medicago sativa) enjoys the title of "King of Forage", and is an important protein forage for the dairy industry as planted in the world. This comprehensive investigation was performed to explore the molecular characterization, phylogenetic relationship, and gene expression analysis of MsTERT under abiotic stress and during seed aging in alfalfa. In this study, MsTERT was identified from the 'Zhongmu 1' alfalfa genome and encoded a coding sequence (CDS) of 3615 bp in length, consisting of telomerase- RNA-Binding Domain (RBD) and Reverse Transcriptase (RT) domains, 1024 amino acids, an isoelectric point of 9.58, and a relative molecular mass of 138.94 kD. Subcellular localization showed that MsTERT was mainly localized in the nucleus and mitochondria. The results of the expression profile showed that MsTERT was observed to respond to various stress conditions such as salt (100 mmol/L NaCl) and drought (20% PEG 6000). Furthermore, exogenous hormones IAA, ABA, and GA3 showed the potential to affect MsTERT expression. Additionally, MsTERT also responded to seed aging. Our results revealed a marginal but significant association between relative telomere length, MsTERT expression, and seed germination percentage, suggesting that the length of telomeres was shortened, and expression of MsTERT decreased with alfalfa seed aged. These results provide some evidence for the hypothesis of relative telomere length and/or TERT expression serving as biomarkers of seed aging. Although this finding is helpful to offer a new way to elucidate the molecular mechanism of vigor loss in alfalfa seed, further investigation is required to elucidate the molecular mechanism by which the MsTERT gene regulates seed vigor.

Origin and Fates of TERT Gene Copies in Polyploid Plants

The gene coding for the telomerase reverse transcriptase (TERT) is essential for the maintenance of telomeres. Previously we described the presence of three TERT paralogs in the allotetraploid plant Nicotiana tabacum, while a single TERT copy was identified in the paleopolyploid model plant Arabidopsis thaliana. Here we examine the presence, origin and functional status of TERT variants in allotetraploid Nicotiana species of diverse evolutionary ages and their parental genome donors, as well as in other diploid and polyploid plant species. A combination of experimental and in silico bottom-up analyses of TERT gene copies in Nicotiana polyploids revealed various patterns of retention or loss of parental TERT variants and divergence in their functions. RT-qPCR results confirmed the expression of all the identified TERT variants. In representative plant and green algal genomes, our synteny analyses show that their TERT genes were located in a conserved locus that became advantageous after the divergence of eudicots, and the gene was later translocated in several plant groups. In various diploid and polyploid species, translocation of TERT became fixed in target loci that show ancient synapomorphy.

Relative Telomere Length and Telomerase Reverse Transcriptase (TERT) Expression Are Associated with Age in Almond (Prunus dulcis [Mill.] D.A.Webb)

While all organisms age, our understanding of how aging occurs varies among species. The aging process in perennial plants is not well-defined, yet can have implications on production and yield of valuable fruit and nut crops. Almond exhibits an age-related disorder known as non-infectious bud failure (BF) that affects vegetative bud development, indirectly affecting kernel yield. This species and disorder present an opportunity to address aging in a commercially relevant and vegetatively propagated perennial crop. The hypothesis tested in this study was that relative telomere length and/or telomerase reverse transcriptase (TERT) expression can serve as biomarkers of aging in almond. Relative telomere lengths and expression of TERT, a subunit of the enzyme telomerase, were measured via qPCR methods using bud and leaf samples collected from distinct age cohorts over a two-year period. Results from this work show a marginal but significant association between both relative telomere length and TERT expression, and age, suggesting that as almonds age, telomeres shorten and TERT expression decreases. This work provides information on potential biomarkers of perennial plant aging, contributing to our knowledge of this process. In addition, these results provide opportunities to address BF in almond breeding and nursery propagation.

Telomerase RNAs in land plants

To elucidate the molecular nature of evolutionary changes of telomeres in the plant order Asparagales, we aimed to characterize telomerase RNA subunits (TRs) in these plants. The unusually long telomere repeat unit in Allium plants (12 nt) allowed us to identify TRs in transcriptomic data of representative species of the Allium genus. Orthologous TRs were then identified in Asparagales plants harbouring telomere DNA composed of TTAGGG (human type) or TTTAGGG (Arabidopsis-type) repeats. Further, we identified TRs across the land plant phylogeny, including common model plants, crop plants, and plants with unusual telomeres. Several lines of functional testing demonstrate the templating telomerase function of the identified TRs and disprove a functionality of the only previously reported plant telomerase RNA in Arabidopsis thaliana. Importantly, our results change the existing paradigm in plant telomere biology which has been based on the existence of a relatively conserved telomerase reverse transcriptase subunit (TERT) associating with highly divergent TRs even between closely related plant taxa. The finding of a monophyletic origin of genuine TRs across land plants opens the possibility to identify TRs directly in transcriptomic or genomic data and/or predict telomere sequences synthesized according to the respective TR template region.

Molecular characterization of Chlamydomonas reinhardtii telomeres and telomerase mutants

This study characterizes the sequence, end structure, and length distribution of Chlamydomonas reinhardtii telomeres and shows that telomerase mutants are defective in telomere maintenance.

Telomeres are repeated sequences found at the end of the linear chromosomes of most eukaryotes and are required for chromosome integrity. Expression of the reverse-transcriptase telomerase allows for extension of telomeric repeats to counteract natural telomere shortening. Although Chlamydomonas reinhardtii, a photosynthetic unicellular green alga, is widely used as a model organism in photosynthesis and flagella research, and for biotechnological applications, the biology of its telomeres has not been investigated in depth. Here, we show that the C. reinhardtii (TTTTAGGG)_n telomeric repeats are mostly nondegenerate and that the telomeres form a protective structure, with a subset ending with a 3′ overhang and another subset presenting a blunt end. Although telomere size and length distributions are stable under various standard growth conditions, they vary substantially between 12 genetically close reference strains. Finally, we identify CrTERT, the gene encoding the catalytic subunit of telomerase and show that telomeres shorten progressively in mutants of this gene. Telomerase mutants eventually enter replicative senescence, demonstrating that telomerase is required for long-term maintenance of telomeres in C. reinhardtii.

The region upstream of the telomerase reverse transcriptase gene is essential for in planta telomerase complementation

Telomerase is essential for the maintenance of telomeres, structures located at the ends of linear eukaryotic chromosomes that are crucial for genomic stability. Telomerase has been frequently explored in mammals because of its activity in many types of cancers, but knowledge in plants is rather sketchy despite plants representing useful models due to peculiarities in their telomeres and telomerase biology. We studied in planta complementation of telomerase in Arabidopsis thaliana mutant plants with disrupted expression of the gene encoding the telomerase protein subunit (AtTERT) and significantly shortened telomeres. We found that the upstream region of AtTERT, previously identified as a putative minimal promoter, was essential for reconstitution of telomerase function, as demonstrated by the full or partial recovery of the telomere phenotype in mutants. In contrast, transformation by the full length AtTERT gene construct resulted in more progressive telomere shortening in mutants and even in wild type plants, despite the high level of AtTERT transcript and telomerase activity detected by in vitro assay. Thus, the telomerase protein subunit putative promoter is essential for in planta telomerase reconstitution and restoration of its catalytical activity. Contributions from other factors, including those tissue-specific, for proper telomerase function are discussed.

Telomeres in Plants and Humans: Not So Different, Not So Similar

Parallel research on multiple model organisms shows that while some principles of telomere biology are conserved among all eukaryotic kingdoms, we also find some deviations that reflect different evolutionary paths and life strategies, which may have diversified after the establishment of telomerase as a primary mechanism for telomere maintenance. Much more than animals, plants have to cope with environmental stressors, including genotoxic factors, due to their sessile lifestyle. This is, in principle, made possible by an increased capacity and efficiency of the molecular systems ensuring maintenance of genome stability, as well as a higher tolerance to genome instability. Furthermore, plant ontogenesis differs from that of animals in which tissue differentiation and telomerase silencing occur during early embryonic development, and the “telomere clock” in somatic cells may act as a preventive measure against carcinogenesis. This does not happen in plants, where growth and ontogenesis occur through the serial division of apical meristems consisting of a small group of stem cells that generate a linear series of cells, which differentiate into an array of cell types that make a shoot and root. Flowers, as generative plant organs, initiate from the shoot apical meristem in mature plants which is incompatible with the human-like developmental telomere shortening. In this review, we discuss differences between human and plant telomere biology and the implications for aging, genome stability, and cell and organism survival. In particular, we provide a comprehensive comparative overview of telomere proteins acting in humans and in Arabidopsis thaliana model plant, and discuss distinct epigenetic features of telomeric chromatin in these species.

Tandem affinity purification of AtTERT reveals putative interaction partners of plant telomerase in vivo

The life cycle of telomerase involves dynamic and complex interactions between proteins within multiple macromolecular networks. Elucidation of these associations is a key to understanding the regulation of telomerase under diverse physiological and pathological conditions from telomerase biogenesis, through telomere recruitment and elongation, to its non-canonical activities outside of telomeres. We used tandem affinity purification coupled to mass spectrometry to build an interactome of the telomerase catalytic subunit AtTERT, using Arabidopsis thaliana suspension cultures. We then examined interactions occurring at the AtTERT N-terminus, which is thought to fold into a discrete domain connected to the rest of the molecule via a flexible linker. Bioinformatic analyses revealed that interaction partners of AtTERT have a range of molecular functions, a subset of which is specific to the network around its N-terminus. A significant number of proteins co-purifying with the N-terminal constructs have been implicated in cell cycle and developmental processes, as would be expected of bona fide regulatory interactions and we have confirmed experimentally the direct nature of selected interactions. To examine AtTERT protein-protein interactions from another perspective, we also analysed AtTERT interdomain contacts to test potential dimerization of AtTERT. In total, our results provide an insight into the composition and architecture of the plant telomerase complex and this will aid in delineating molecular mechanisms of telomerase functions.

Molecular cloning and measurement of telomerase reverse transcriptase (TERT) transcription patterns in tissues of European hake (Merluccius merluccius) and Atlantic cod (Gadus morhua) during aging

Telomerase is a reverse transcriptase ribonucleoprotein that maintains the ends of linear chromosomes. This enzyme plays a major role in cell processes like proliferation, differentiation and tumorigenesis, being associated with aging and survival of species. In this study, the gene coding for TERT (Telomerase Reverse Transcriptase) of two commercial fish species, European hake (Merluccius merluccius) and Atlantic cod (Gadus morhua), has been partially cloned. A fragment of 1581bp (hake) and 633bp (cod) showed high homology (identity 74%, query cover 99%, E-value=0) with known Perciformes TERT sequences. TERT transcription patterns were assessed by qRT-PCR in different tissues of hake (brain, ovary, testis, muscle, skin, gills, liver and kidney) and cod (brain, muscle and skin) of different sizes/ages in order to understand its role in the physiological aging of teleosts. TERT was found to be ubiquitously transcribed in all tissues and size/age groups studied in both species. Significantly higher relative transcription levels (p<0.05) were found with increasing size/age of M. merluccius in the kidney, muscle, skin and gonad, the latter exhibiting particularly high relative transcription levels. Male hakes showed higher TERT relative transcription levels in the brain, gonad and liver than females, although these differences were not statistically significant (p<0.05). In G. morhua, higher TERT relative transcription levels were recorded in the muscle and brain of fry and juvenile individuals. Therefore, TERT relative transcription pattern exhibited a higher telomerase demand in early developmental stages and also in mature stages, suggesting tissue renewal or regeneration processes as a conserved mechanism for maintaining long-term cell proliferation capacity and preventing senescence. Thus, it can be concluded that TERT relative transcription level was species and tissue specific and changed with the age of fishes.

Structure-function relationships during transgenic telomerase expression in Arabidopsis

Although telomerase (EC 2.7.7.49) is important for genome stability and totipotency of plant cells, the principles of its regulation are not well understood. Therefore, we studied subcellular localization and function of the full-length and truncated variants of the catalytic subunit of Arabidopsis thaliana telomerase, AtTERT, in planta. Our results show that multiple sites in AtTERT may serve as nuclear localization signals, as all the studied individual domains of the AtTERT were targeted to the nucleus and/or the nucleolus. Although the introduced genomic or cDNA AtTERT transgenes display expression at transcript and protein levels, they are not able to fully complement the lack of telomerase functions in tert -/- mutants. The failure to reconstitute telomerase function in planta suggests a more complex telomerase regulation in plant cells than would be expected based on results of similar experiments in mammalian model systems.

Developmental silencing of the AtTERT gene is associated with increased H3K27me3 loading and maintenance of its euchromatic environment

Telomerase, an enzyme responsible for the maintenance of linear chromosome ends, is precisely regulated during plant development. In animals, involvement of the epigenetic state of the telomerase reverse transcriptase (TERT) gene in the complex regulation of telomerase activity has been reported. To reveal whether epigenetic mechanisms participate in the regulation of plant telomerase, the relationship between telomerase activity in tissues of Arabidopsis thaliana and DNA methylation and histone modifications in the A. thaliana TERT (AtTERT) upstream region was studied. As expected, a gradual decrease of telomerase activity during leaf maturation was observed. A different pattern with a more progressive loss of telomerase activity and AtTERT transcription during leaf development was revealed in MET1 gene-knockout mutants. Analysis of DNA methylation in the AtTERT upstream region showed low levels of methylated cytosines without notable differences between telomerase-positive and telomerase-negative wild-type tissues. Surprisingly, a high level of CG methylation was found in the AtTERT coding region, although this type of methylation is a characteristic attribute of constitutively expressed genes. Analysis of chromatin modifications in the AtTERT upstream region and in exon 5 showed increased loading of the H3K27me3 mark in the telomerase-negative mature leaf compared to telomerase-positive seedlings, whereas H3K4me3, H3K9Ac, and H3K9me2 were approximately at the same level. Consistently, the chromatin structure of the AtTERT gene was maintained. These results are discussed in the context of the general involvement of epigenetic mechanisms in the regulation of gene expression and with respect to similar studies performed in animal models.

Insights into the evolution of mammalian telomerase: platypus TERT shares similarities with genes of birds and other reptiles and localizes on sex chromosomes

Background

The TERT gene encodes the catalytic subunit of the telomerase complex and is responsible for maintaining telomere length. Vertebrate telomerase has been studied in eutherian mammals, fish, and the chicken, but less attention has been paid to other vertebrates. The platypus occupies an important evolutionary position, providing unique insight into the evolution of mammalian genes. We report the cloning of a platypus TERT (OanTERT) ortholog, and provide a comparison with genes of other vertebrates.

Results

The OanTERT encodes a protein with a high sequence similarity to marsupial TERT and avian TERT. Like the TERT of sauropsids and marsupials, as well as that of sharks and echinoderms, OanTERT contains extended variable linkers in the N-terminal region suggesting that they were present already in basal vertebrates and lost independently in ray-finned fish and eutherian mammals. Several alternatively spliced OanTERT variants structurally similar to avian TERT variants were identified. Telomerase activity is expressed in all platypus tissues like that of cold-blooded animals and murine rodents. OanTERT was localized on pseudoautosomal regions of sex chromosomes X3/Y2, expanding the homology between human chromosome 5 and platypus sex chromosomes. Synteny analysis suggests that TERT co-localized with sex-linked genes in the last common mammalian ancestor. Interestingly, female platypuses express higher levels of telomerase in heart and liver tissues than do males.

Conclusions

OanTERT shares many features with TERT of the reptilian outgroup, suggesting that OanTERT represents the ancestral mammalian TERT. Features specific to TERT of eutherian mammals have, therefore, evolved more recently after the divergence of monotremes.

Molecular analysis of T-DNA insertion mutants identified putative regulatory elements in the AtTERT gene

Analysis of plants bearing a T-DNA insertion is a potent tool of modern molecular biology, providing valuable information about the function and involvement of genes in metabolic pathways. A collection of 12 Arabidopsis thaliana lines with T-DNA insertions in the gene coding for the catalytic subunit of telomerase (AtTERT) and in adjacent regions was screened for telomerase activity [telomere repeat amplification protocol (TRAP) assay], telomere length (terminal restriction fragments), and AtTERT transcription (quantitative reverse transcription-PCR). Lines with the insertion located upstream of the start codon displayed unchanged telomere stability and telomerase activity, defining a putative minimal AtTERT promoter and the presence of a regulatory element linked to increased transcription in the line SALK_048471. Lines bearing a T-DNA insertion inside the protein-coding region showed telomere shortening and lack of telomerase activity. Transcription in most of these lines was unchanged upstream of the T-DNA insertion, while it was notably decreased downstream. The expression profile varied markedly in mutant lines harbouring insertions at the 5' end of AtTERT which showed increased transcription and abolished tissue specificity. Moreover, the line FLAG_385G01 (T-DNA insertion inside intron 1) revealed the presence of a highly abundant downstream transcript with normal splicing but without active telomerase. The role of regulatory elements found along the AtTERT gene is discussed in respect to natural telomerase expression and putative intron-mediated enhancement.

Cloning and characterization of telomerase reverse transcriptase gene in Trichinella spiralis

The telomerase reverse transcriptase (TERT) is primarily known for its ability to elongate telomeres for maintaining chromosomal integrity and delaying cellular senescence. It plays an important role in cell proliferation, differentiation, tumorigenesis, and aging. Telomerase includes two core components-an internal RNA moiety acting as a template of DNA extension and a catalytic subunit (TERT) which provides catalytic activity. Here, we described the cloning, sequence, and characterization of the TERT gene from Trichinella spiralis (T. spiralis). The prediction results of amino acid sequence showed that it possessed all the motifs characteristics of the TERT family members. T. spiralis TERT (Ts_TERT) cDNA contains an open reading frame encoding a protein with 1,201 amino acids with moleculer mass of 139 kDa and isoelectric point of 9.673, and the protein contains the conserved reverse transcriptase motifs 1, 2, A, B, C, D, and E, as well as the TERT-specific T motifs and the N-terminal conserved motifs GQ, CP, and QFP. While RT-PCR analysis indicates that TERT mRNA is expressed in T. spiralis adult worm, newborn larvae, and muscle larvae.

Season- and age-associated telomerase activity in Ginkgo biloba L

Telomeres have lately received considerable attention in the development of broad-leaved tree species. In order to determine tissue-, sex-, season- and age-specific changes in telomerase activity in ginkgo trees, analyses of the telomerase repeat amplification protocol were carried out. In all of the tissues detected (embryonal callus, microspore tissues and leaves) telomerase activity was found, with differences between these activities statistically significant (P < 0.05). The highest telomerase activity was found in embryonal callus, suggesting that ginkgo trees have tissue-specific telomerase activity. Tissues containing high levels of dividing cells also have high levels of telomerase activity. No significant difference of telomerase activity was found between male and female trees (P > 0.05). In the annual development cycle, the highest telomerase activity was found in April and a decreasing trend over time in the four age groups studied: 10, 20, 70 and 700 year. The most obvious decline appeared in trees of the 700 year old group, suggesting that ginkgo trees have season-specific telomerase activities and trees of various ages react differently to seasonal changes. The mean annual telomerase activity showed a regular decreasing trend in all leaf samples analyzed from 10 to 700 year old ginkgo trees. We conclude that maintenance of telomere length depends on season- and age- associated telomerase activity. An optimal telomere length is regulated and maintained by telomerase in Ginkgo biloba L.

Comparative biology of telomeres: where plants stand

Telomeres are essential structures at the ends of eukaryotic chromosomes. Work on their structure and function began almost 70 years ago in plants and flies, continued through the Nobel Prize winning work on yeast and ciliates, and goes on today in many model and non-model organisms. The basic molecular mechanisms of telomeres are highly conserved throughout evolution, and our current understanding of how telomeres function is a conglomeration of insights gained from many different species. This review will compare the current knowledge of telomeres in plants with other organisms, with special focus on the functional length of telomeric DNA, the search for TRF homologs, the family of POT1 proteins, and the recent discovery of members of the CST complex.

Eimeria tenella: cloning and characterization of telomerase reverse transcriptase gene

Telomerase is essential for maintaining telomere length and chromosome stability in most eukaryotic organisms. The telomerase ribonucleoprotein complex consists of two essential components, the intrinsic telomerase RNA and the catalytic telomerase reverse transcriptase protein. Here we describe the cloning, sequencing and characterization of the telomerase reverse transcriptase catalytic subunit from the apicomplexan protozoon Eimeria tenella. The amino acid sequence predicted from it has all the signature motifs of the TERT family members. The E. tenella TERT cDNA contains an open reading frame encoding a protein with 1497 amino acids predicted size of 172kDa and isoelectric point of 9.344. It contains the conserved reverse transcriptase motifs 1, 2, A, B, C, D and E as well as the TERT-specific T motif and the N-terminal conserved motifs GQ, CP and QFP. RT-PCR analysis indicated that E. tenella TERT mRNA expressed in sporozoites and merozoites phase while not in unsporulated oocysts. At the same time, the result of TRAP assay indicated that marked telomerase activity were detected in sporozoites and merozoites. In contrast, telomerase activity was not detected in unsporulated oocysts.

Arabidopsis replication protein A 70a is required for DNA damage response and telomere length homeostasis

Replication protein A1 (RPA1/RPA70) forms a heterotrimeric complex together with RPA2/RPA32 and RPA3/RPA14 subunits which plays essential roles in various aspects of DNA metabolism including replication, repair, recombination and telomere maintenance. Compared with RPA70 in yeast and mammals, limited information is available about the factor in plants. In this study, we analyzed the functions of AtRPA70a, which is most similar to human RPA70 among four paralogs in Arabidopsis thaliana. RNA blot analysis showed that AtRPA70a is expressed ubiquitously in plant organs containing differentiated and meristematic tissues, while its expression was up-regulated in response to DNA damage stress. Yeast two-hybrid and co-immunoprecipitation analyses showed that AtRPA70a interacted preferentially with Arabidopsis RPA32a, one of two paralogs. Inactivation of AtRPA70a by T-DNA insertion did not affect growth under normal conditions, but resulted in increased sensitivity to genotoxic agents such as methylmethane sulfonate, bleomycin and hydroxyurea. Terminal restriction fragment analysis revealed that telomere lengths in an AtRPA70a-deficient line were significantly larger than in the wild type, whereas those in the mutant expressing antisense AtTERT (telomerase catalytic subunit gene) were shortened during successive generations. These results demonstrate that AtRPA70a is involved in repair of double-strand DNA breaks and possibly contributes to telomerase-dependent telomere length regulation.

Arabidopsis POT1A interacts with TERT-V(I8), an N-terminal splicing variant of telomerase

Chromosome integrity is maintained via the actions of ribonucleoprotein complexes that can add telomeric repeats or can protect the chromosome end from being degraded. POT1 (protection of telomeres 1), a class of single-stranded-DNA-binding proteins, is a regulator of telomeric length. The Arabidopsis genome contains three POT1 homologues: POT1A, POT1B and POT1C. Using yeast two-hybrid assays to identify components of a potential POT1A complex, we retrieved three interactors: the N-terminus of the telomerase, a protein kinase and a plant-specific protein. Further analysis of the interaction of POT1 proteins with telomerase showed that this interaction is specific to POT1A, suggesting a specific role for this paralogue. The interaction is specific to the N-terminal region of the telomerase, which can be encoded by splicing variants. This interaction indicates possible mechanisms for telomerase regulation by alternative splicing and by POT1 proteins.

Identification and characterization of sea squirt telomerase reverse transcriptase

Telomerase is essential for maintaining telomere length and chromosome stability in most eukaryotic organisms. The telomerase ribonucleoprotein complex consists of two essential components, the catalytic telomerase reverse transcriptase protein (TERT) and the intrinsic telomerase RNA. The sea squirts, as urochordates, occupy a key position in the phylogenetic tree of the chordates: they diverged from the other chordates just before the lineage of vertebrates, and thus provide special insight into the origin and evolution of vertebrate genes. Here, we report the cloning and characterization of TERT genes from two sea squirts, Ciona intestinalis and Ciona savignyi. The C. intestinalis TERT (CinTERT) gene encodes 907 amino acids and consists of 17 exons, which are similar to vertebrate TERT genes. The C. savignyi TERT (CsaTERT) gene encodes 843 amino acids, but surprisingly does not contain any introns. Both Ciona TERTs contain all of the reverse transcriptase (RT) motifs (1, 2, A, B, C, D, and E) that are typically present in telomerase and viral RTs. Interestingly, the alignment of Ciona and vertebrate TERT sequences reveals a previously unknown motif, named motif 3, located between motifs 2 and A. The Ciona TERT gene is expressed in all tissues analyzed except the brain and heart. This is the first report of the TERT gene in invertebrate chordates.

Molecular cloning and characterization of the zebrafish (Danio rerio) telomerase catalytic subunit (telomerase reverse transcriptase, TERT)

Telomerase is an enzyme composed of a catalytic subunit (TERT) and RNA template (TR), which specifically elongates telomeres and prevents cellular senescence. Although telomerase cannot be detected in most human somatic tissues, including the nervous system, it can be detected in teleost tissues. To facilitate the investigation of telomerase function in the teleost visual system, the coding sequence of zebrafish TERT is revealed and cloned. Immunoblot, immunohistochemistry, reverse transcription polymerase chain reaction (RT-PCR), and telomeric repeats amplification protocol (TRAP) assay are used to assess the expression of telomerase at mRNA, protein, and functional levels in zebrafish retina. Based on the amino acid sequence of mouse TERT, a full-length telomerase reverse transcriptase cDNA of zebrafish has been isolated and cloned. The deduced protein sequence contains 1,091 amino acid residues and a predicted molecular mass of 126 kDa. Multiple alignment shows that the protein sequence contains the conserved motifs and residues found in TERT of other species. RT-PCR and TRAP assay has detected TERT mRNA expression and telomerase activity, respectively, in all tissues examined, including the retina and the brain. The presence of telomerase activity indicates that a fully functional form of telomerase can be found in the retina. Immunohistochemistry reveals that most neurons in zebrafish retina express TERT in the cell nucleus. The presence of telomerase in different tissues may be associated with the indeterminate growth of teleost. However, teleost retinal neurons are post-mitotic and do not further divide under normal situation. The expression of telomerase activity and TERT in retina implies that telomerase has functions other than the elongation of telomere. These findings could provide new insights on telomerase function in the nervous system.

Asparagales telomerases which synthesize the human type of telomeres

The order of monocotyledonous plants Asparagales is attractive for studies of telomere evolution as it includes three phylogenetically distinct groups with telomeres composed of TTTAGGG (Arabidopsis-type), TTAGGG (human-type) and unknown alternative sequences, respectively. To analyze the molecular causes of these switches in telomere sequence (synthesis), genes coding for the catalytic telomerase subunit (TERT) of representative species in the first two groups have been cloned. Multiple alignments of the sequences, together with other TERT sequences in databases, suggested candidate amino acid substitutions grouped in the Asparagales TERT synthesizing the human-type repeat that could have contributed to the changed telomere sequence. Among these, mutations in the C motif are of special interest due to its functional importance in TERT. Furthermore, two different modes of initial elongation of the substrate primer were observed in Asparagales telomerases producing human-like repeats, which could be attributed to interactions between the telomerase RNA subunit (TR) and the substrate.

The putative telomerase reverse transcriptase component of Leishmania amazonensis: gene cloning and characterization

The Leishmania amazonensis telomerase gene was cloned by a polymerase chain reaction-based strategy using primers designed from a Leishmania major sequence that shared similarities with conserved telomerase motifs. The genes from three other species were cloned for comparative purposes. A ClustalW multiple-sequence alignment demonstrated that the Leishmania telomerases show greater homology with each other than with the proteins of other kinetoplastids and eukaryotes. Characterization experiments indicated that the putative Leishmania telomerase gene was probably in single copy and located in the largest chromosomes. A single messenger ribonucleic acid transcript was found in promastigotes. Phylogenetic analysis suggested that Leishmania telomerase might represent a liaison between the oldest and the newest branches of telomerases.

The telomerase inhibitor telomestatin induces telomere shortening and cell death in Arabidopsis

The cellular response to telomere dysfunction in plants was investigated with the use of telomestatin, an inhibitor of human telomerase activity. Telomestatin bound to plant telomeric repeat sequence, and inhibited telomerase activity in suspension-cultured cells of Arabidopsis thaliana and Oryza sativa (rice) in a dose-dependent manner. The inhibitor did not affect transcript level of the TERT gene, which encodes the catalytic subunit of telomerase, in the plant cells. Inhibition of telomerase activity by telomestatin resulted in rapid shortening of telomeres and the induction of cell death by an apoptosis-like mechanism in Arabidopsis cells. These results suggest that telomerase contributes to the survival of proliferating plant cells by maintaining telomere length, and that telomere erosion triggers cell death.

DNA repair and recombination functions in Arabidopsis telomere maintenance

In this review, we discuss recent advances in the knowledge of plant telomere maintenance, focusing on the model plant Arabidopsis thaliana and, in particular, on the roles of proteins involved in DNA repair and recombination. The question of the interrelationships between DNA repair and recombination pathways and proteins with telomere function and maintenance is of increasing interest and has been the subject of a number of recent reviews (Cech 2004, d'Adda di Fagagna et al. 2004, Hande 2004, Harrington 2004, Maser and DePinho 2004). Understanding of telomere biology, DNA repair and recombination in plants has rapidly progressed over the last decade, substantially due to genetic approaches in Arabidopsis, and we feel that this is an appropriate time to review current knowledge in this field. A number of recent reviews have dealt more generally with the subject of plant telomere structure and evolution (Riha et al. 2001, McKnight et al. 2002, Riha and Shippen 2003b, McKnight and Shippen 2004, Fajkus et al. 2005) and we thus focus specifically on plant telomere biology in the context of DNA repair and recombination in Arabidopsis.

Telomeres in evolution and evolution of telomeres

This paper examines telomeres from an evolutionary perspective. In the monocot plant order Asparagales two evolutionary switch-points in telomere sequence are known. The first occurred when the Arabidopsis-type telomere was replaced by a telomere based on a repeat motif more typical of vertebrates. The replacement is associated with telomerase activity, but the telomerase has low fidelity and this may have implications for the binding of telomeric proteins. At the second evolutionary switch-point, the telomere and its mode of synthesis are replaced by an unknown mechanism. Elsewhere in plants (Sessia, Vestia, Cestrum) and in arthropods, the telomere "typical" of the group is lost. Probably many other groups with "unusual" telomeres will be found. We question whether telomerase is indeed the original end-maintenance system and point to other candidate processes involving t-loops, t-circles, rolling circle replication and recombination. Possible evolutionary outcomes arising from the loss of telomerase activity in alternative lengthening of telomere (ALT) systems are discussed. We propose that elongation of minisatellite repeats using recombination/replication processes initially substitutes for the loss of telomerase function. Then in more established ALT groups, subtelomeric satellite repeats may replace the telomeric minisatellite repeat whilst maintaining the recombination/replication mechanisms for telomere elongation. Thereafter a retrotransposition-based end-maintenance system may become established. The influence of changing sequence motifs on the properties of the telomere cap is discussed. The DNA and protein components of telomeres should be regarded--as with any other chromosome elements--as evolving and co-evolving over time and responding to changes in the genome and to environmental stresses. We describe how telomere dysfunction, resulting in end-to-end chromosome fusions, can have a profound effect on chromosome evolution and perhaps even speciation.

Telomere structure and shortening in telomerase-deficient Trypanosoma brucei

Telomerase consists of a reverse transcriptase (TERT) and an RNA that contains a template for telomere-repeat extension. Telomerase is required to prevent telomere erosion and its activity or lack thereof is important for tumorigenesis and ageing. Telomerase has been identified in numerous organisms but it has not been studied in kinetoplastid protozoa. Trypanosoma brucei, the causative agent of African sleeping sickness, evades the host immune response by frequently changing its variant surface glycoprotein (VSG). The single expressed VSG is transcribed from one of ∼20 subtelomeric ‘Expression Sites’, but the role telomeres might play in regulating VSG transcription and switching is unknown. We identified and sequenced the T.brucei TERT gene. Deleting TERT resulted in progressive telomere shortening of 3–6 bp per generation. In other organisms, the rate of telomere shortening is proportional to the length of the terminal 3′ single-strand overhang. In T.brucei, G-overhangs were undetectable (<30 nt) by in-gel hybridization. The rate of telomere shortening therefore, agrees with the predicted shortening due to the end replication problem, and is consistent with our observation that G-overhangs are short. Trypanosomes whose telomere length can be manipulated provide a new tool to investigate the role of telomeres in antigenic variation.

The unusually large Plasmodium telomerase reverse-transcriptase localizes in a discrete compartment associated with the nucleolus

Telomerase replicates chromosome ends, a function necessary for maintaining genome integrity. We have identified the gene that encodes the catalytic reverse transcriptase (RT) component of this enzyme in the malaria parasite Plasmodium falciparum (PfTERT) as well as the orthologous genes from two rodent and one simian malaria species. PfTERT is predicted to encode a basic protein that contains the major sequence motifs previously identified in known telomerase RTs (TERTs). At ∼2500 amino acids, PfTERT is three times larger than other characterized TERTs. We observed remarkable sequence diversity between TERT proteins of different Plasmodial species, with conserved domains alternating with hypervariable regions. Immunofluorescence analysis revealed that PfTERT is expressed in asexual blood stage parasites that have begun DNA synthesis. Surprisingly, rather than at telomere clusters, PfTERT typically localizes into a discrete nuclear compartment. We further demonstrate that this compartment is associated with the nucleolus, hereby defined for the first time in P.falciparum.

Techniques in plant telomere biology

The role model systems have played in understanding telomere biology has been enormous, and understanding has rapidly transferred to human telomere research. Most work using model organisms to study telomerase and nontelomerase-based telomere-maintenance systems has centered on yeasts, ciliates, and insects. But it is now timely to put considerably more effort into plant models for a number of reasons: (i) the rice and Arabidopsis genome sequencing projects make data mining possible; (ii) extensive collections of insertion mutants of Arabidopsis thaliana enable phenotypic effects of protein gene knockouts to be analyzed, including for those genes involved in telomere structure, function (including, for example, in meiosis), and maintenance; and (iii) the variability of plant telomeres is considerable and ranges from the telomerase-mediated synthesis of the Arabidopsis-type (TTTAGGG) and vertebrate-type (TTAGGG) repeats to sequences synthesized by telomerase-independent mechanism(s) that are still to be discovered. Here we describe how the understanding of telomere biology has been advanced by methods used to isolate telomeric sequences and prove that the putative sequences isolated are indeed telomeric. We show how assays designed to prove the activity of telomerase [e.g., telomeric repeat amplification protocol (TRAP)] lead not only to an understanding of telomere structure and function, but also to the understanding of cell activity in development and in the cell cycle. We review how assays designed to reveal protein/protein and protein/nucleic acid interactions promote understanding of the structure and activities of plant telomeres. Together, the data are making significant contributions to telomere biology in general and could have medical implications.

Characterization of Oryza sativa telomerase reverse transcriptase and possible role of its phosphorylation in the control of telomerase activity

Telomerase reverse transcriptase (TERT) has been characterized in the dicotyledon Arabidopsis thaliana. A TERT homolog has now been identified in the monocotyledon rice (Oryza sativa L.) on the basis of its predicted homology to the A. thaliana enzyme (AtTERT). At least five alternatively spliced transcripts of the rice TERT (OsTERT) gene were detected. The full-length OsTERT protein shares structural features with TERTs of other species, including a calculated molecular size of 144 kDa, an isoelectric point of 9.6, and conserved sequence motifs. Phylogenetic analysis showed that OsTERT clusters with AtTERT and is more related to the human and mouse enzymes than to those of yeast and ciliated protozoa, consistent with the evolutionary relations among these eukaryotes. Telomerase activity was abundant in shoot apices and cultured cells but was low or absent in leaves or roots of rice plants, whereas similarly spliced OsTERT transcripts were detected in all tissues examined and cultured cells. Similar to mouse and human TERT proteins, OsTERT contains two putative phosphorylation sites for Akt kinase. Incubation of a rice cell extract with Akt or with protein phosphatase 2A potentiated or inhibited telomerase activity, respectively, whereas Akt did not affect the activity in Arabidopsis cell extract. In addition, the kinase activated the telomerase in a leaf extract. The mechanism of telomerase regulation in rice thus appears to differ from that in Arabidopsis (which is mediated predominantly at the level of AtTERT transcription), possibly reflecting the taxonomic distance between monocotyledons and dicotyledons.

Telomere structure, function and maintenance in Arabidopsis

The stability of eukaryotic genomes is provided in part by the integrity of telomeres, the nucleoprotein caps on the ends of chromosome. Recent studies reveal that proper telomere architecture is required for long-term proliferation capacity. Here we describe molecular mechanisms that protect and maintain chromosome ends and discuss why Arabidopsis is emerging as a powerful new model for elucidating fundamental aspects of telomere biology.

Improvement of Culture Conditions and Evidence for Nuclear Transformation by Homologous Recombination in a Red Alga, Cyanidioschyzon merolae 10D

Although the nuclear genome sequence of Cyanidioschyzon merolae 10D, a unicellular red alga, was recently determined, DNA transformation technology that is important as a model plant system has never been available thus far. In this study, improved culture conditions resulted in a faster growth rate of C. merolae in liquid medium (doubling time = 9.2 h), and colony formation on gellan gum plates. Using these conditions, spontaneous mutants (5-fluoroortic acid resistant) deficient in the UMP synthase gene were isolated. The lesions were then restored by introducing the wild-type UMP synthase gene into the cells suggesting DNA transformation by homologous recombination.

Yeast telomerase is capable of limited repeat addition processivity

Telomerase is a ribonucleoprotein reverse transcriptase responsible for the maintenance of one strand of telomere terminal repeats. Telomerase-mediated sequence addition is dictated by a short 'template' region of the RNA component. Despite the short template segment, telomerases from many organisms have been shown to mediate the synthesis of long extension products. This synthesis presumably depends on two types of translocation events: simultaneous translocation of the RNA-DNA duplex relative to the active site after each nucleotide incorporation (type I or nucleotide addition processivity), and translocation of the RNA relative to the DNA product after each round of repeat synthesis (type II or repeat addition processivity). In contrast, telomerases from yeasts have been shown to synthesize mostly short products, implying a defect in one or both types of translocation. In this report, we analyzed the processivity of yeast telomerase in vitro, and identified two position-specific elongation barriers within the 5' region of the RNA template that can account for the synthesis of incomplete first round products. These barriers respond differently to variations in nucleotide concentration, primer sequence and mutations in the catalytic protein subunit, consistent with their having distinct mechanistic bases. In addition, by using optimal primers and high concentrations of dGTP, we were able to detect significant type II translocation by the yeast enzyme. Thus, the difference between the elongation property of yeast and other telomerases appears to be quantitative rather than qualitative. Our results suggest that yeast may be a useful system for investigating the physiologic significance of repeat addition processivity.

A conserved telomerase motif within the catalytic domain of telomerase reverse transcriptase is specifically required for repeat addition processivity

Telomerase is a ribonucleoprotein reverse transcriptase responsible for the maintenance of one strand of the telomere terminal repeats. The catalytic protein subunit of the telomerase complex, known as TERT, possesses a reverse transcriptase (RT) domain that mediates nucleotide addition. The RT domain of TERT is distinguishable from retroviral and retrotransposon RTs in having a sizable insertion between conserved motifs A and B', within the so-called fingers domain. Sequence analysis revealed the existence of conserved residues in this region, named IFD (insertion in fingers domain). Mutations of some of the conserved residues in Saccharomyces cerevisiae TERT (Est2p) abolished telomerase function in vivo, testifying to their importance. Significant effects of the mutations on telomerase activity in vitro were observed, with most of the mutants exhibiting a uniform reduction in activity regardless of primer sequence. Remarkably, one mutant manifested a primer-specific defect, being selectively impaired in extending primers that form short hybrids with telomerase RNA. This mutant also accumulated products that correspond to one complete round of repeat synthesis, implying an inability to effect the repositioning of the DNA product relative to the RNA template that is necessary for multiple repeat addition. Our results suggest that the ability to stabilize short RNA-DNA hybrids is crucial for telomerase function in vivo and that this ability is mediated in part by a more elaborate fingers domain structure.

Mitotic and Postmitotic Senescence in Plants

Plants exhibit both mitotic and postmitotic senescence. Mitotic senescence, also known as proliferative senescence, occurs when germline-like meristem cells lose their ability to undergo mitotic cell division. Unlike replicative senescence in yeast and human cells in culture, mitotic senescence in plants is not controlled by telomere shortening. Postmitotic senescence, an active degenerative process, occurs in organs such as leaves and floral petals. Substantial progress has been made toward understanding the molecular mechanisms of postmitotic senescence (especially leaf senescence). Leaf senescence is a form of programmed cell death that can be regulated by an array of endogenous factors and environmental cues. Gene expression is required in order for leaf cells to die. In Arabidopsis thaliana, up to 2500 genes (including more than 130 that encode transcription factors) are transcribed during leaf senescence. Mutant analysis and functional genomics approaches have revealed important roles for several of these genes in leaf senescence. In addition to summarizing our current understanding of senescence in plants at the molecular level, this Review compares mechanisms of senescence in yeast and animal systems.

Expression of the telomeric repeat binding factor gene NgTRF1 is closely coordinated with the cell division program in tobacco BY-2 suspension culture cells

Telomeres are vital for preserving chromosome integrity during cell division. Several genes encoding potential telomere-binding proteins have recently been identified in higher plants, but nothing is known about their function or regulation during cell division. In this study, we have isolated and characterized a cDNA clone, pNgTRF1, encoding a putative double-stranded telomeric repeat binding factor of Nicotiana glutinosa, a diploid tobacco plant. The predicted protein sequence of NgTRF1 (Mr = 75,000) contains a single Myb-like domain with significant homology to a corresponding motif in human TRF1/Pin2 and TRF2. Gel retardation assays revealed that bacterially expressed full-length NgTRF1 was able to form a specific complex only with probes containing three or more contiguous telomeric TTTAGGG repeats. The Myb-like domain of NgTRF1 is essential, but not sufficient, to bind the telomeric repeat sequence. The glutamine-rich extreme C-terminal region, which does not exist in animal proteins, was additionally required to form a specific telomere-protein complex. The dissociation constant (Kd) of the Myb motif plus the glutamine-rich domain of NgTRF1 to the two-telomeric repeat sequence was evaluated to be 4.5 +/- 0.2 x 10-9 m, which is comparable to that of the Myb domain of human TRF1. Expression analysis showed that NgTRF1 gene activity was inversely correlated with the cell division capacity of tobacco root cells and during the 9-day culture period of BY-2 suspension cells, while telomerase activity was positively correlated with cell division. In synchronized BY-2 cells, NgTRF1 was selectively expressed in G1 phase, whereas telomerase activity peaked in S phase. These findings suggest that telomerase activity and NgTRF1 expression are differentially regulated in an opposing fashion during growth and cell division in tobacco plants. The possible physiological functions of NgTRF1 in tobacco cells are also discussed.

Conserved N-terminal motifs of telomerase reverse transcriptase required for ribonucleoprotein assembly in vivo

Telomerase is a ribonucleoprotein (RNP) reverse transcriptase responsible for the maintenance of one strand of the telomere terminal repeats. The key protein subunit of the telomerase complex, known as TERT, possesses reverse transcriptase (RT)-like motifs that directly mediate nucleotide addition. The RT motifs are located in the C-terminal region of the polypeptide. Sequence alignments also revealed the existence of four conserved motifs (named GQ, CP, QFP, and T) in the N-terminal region of TERT. The GQ motif of yeast TERT has been demonstrated previously to be essential for telomerase catalysis and may participate in RNP formation. In this report, we show that substitution of conserved residues in the CP, QFP, and T motifs of yeast TERT also impairs both telomere maintenance and telomerase activity, thus confirming the validity of the sequence alignment. The extent of telomere shortening correlates with the extent of reduction in the level of telomerase activity, TERT protein, and TERT-associated TLC1 RNA. Overexpression of the mutant proteins does not result in telomere shortening, implying that assembly rather than catalytic function was affected. This notion was further supported by comparing the efficiency of RNP formation in the wild type and the overexpression strains. Taken together, our results show that three of the four N-terminal motifs are required for efficient telomerase RNP formation in vivo but not for the enzymatic function of telomerase. We also show that the majority of telomerase-associated TLC1 RNA has a more upstream 3' end than previously reported, consistent with additional processing events during RNP maturation.

N-terminal domain of yeast telomerase reverse transcriptase: recruitment of Est3p to the telomerase complex

Telomerase is a reverse transcriptase that maintains chromosome ends. The N-terminal half of the catalytic protein subunit (TERT) contains three functional domains (I, II, and III) that are conserved among TERTs but not found in other reverse transcriptases. Guided by an amino acid sequence alignment of nine TERT proteins, mutations were introduced into yeast TERT (Est2p). In support of the proposed alignment, mutation of virtually all conserved residues resulted in loss-of-function or temperature sensitivity, accompanied by telomere shortening. Overexpression of telomerase component Est3p led to allele-specific suppression of the temperature-sensitive mutations in region I, suggesting that Est3p interacts with this protein domain. As predicted by the genetic results, a lethal mutation in region I resulted in loss of Est3p from the telomerase complex. We conclude that Est2p region I is required for the recruitment of Est3p to yeast telomerase. Given the phylogenetic conservation of region I of TERT, this protein domain may provide the equivalent function in all telomerases.

Functional analysis of the C-terminal extension of telomerase reverse transcriptase. A putative "thumb" domain

Telomerase is an RNA-protein complex responsible for the extension of one strand of telomere terminal repeats. The catalytic protein subunit of telomerase, known generically as telomerase reverse transcriptase (TERT), exhibits significant homology to reverse transcriptases (RTs) encoded by retroviruses and retroelements. The mechanisms of telomerase may therefore be similar to those of the conventional reverse transcriptases. In this report, we explore potential similarity between these two classes of proteins in a region with no evident sequence similarity. Previous analysis has implicated a C-terminal domain of retroviral RTs (known as the "thumb" domain) in template-primer binding and in processivity control. The equivalent region of TERTs, although similar to one another, does not exhibit significant sequence homology to retroviral RTs. However, we found that removal of this region of yeast TERT similarly resulted in a decrease in the stability of telomerase-DNA complex and in the processivity of telomerase-mediated nucleotide addition. Moreover, the C-terminal domain of TERT exhibits a nucleic acid binding activity when recombinantly expressed and purified. Finally, amino acid substitutions of conserved residues in this region of TERT were found to impair telomerase activity and processivity. We suggest that mechanistic similarity between telomerase and retroviral RTs may extend beyond the regions with apparent sequence similarity.

Human telomerase and its regulation

The telomere is a special functional complex at the end of linear eukaryotic chromosomes, consisting of tandem repeat DNA sequences and associated proteins. It is essential for maintaining the integrity and stability of linear eukaryotic genomes. Telomere length regulation and maintenance contribute to normal human cellular aging and human diseases. The synthesis of telomeres is mainly achieved by the cellular reverse transcriptase telomerase, an RNA-dependent DNA polymerase that adds telomeric DNA to telomeres. Expression of telomerase is usually required for cell immortalization and long-term tumor growth. In humans, telomerase activity is tightly regulated during development and oncogenesis. The modulation of telomerase activity may therefore have important implications in antiaging and anticancer therapy. This review describes the currently known components of the telomerase complex and attempts to provide an update on the molecular mechanisms of human telomerase regulation.

Cloning and characterization of rice (Oryza sativa L) telomerase reverse transcriptase, which reveals complex splicing patterns

Plant chromosomes terminate in telomeres as in other eukaryotes. Telomeres are vital to genome stability and their malfunctioning is lethal. One of the core components of the telomere complex is telomerase. The enzyme activity depends on RNA (TER) and reverse transcriptase (TERT) subunits. We describe here the isolation, sequencing and characterization of the telomerase reverse transcriptase catalytic subunit from the monocot plant Oryza sativa L. (OsTERT). A single copy of this gene is present in the rice genome. The protein predicted from the OsTERT sequence has all the signature motifs of the TERT family members. Our data indicate that rice telomerase activity is developmentally regulated and is high in in vitro tissue and cell culture. However, steady-state transcript levels of the TERT gene do not seem to correlate with enzyme activity. Northern and RT-PCR analyses of the OsTERT gene transcript profile show multiple differentially spliced transcripts in both telomerase-positive and telomerase-negative tissues. Based on quantitative analysis of these transcripts, we speculate that the overall balance between the quantities of particular alternatively spliced transcripts may determine whether the TERT protein(s) is active or not. The diversity of splicing variants detected suggests that, as recently discovered for mammalian TERT proteins, rice TERT protein variants may perform functions other than telomere maintenance.

Identification of Ku70 and Ku80 homologues in Arabidopsis thaliana: evidence for a role in the repair of DNA double-strand breaks

In higher organisms such as mammals and plants, DNA double-strand breaks (DSBs) are repaired preferentially by non-homologous end joining (NHEJ) rather than by homologous recombination. The NHEJ pathway is mediated by Ku, a heterodimer of approximately 70 and 80 kDa subunits, which contributes to various aspects of the metabolism of DNA ends in eukaryotic cells. On the basis of their predicted sequence similarity to human Ku70 and Ku80, cDNAs encoding the first plant homologues of these proteins (AtKu70 and AtKu80, respectively) have now been isolated from Arabidopsis thaliana. AtKu70 and AtKu80 share 28.6 and 22.5% amino acid sequence identity with human Ku70 and Ku80, respectively. Yeast two-hybrid analysis demonstrated that AtKu70 and AtKu80 form a heterodimer, and electrophoretic mobility-shift assays revealed that this heterodimer binds to double-stranded telomeric and non-telomeric DNA sequences, but not to single-stranded DNA. The AtKu heterodimer also possesses single-stranded DNA-dependent ATPase and ATP-dependent DNA helicase activities. Reverse transcription and the polymerase chain reaction revealed that AtKu70 and AtKu80 genes are expressed widely but at low levels in plant tissues. The expression of these two genes in cultured cells was markedly increased in response to the generation of DSBs by bleomycin or methylmethane sulfonate. These results suggest that the evolutionarily conserved Ku70-Ku80 heterodimer functions in DSB repair by the NHEJ pathway in A. thaliana.

Cell cycle-dependent regulation of telomerase activity by auxin, abscisic acid and protein phosphorylation in tobacco BY-2 suspension culture cells

Telomerase is a specialized RNA-directed DNA polymerase that adds telomeric repeats onto the ends of linear eukaryotic chromosomes. It was recently reported that the low, basal level of telomerase activity markedly increased at early S-phase of the cell cycle, and auxin further increased the S-phase-specific telomerase activity in tobacco BY-2 cells. In this study we show that abscisic acid (ABA), a phytohormone known to induce the cyclin-dependent protein kinase inhibitor, effectively abolished both the auxin- and S-phase-specific activation of telomerase in a concentration- and time-dependent fashion in synchronized tobacco BY-2 cells. These results suggest that there exists a hormonal cross-talk between auxin and ABA for the regulation of telomerase activity during the cell cycle of tobacco cells. Treatment of synchronized BY-2 cells with the protein kinase inhibitor staurosporine or H-7 effectively prevented the S-phase-specific activation of telomerase activity. By contrast, when okadaic acid or cantharidin, potent inhibitors of protein phosphatase 2A (PP2A), was applied to the cells, the S-phase-specific high level of telomerase activity was continuously maintained in the cell cycle for at least 14 h after release from M-phase arrest. Incubation of tobacco cell extracts with exogenous PP2A rapidly abrogated in vitro telomerase activity, while okadaic acid and cantharidin blocked the action of PP2A, effectively restoring in vitro telomerase activity. Taken together, these findings are discussed in the light of the suggestion that antagonistic functions of auxin and ABA, and reciprocal phosphorylation and dephosphorylation of telomerase complex, are necessarily involved in the cell cycle-dependent modulation of telomerase activity in tobacco cells.

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.

Tetrahymena proteins p80 and p95 are not core telomerase components

Telomeres provide stability to eukaryotic chromosomes and consist of tandem DNA repeat sequences. Telomeric repeats are synthesized and maintained by a specialized reverse transcriptase, termed telomerase. Tetrahymena thermophila telomerase contains two essential components: Tetrahymena telomerase reverse transcriptase (tTERT), the catalytic protein component, and telomerase RNA that provides the template for telomere repeat synthesis. In addition to these two components, two proteins, p80 and p95, were previously found to copurify with telomerase activity and to interact with tTERT and telomerase RNA. To investigate the role of p80 and p95 in the telomerase enzyme, we tested the interaction of p80, p95, and tTERT in several different recombinant expression systems and in Tetrahymena extracts. Immunoprecipitation of recombinant proteins showed that although p80 and p95 associated with each other, they did not associate with tTERT. In in vitro transcription and translation lysates, tTERT was associated with telomerase activity, but p80 and p95 were not. p80 bound telomerase RNA as well as several other unrelated RNAs, suggesting p80 has a general affinity for RNA. Immunoprecipitations from Tetrahymena extracts also showed no evidence for an interaction between the core tTERT/telomerase RNA complex and the p80 and p95 proteins. These data suggest that p80 and p95 are not associated with the bulk of active telomerase in Tetrahymena.

A stem-loop of Tetrahymena telomerase RNA distant from the template potentiates RNA folding and telomerase activity

The ribonucleoprotein enzyme telomerase adds telomeric repeats to the ends of linear chromosomes. The Tetrahymena telomerase reverse transcriptase (TERT) protein and the telomerase RNA can be reconstituted into an active complex in vitro in rabbit reticulocyte lysates. We have probed the structure of the telomerase RNA in the reconstituted complex with RNases T1 and V1. Upon TERT binding to the RNA, sites of both protection and enhancement of cleavage were observed, suggesting potential protein-binding sites and conformational changes in the RNA. Especially prominent was a large region of RNase V1 protection in stem-loop IV. A number of loop IV mutants still bound TERT but showed drastic decreases in the level of telomerase activity and the loss of protein-dependent folding of the pseudoknot region of the telomerase RNA. The telomerase activity defect and the misfolding of the pseudoknot were partially separable, leading to the proposal of two functions for stem-loop IV: to aid in the folding of the pseudoknot and to function more directly in the active site of telomerase. Thus an RNA element far from the template makes a major contribution to Tetrahymena telomerase enzyme activity.

Analysis of telomerase processivity: mechanistic similarity to HIV-1 reverse transcriptase and role in telomere maintenance

The key protein subunit of the telomerase complex, known as TERT, possesses a reverse transcriptase (RT)-like domain that is conserved in enzymes encoded by retroviruses and retroelements. Structural and functional analysis of HIV-1 RT suggests that RT processivity is governed, in part, by the conserved motif C, motif E, and a C-terminal domain. Mutations in analogous regions of the yeast TERT were found to have anticipated effects on telomerase processivity in vitro, suggesting a great deal of mechanistic and structural similarity between TERT and retroviral RTs, and a similarity that goes beyond the homologous domain. A close correlation was uncovered between telomerase processivity and telomere length in vivo, suggesting that enzyme processivity is a limiting factor for telomere maintenance.

A mutant of Tetrahymena telomerase reverse transcriptase with increased processivity

The protein catalytic subunit of telomerase (TERT) is a reverse transcriptase (RT) that utilizes an internal RNA molecule as a template for the extension of chromosomal DNA ends. In all retroviral RTs there is a conserved tyrosine two amino acids preceding the catalytic aspartic acids in motif C, a motif that is critical for catalysis. In TERTs, however, this position is a leucine, valine, or phenylalanine. We developed and characterized a robust in vitro reconstitution system for Tetrahymena telomerase and tested the effects of amino acid substitutions on activity. Substitution of the retroviral-like tyrosine in motif C did not change overall enzymatic activity but increased processivity. This increase in processivity correlated with an increased affinity for telomeric DNA primer. Substitution of an alanine did not increase processivity, while substitution of a phenylalanine had an intermediate effect. The data suggest that this amino acid is involved in interactions with the primer in telomerase as in other RTs, and show that mutating an amino acid to that conserved in retroviral RTs makes telomerase more closely resemble these other RTs.