Saccharomyces telomeres acquire single-strand TG1-3 tails late in S phase

Single-stranded DNA arising at telomeres in cdc13 mutants may constitute a specific signal for the RAD9 checkpoint

A cdc13 temperature-sensitive mutant of Saccharomyces cerevisiae arrests in the G2 phase of the cell cycle at the restrictive temperature as a result of DNA damage that activates the RAD9 checkpoint. The DNA lesions present after a failure of Cdc13p function appear to be located almost exclusively in telomere-proximal regions, on the basis of the profile of induced mitotic recombination. cdc13 rad9 cells dividing at the restrictive temperature contain single-stranded DNA corresponding to telomeric and telomere-proximal DNA sequences and eventually lose telomere-associated sequences. These results suggest that the CDC13 product functions in telomere metabolism, either in the replication of telomeric DNA or in protecting telomeres from the double-strand break repair system. Moreover, since cdc13 rad9 cells divide at a wild-type rate for several divisions at the restrictive temperature while cdc13 RAD9 cells arrest in G2, these results also suggest that single-stranded DNA may be a specific signal for the RAD9 checkpoint.

Telomerase RNA localized in the replication band and spherical subnuclear organelles in hypotrichous ciliates

The intranuclear distribution of telomere DNA-binding protein and telomerase RNA in hypotrichous ciliates was revealed by indirect fluorescent antibody staining and in situ hybridization. The Oxytricha telomere protein colocalized with DNA, both being dispersed throughout the macronucleus except for numerous spherical foci that contained neither DNA nor the protein. Surprisingly, the telomerase RNA was concentrated in these foci; therefore, much of telomerase does not colocalize with telomeres. These foci persist through the cell cycle. They may represent sites of assembly, transport or stockpiling of telomerase and other ribonucleoproteins. During S phase, the macronuclear DNA replication machinery is organized into a disc-shaped structure called the replication band. Telomerase RNA is enriched in the replication band as judged by fluorescence intensity. We conclude that the localization of a subfraction of telomerase is coordinated with semiconservative DNA replication.

Gene disruption of a G4-DNA-dependent nuclease in yeast leads to cellular senescence and telomere shortening

The yeast gene KEM1 (also named SEP1/DST2/XRN1/RAR5) produces a G4-DNA-dependent nuclease that binds to G4 tetraplex DNA structure and cuts in a single-stranded region 5' to the G4 structure. G4-DNA generated from yeast telomeric oligonucleotides competitively inhibits the cleavage reaction, suggesting that this enzyme may interact with yeast telomeres in vivo. Homozygous deletions of the KEM1 gene in yeast block meiosis at the pachytene stage, which is consistent with the hypothesis that G4 tetraplex DNA may be involved in homologous chromosome pairing during meiosis. We conjectured that the mitotic defects of kem1/sep1 mutant cells, such as a higher chromosome loss rate, are also due to failure in processing G4-DNA, especially at telomeres. Here we report two phenotypes associated with a kem1-null allele, cellular senescence and telomere shortening, that provide genetic evidence that G4 tetraplex DNA may play a role in telomere functioning. In addition, our results reveal that chromosome ends in the same cells behave differently in a fashion dependent on the KEM1 gene product.

Phosphorylation of the Oxytricha telomere protein: possible cell cycle regulation

In the macronucleus of the ciliate Oxytricha nova, telomeres end with single-stranded (T4G4)2 DNA bound to a heterodimeric telomere protein (alpha beta). Both the alpha and beta subunits (alpha-TP and beta-TP) were phosphorylated in asynchronously growing Oxytricha; beta-TP was phosphorylated to a much higher degree. In vitro, mouse cyclin-dependent kinases (Cdks) phosphorylated beta-TP in a lysine-rich domain that is not required for specific DNA binding but is implicated in higher order structure formation of telomeres. Therefore, phosphorylation of beta-TP could modulate a function of the telomere protein that is separate from specific DNA binding. Phosphoamino acid analysis revealed that the mouse Cdks modify predominantly threonine residues in beta-TP, consistent with the observation that beta-TP contains two consensus Cdk recognition sequences containing threonine residues. In Xenopus egg extracts that undergo cell cycling, beta-TP was phosphorylated in M phase and dephosphorylated in interphase. This work provides the first direct evidence of phosphorylation at telomeres in any organism, as well as indirect evidence for cell cycle regulation of telomere phosphorylation. The Cdc2/cyclin A and Cdc2/cyclin B kinases are required for major mitotic events. An attractive model is that phosphorylation of beta-TP by these kinases is required for the breakdown of telomere associations with each other and/or with nuclear structures prior to nuclear division.

Telomere structure and function

Telomeres, the termini of linear eukaryotic chromosomes, contain specific DNA sequences that are widely conserved. These sequences not only recruit telomere-specific proteins, but also give telomeric DNA the ability to fold into four-stranded DNA structures. Recent structural studies have shown that the repertoire of quadruplexes formed by the G-rich strand is larger than had been envisaged. Even more surprising is a novel four-stranded structure formed by the C-rich strand, called the i-tetraplex. Genetic and biochemical analyses have been used to identify proteins involved in telomeric DNA packaging and organization. The possibility that four-stranded structures may play a role in telomere function has been strengthened by the discovery that telomeric proteins can bind to and promote the formation of G-quadruplexes.

RAP1 stimulates single- to double-strand association of yeast telomeric DNA: implications for telomere-telomere interactions

Repressor Activator Protein 1 (RAP1) of Saccharomyces cerevisiae is an abundant nuclear protein implicated in telomere length maintenance, transactivation, and in the establishment of silent chromatin domains. The RAP1 binding site 5' of the yeast HIS4 gene is also a region of hyperrecombination in meiosis. We report here that as RAP1 binds its recognition consensus, it appears to untwist double-stranded DNA, which we detect as the introduction of a negative supercoil in circularization assays. Coincident with the RAP1-dependent untwisting, we observe stimulation of the association of a single-stranded yeast telomeric sequence with its homologous double-stranded sequence in a supercoiled plasmid. This unusual distortion of the DNA double helix by RAP1 may contribute to the RAP1-dependent enhancement of recombination rates and promote non-duplex strand interactions at telomeres.

Transcription of a yeast telomere alleviates telomere position effect without affecting chromosome stability

Telomeres are required for the stable maintenance of chromosomes in the yeast Saccharomyces cerevisiae. Telomeres also repress the expression of genes in their vicinity, a phenomenon known as telomere position effect. In an attempt to construct a conditional telomere, an inducible promoter was introduced adjacent to a single telomere of a chromosome such that transcription could be induced toward the end of the chromosome. Transcription toward two other essential chromosomal elements, centromeres and origins of replication, eliminates their function. In contrast, transcription toward a telomere did not affect the stability function of the telomere as measured by the loss rate of the transcribed chromosome. Transcription proceeded through the entire length of the telomeric tract and caused a modest reduction in the average length of the transcribed telomere. Transcription of the telomere substantially reduced the frequency of cells in which an adjacent URA3 gene was subject to telomere position effect. These results indicate that telomere position effect can be alleviated without compromising chromosome stability.

Isolation and characterization of two Saccharomyces cerevisiae genes that encode proteins that bind to (TG1-3)n single strand telomeric DNA in vitro

By screening lambda gt11 libraries with a radiolabeled (TG1-3)n oligonucleotide, two Saccharomyces cerevisiae genes were identified that encode polypeptides that recognize the single-stranded telomeric repeat sequence (TG1-3)n. The first gene, NSR1, a previously identified gene, encodes a protein involved in ribosomal RNA maturation and possibly in transport of proteins into the nucleus. The second gene, GBP2 (G-strand Binding Protein), is an anonymous open reading frame from chromosome III. These two genes contain RNA recognition motifs (RRMs) that are found in proteins that interact with RNA. Both Nsr1p and Gbp2p bind specifically to yeast single strand (TG1-3)n DNA in vitro. To test whether these two proteins associate with telomeres in vivo, strains were constructed in which one or both of these genes were either disrupted or overexpressed. None of these alterations affected telomere length or telomere position effect. The potential role of these two (TG1-3)n binding proteins is discussed.

In vitro identification of a protein of Saccharomyces cerevisiae that interacts specifically with the G-rich DNA strand of the telomere

The telomeres of Saccharomyces cerevisiae consist of a repeated G2-3T(GT)1-6 DNA sequence that forms a complex with proteins. To date only the RAP1 protein has been shown to bind to the simple sequences in yeast telomeric DNA, as well as to non-telomeric regulatory sites. We have used synthetic oligodeoxyribonucleotides, both double- and single-stranded, to identify specific yeast telomeric proteins in a partially purified yeast extract. Using the gel shift assay, we detected a binding activity that is stable at high ionic strength and that recognizes specifically the G-rich protrusion of a double-stranded synthetic yeast telomere, as well as the G-rich single strand. This is the first evidence of a purely telomeric protein in that it binds to the single-stranded telomeric protrusion of the yeast chromosome.

Promotion of parallel DNA quadruplexes by a yeast telomere binding protein: a circular dichroism study

Repressor-activator protein 1 (RAP1) has an essential role in the maintenance of yeast telomeres. Yeast telomeric DNA consists of simple repeated G-rich sequences that are bound by RAP1. We have found that RAP1, in addition to its known binding activity for double-stranded DNA, interacts with the G-rich strand containing guanine base (G)-tetrads. We show here using circular dichroism spectroscopy that RAP1 promotes the formation of one particular type of DNA quadruplex, parallel G4-DNA. Furthermore, RAP1 is able to bind to both preformed parallel and antiparallel DNA quadruplexes. These results have implications for the possible use of DNA quadruplexes in telomere-telomere association in vivo.

TEL+CEN antagonism on plasmids involves telomere repeat sequences tracts and gene products that interact with chromosomal telomeres

In Saccharomyces cerevisiae, circular plasmids that include either a centromere (CEN-plasmids) or a telomere sequence (TEL-plasmids) segregate more efficiently than circular ARS-plasmids. In contrast, circular plasmids that include both telomere and centromere sequences were unstable, a property we term TEL+CEN antagonism. TEL+CEN antagonism required a telomere repeat tract longer than 49 bp although the distance and relative orientation of the centromere and telomere sequences was not critical. TEL+CEN antagonism was alleviated in strains carrying different rap1 alleles including rap1ts, rap1s, and rap1t alleles. Mutations SIR2, SIR3, SIR4, NAT1 and ARD1, genes that influence transcriptional silencing at telomeres and at the silent mating type loci, abolished TEL+CEN antagonism Mutation of SIR1 also partially alleviated TEL-CEN antagonism. In some sir mutant strains short yeast artificial chromosomes (YACs), which are normally unstable, became more stable, suggesting that the same mechanism that caused TEL+CEN antagonism on circular plasmids may contribute to the instability of short linear plasmids.

Internal tracts of telomeric DNA act as silencers in Saccharomyces cerevisiae

Telomeric position effect (TPE) refers to the ability of telomeres to repress the transcription of genes in their vicinity. Internal stretches of C1-3A DNA, the sequence found at Saccharomyces telomeres, also repressed transcription of nearby genes. This repression, hereafter called C1-3A-based silencing, was observed at several chromosomal loci, including on a circular chromosome. The magnitude of C1-3A-based silencing was increased by both proximity to a telomere and increased length of the C1-3A tracts. C1-3A-based silencing was affected by many of the same genes and conditions that influence TPE and acted in an orientation-independent manner. Thus, in yeast, an expanded array of a simple repetitive DNA, C1-3A, is sufficient to promote the assembly of a transcriptionally silent chromosomal domain.

The DNA of ciliated protozoa

Ciliates contain two types of nuclei: a micronucleus and a macronucleus. The micronucleus serves as the germ line nucleus but does not express its genes. The macronucleus provides the nuclear RNA for vegetative growth. Mating cells exchange haploid micronuclei, and a new macronucleus develops from a new diploid micronucleus. The old macronucleus is destroyed. This conversion consists of amplification, elimination, fragmentation, and splicing of DNA sequences on a massive scale. Fragmentation produces subchromosomal molecules in Tetrahymena and Paramecium cells and much smaller, gene-sized molecules in hypotrichous ciliates to which telomere sequences are added. These molecules are then amplified, some to higher copy numbers than others. rDNA is differentially amplified to thousands of copies per macronucleus. Eliminated sequences include transposonlike elements and sequences called internal eliminated sequences that interrupt gene coding regions in the micronuclear genome. Some, perhaps all, of these are excised as circular molecules and destroyed. In at least some hypotrichs, segments of some micronuclear genes are scrambled in a nonfunctional order and are recorded during macronuclear development. Vegetatively growing ciliates appear to possess a mechanism for adjusting copy numbers of individual genes, which corrects gene imbalances resulting from random distribution of DNA molecules during amitosis of the macronucleus. Other distinctive features of ciliate DNA include an altered use of the conventional stop codons.

Telomere maintenance and gene repression: a common end?

In yeast, the study of the teleomere has recently provided new information on the requirements for chromosome stability, on elements influencing nuclear architecture and on position-effect variegation.

A conserved sequence motif within the exceptionally diverse telomeric sequences of budding yeasts

Telomeric DNA sequences have generally been found to be remarkably conserved in evolution, typically consisting of repeated, very short sequence units containing clusters of G residues. Recently however the telomeric DNA of the asexual yeast Candida albicans was shown to consist of much longer repeat units. Here we report the identification of seven additional telomeric sequences from sexual and asexual budding yeast species. The telomeric repeat units from this group of relatively closely related species show more phylogenetic diversity in length (8-25 bp), sequence, and composition than has been seen previously throughout a wide phylogenetic range of other eukaryotes. We also show that certain strains of the asexual diploid species Candida tropicalis have two forms of telomeric repeats, which appear to differ by a single base pair. Despite their great diversity, the telomeric repeat units of C. albicans, Saccharomyces cerevisiae, and all of the species we have examined in this report share a conserved approximately 6-bp motif of T and G residues resembling more typical telomeric sequences.

Telomeric DNA sequence and structure following de novo telomere synthesis in Euplotes crassus

To learn more about the mechanism of de novo telomere synthesis, we have characterized the sequence and structure of newly synthesized telomeres from Euplotes crassus. E. crassus is a particularly useful organism for studying telomere synthesis because millions of telomeres are made in each cell at a well-defined time during the sexual stage of the life cycle. These newly synthesized telomeres are approximately 50 bp longer than mature macronuclear telomeres. We have investigated the structure of the newly synthesized telomeres and have found that they are much more heterogeneous in length than mature telomeres. Most of the heterogeneity is present on the G-rich strand, indicating that the length of this strand is rather loosely controlled. In contrast, the length of the C-rich strand is much less variable, suggesting that synthesis of this strand is the more precisely regulated step in telomere addition. The G-rich strand exhibits variability both in the total number of G4T4 repeats and in the identity of the terminal nucleotide. In most cases, the G-rich strnd extends beyond the C-rich strand to leave a 3' overhang. While the size of this overhang is variable, the median length is 10 nucleotides. This research provides the first detailed picture of a newly synthesized telomere and has allowed us to formulate a model to describe the various steps involved in de novo telomere synthesis.

The saccharomyces PIF1 DNA helicase inhibits telomere elongation and de novo telomere formation

A screen to detect yeast mutants that frequently lost expression of subtelomeric genes identified two mutations in PIF1, a gene known to encode a 5' to 3' DNA helicase. The loss of expression of subtelomeric genes in pif1 cells was due to deletion of the subtelomeric regions of the chromosomes and the generation of new telomeres at proximal sites. In pif1 mutants, de novo telomere formation usually occurred at sites with very little homology to telomeric DNA. De novo telomere formation after HO-induced chromosome breakage also occurred at elevated frequencies in pif1 cells. Moreover, mutations in PIF1 caused all telomeres to lengthen. These results suggest that the PIF1 helicase is an inhibitor of both de novo telomere formation and telomere elongation.

Telomeric DNA sequence and structure following de novo telomere synthesis in Euplotes crassus

To learn more about the mechanism of de novo telomere synthesis, we have characterized the sequence and structure of newly synthesized telomeres from Euplotes crassus. E. crassus is a particularly useful organism for studying telomere synthesis because millions of telomeres are made in each cell at a well-defined time during the sexual stage of the life cycle. These newly synthesized telomeres are approximately 50 bp longer than mature macronuclear telomeres. We have investigated the structure of the newly synthesized telomeres and have found that they are much more heterogeneous in length than mature telomeres. Most of the heterogeneity is present on the G-rich strand, indicating that the length of this strand is rather loosely controlled. In contrast, the length of the C-rich strand is much less variable, suggesting that synthesis of this strand is the more precisely regulated step in telomere addition. The G-rich strand exhibits variability both in the total number of G4T4 repeats and in the identity of the terminal nucleotide. In most cases, the G-rich strnd extends beyond the C-rich strand to leave a 3' overhang. While the size of this overhang is variable, the median length is 10 nucleotides. This research provides the first detailed picture of a newly synthesized telomere and has allowed us to formulate a model to describe the various steps involved in de novo telomere synthesis.

SIR3 and SIR4 proteins are required for the positioning and integrity of yeast telomeres

Heritable inactivation of genes occurs in specific chromosomal domains located at the silent mating type loci and at telomeres of S. cerevisiae. The SIR genes (for silent information regulators) are trans-acting factors required for this repression mechanism. We show here that the SIR3 and SIR4 gene products have a sub-nuclear localization similar to the telomere-associated RAP1 protein, which is found primarily in foci at the nuclear periphery of fixed yeast spheroplasts. In strains deficient for either SIR3 or SIR4, telomeres lose their perinuclear localization, as monitored by RAP1 immunofluorescence. The length of the telomeric repeat shortens in sir3 and sir4 mutant strains, and the mitotic stability of chromosome V is reduced. These data suggest that SIR3 and SIR4 are required for both the integrity and subnuclear localization of yeast telomeres, the loss of which correlates with loss of telomere-associated gene repression.

Characterization of a G-quartet formation reaction promoted by the beta-subunit of the Oxytricha telomere-binding protein

Telomeres, the ends of linear chromosomes, typically consists of tandem repeats of a simple guanine-rich sequence. Telomeric DNA is able to form intermolecular G-quartet structures. The beta-subunit of the Oxytricha telomere-binding protein acts as a molecular chaperone to promote the formation of dimers and specific higher order complexes of telomeric DNA stabilized by G-quartets; these reactions occur under physiological conditions in vitro. In the present article, we show that, at saturating protein concentrations (> or = 200 nM), beta-mediated G-quartet formation is a first-order reaction with respect to DNA concentration, with k approximately 1 h-1 at 37 degrees C. In contrast, the protein-independent reaction is a second-order reaction. The beta-subunit enhances the rate of G-quartet formation by 10(5)-10(6)-fold at a telomeric DNA concentration of 20 nM. The beta-mediated higher order complexes are identified as parallel four-stranded tetramers of telomeric DNA (G4-DNA). Poly-L-lysine also promotes formation of the tetramers, but not dimers. These DNA structures were studied by irreversible thermal melting experiments and probed by annealing to different complementary strands. Guanine residues important for structure formation were analyzed by methylation interference experiments. On the basis of these data, models for the beta-mediated structures are proposed, and possible mechanisms for the beta-mediated reaction are discussed. In addition, we found that the beta-subunit promotes the annealing of two complementary strands into a duplex, as do many other basic proteins. However, not all proteins with annealing-promoting activity are active in the formation of G-quartet structures. The activity of the telomere protein in promoting the formation of telomeric DNA structures may enable chromosome-chromosome association or the regulation of telomerase activity in vivo.

Telomeres and telomerases

The ends of eukaryotic chromosomes are defined by specialized nucleoprotein complexes called telomeres. Telomeres impart stability to the genome and are of general interest due to their unique structure and unconventional mode of synthesis. Recent work has identified new components of the telomere complex and expanded our understanding of the role of terminal structures in maintaining cell viability.

The beta subunit of Oxytricha telomere-binding protein promotes G-quartet formation by telomeric DNA

Telomeres, the ends of linear chromosomes, typically consist of tandem repeats of simple G-rich sequences. At high concentrations, single-stranded telomeric DNA can form dimers and tetramers involving G-quartets. We show that under physiological conditions, the beta subunit of the Oxytricha telomere-binding protein greatly accelerates G-quartet formation. The reaction occurs with oligonucleotides ending in the Oxytricha (T4G4T4G4) and Tetrahymena (T2G4T2G4) telomeric sequences; the sequence preceding these telomeric repeats can be nontelomeric, single-, or double-stranded. Protein deletion analysis indicates that the carboxy-terminal highly basic domain of the beta subunit, which is dispensible for telomeric complex formation, is sufficient for mediating G-quartet formation. The finding that a telomeric protein acts as a molecular chaperone for G-quartet formation provides a strong argument that such DNA structures exist in vivo at chromosome telomeres.

Telomeric DNA binding proteins

The physical ends of eukaryotic chromosomes form a specialized nucleoprotein complex composed of DNA and DNA binding proteins. This nucleoprotein complex, termed the telomere, is essential for chromosome stability. In most organisms, the DNA portion of the nucleo-protein complex consists of simple tandem DNA repeats with one strand guanine rich. The protein portion of the complex is less well understood. The experiments presented in two recent papers represent different stages in the characterization of the telomeric DNA binding proteins. The first paper presents a structure-function study of the Oxytricha telomeric DNA binding proteins and the second paper shows the identification and initial characterization of a telomeric DNA binding activity from Xenopus laevis. These two reports provided valuable information in understanding the structure and function of telomeres.

Origin activation and formation of single-strand TG1-3 tails occur sequentially in late S phase on a yeast linear plasmid

In order to understand the mechanisms leading to the complete duplication of linear eukaryotic chromosomes, the temporal order of the events involved in replication of a 7.5-kb Saccharomyces cerevisiae linear plasmid called YLpFAT10 was determined. Two-dimensional agarose gel electrophoresis was used to map the position of the replication origin and the direction of replication fork movement through the plasmid. Replication began near the center of YLpFAT10 at the site in the 2 microns sequences that corresponds to the 2 microns origin of DNA replication. Replication forks proceeded bidirectionally from the origin to the ends of YLpFAT10. Thus, yeast telomeres do not themselves act as origins of DNA replication. The time of origin utilization on YLpFAT10 and on circular 2 microns DNA in the same cells was determined both by two-dimensional gel electrophoresis and by density transfer experiments. As expected, 2 microns DNA replicated in early S phase. However, replication of YLpFAT10 occurred in late S phase. Thus, the time of activation of the 2 microns origin depended upon its physical context. Density transfer experiments established that the acquisition of telomeric TG1-3 single-strand tails, a predicted intermediate in telomere replication, occurred immediately after the replication forks approached the ends of YLpFAT10. Thus, telomere replication may be the very last step in S phase.

Origin activation and formation of single-strand TG1-3 tails occur sequentially in late S phase on a yeast linear plasmid

In order to understand the mechanisms leading to the complete duplication of linear eukaryotic chromosomes, the temporal order of the events involved in replication of a 7.5-kb Saccharomyces cerevisiae linear plasmid called YLpFAT10 was determined. Two-dimensional agarose gel electrophoresis was used to map the position of the replication origin and the direction of replication fork movement through the plasmid. Replication began near the center of YLpFAT10 at the site in the 2 microns sequences that corresponds to the 2 microns origin of DNA replication. Replication forks proceeded bidirectionally from the origin to the ends of YLpFAT10. Thus, yeast telomeres do not themselves act as origins of DNA replication. The time of origin utilization on YLpFAT10 and on circular 2 microns DNA in the same cells was determined both by two-dimensional gel electrophoresis and by density transfer experiments. As expected, 2 microns DNA replicated in early S phase. However, replication of YLpFAT10 occurred in late S phase. Thus, the time of activation of the 2 microns origin depended upon its physical context. Density transfer experiments established that the acquisition of telomeric TG1-3 single-strand tails, a predicted intermediate in telomere replication, occurred immediately after the replication forks approached the ends of YLpFAT10. Thus, telomere replication may be the very last step in S phase.

A Xenopus egg factor with DNA-binding properties characteristic of terminus-specific telomeric proteins

We have identified a Xenopus laevis protein factor that specifically recognizes vertebrate telomeric repeats at DNA ends. This factor, called Xenopus telomere end factor (XTEF), is detected predominantly in extracts of Xenopus eggs and ovaries, which are estimated to contain sufficient XTEF to bind approximately 3 x 10(7) DNA ends. In contrast, XTEF is much less abundant (approximately 90 per cell) in extracts of somatic cell nuclei. Mobility retardation analysis of the XTEF activity in egg extracts indicates that this factor binds the vertebrate telomeric repeat sequence (TTAGGG)2 when present in a single-stranded 3' overhang. Single-stranded 3' extensions of (TTTGGG)2, (AAAGGG)2, (TTACCC)2, or a nonrepetitive sequence fail to bind XTEF efficiently, whereas changes in the double-stranded sequence 5' to the TTAGGG repeat tail are tolerated. TTAGGG repeats are not recognized at internal position, at a 5' protruding end, or in double-stranded DNA. In addition, the factor does not bind RNA with single-stranded UUAGGG repeats at a 3' end. XTEF-DNA complexes form and are stable in high salt. The DNA-binding properties of XTEF resemble the characteristics of a class of terminus-specific telomere proteins identified previously in hypotrichous ciliates.

Telomeres and the functional architecture of the nucleus

The single molecule of DNA that constitutes a eukaryotic chromosome begins and ends with a stretch of repetitive DNA known as a telomere. These sequences appear to be necessary to preserve the integrity of the genetic material through the cell cycle. Telomeric DNA is organized into regions of non-nucleosomal chromatin called the telosome, which can interact with other telosomes and with the nuclear envelope. This review focuses on cytological evidence for these interactions and on recent insights into the molecular organization of the telomeric complex.