In vitro assembly of human H/ACA small nucleolar RNPs reveals unique features of U17 and telomerase RNAs

Insight into the Protein Components of the Box H/ACA RNP

Among eukaryotic organisms a vast majority of Box H/ACA ribonucleoproteins (RNPs) are responsible for the post-transcriptional introduction of pseudouridine (Psi) into ribosomal RNAs (rRNA) and spliceosomal small nuclear RNAs (snRNA), thus influencing protein translation and pre-mRNA splicing, respectively. Additionally, a few distinct Box H/ACA RNPs are involved in the processing of rRNA, and the stabilization of vertebrate telomerase RNA. Thus, whether directly or indirectly, Box H/ACA RNPs impact major steps of gene expression, as well as play a role in maintaining genome integrity. Box H/ACA RNPs each consist of a unique Box H/ACA RNA and a set of four common core proteins. While the RNA component is responsible for dictating site-specificity, the four core proteins impact numerous aspects of RNP function including both stability and catalytic potential. Interestingly, mutations have been identified in the core proteins of the Box H/ACA RNP, resulting in a rare inherited bone marrow failure syndrome referred to as dyskeratosis congenita. This review discusses our current understanding of the roles of the protein components of the Box H/ACA RNP, and provides a framework to understand how mutations in the Box H/ACA RNP contribute to disease pathology.

Mutations in the telomerase component NHP2 cause the premature ageing syndrome dyskeratosis congenita

Dyskeratosis congenita is a premature aging syndrome characterized by muco-cutaneous features and a range of other abnormalities, including early greying, dental loss, osteoporosis, and malignancy. Dyskeratosis congenita cells age prematurely and have very short telomeres. Patients have mutations in genes that encode components of the telomerase complex (dyskerin, TERC, TERT, and NOP10), important in the maintenance of telomeres. Many dyskeratosis congenita patients remain uncharacterized. Here, we describe the analysis of two other proteins, NHP2 and GAR1, that together with dyskerin and NOP10 are key components of telomerase and small nucleolar ribonucleoprotein (snoRNP) complexes. We have identified previously uncharacterized NHP2 mutations that can cause autosomal recessive dyskeratosis congenita but have not found any GAR1 mutations. Patients with NHP2 mutations, in common with patients bearing dyskerin and NOP10 mutations had short telomeres and low TERC levels. SiRNA-mediated knockdown of NHP2 in human cells led to low TERC levels, but this reduction was not observed after GAR1 knockdown. These findings suggest that, in human cells, GAR1 has a different impact on the accumulation of TERC compared with dyskerin, NOP10, and NHP2. Most of the mutations so far identified in patients with classical dyskeratosis congenita impact either directly or indirectly on the stability of RNAs. In keeping with this effect, patients with dyskerin, NOP10, and now NHP2 mutations have all been shown to have low levels of telomerase RNA in their peripheral blood, providing direct evidence of their role in telomere maintenance in humans.

Nop10 is a conserved H/ACA snoRNP molecular adaptor

The H/ACA class of small nucleolar ribonucleoproteins (snoRNPs) is primarily responsible for catalyzing the isomerization of uridine to pseudouridine (Psi) in ribosomal and other cellular RNAs. Each H/ACA snoRNP consist of four conserved proteins, Cbf5 (the Psi-synthase), Gar1, Nhp2 (L7Ae in archaea) and Nop10, that assemble onto a unique RNA component (the snoRNA). The smallest of these proteins, Nop10 ( approximately 7 kDa), has an essential role in the assembly and activity of these particles and binds directly to the Psi-synthase to form the minimal active enzyme in archaea. To better understand the conserved function of this protein, we characterized the NMR structure and dynamics of Nop10 proteins from both archaea and yeast. We show that archaeal Nop10 contains a highly stable Zn2+ binding motif that is replaced in eukaryotes by a smaller meta-stable beta-hairpin, while a highly conserved and conformationally dynamic linker connects these motifs to a nascent alpha-helical structure. Our structural analysis and NMR relaxation data show that these motifs do not interact with each other and tumble independently in solution. Several residues within the archaeal Nop10 Zn2+ binding motif have clear structural and functional roles and are conserved in eukaryotes, yet remain disordered in the free yeast Nop10. We propose that the dynamic structure of Nop10 facilitates an induced-fit recognition with the H/ACA Psi-synthase and allows it to act as a molecular adaptor for guiding snoRNP assembly in similar fashion in all archaea and eukaryotic organisms.

Identification of ATPases pontin and reptin as telomerase components essential for holoenzyme assembly

Telomerase is a multisubunit ribonucleoprotein (RNP) complex that adds telomere repeats to the ends of chromosomes. Three essential telomerase components have been identified thus far: the telomerase reverse transcriptase (TERT), the telomerase RNA component (TERC), and the TERC-binding protein dyskerin. Few other proteins are known to be required for human telomerase function, limiting our understanding of both telomerase regulation and mechanisms of telomerase action. Here, we identify the ATPases pontin and reptin as telomerase components through affinity purification of TERT from human cells. Pontin interacts directly with both TERT and dyskerin, and the amount of TERT bound to pontin and reptin peaks in S phase, evidence for cell-cycle-dependent regulation of TERT. Depletion of pontin and reptin markedly impairs telomerase RNP accumulation, indicating an essential role in telomerase assembly. These findings reveal an unanticipated requirement for additional enzymes in telomerase biogenesis and suggest alternative approaches for inhibiting telomerase in cancer.

Dyskeratosis Congenita: a historical perspective

"Dyskeratosis Congenita (DC) also known as Zinsser-Engman-Cole syndrome is a rare multi-system bone marrow failure syndrome characterised by mucocutaneous abnormalities and an increased predisposition to cancer". This is a common definition of DC but how did this definition arise? The aim of this review is to follow the development of DC and associated diseases from its first reported description in the early 20th century to the current understanding of the genes involved and its pathophysiology in 2007 in a chronological order. Although this review is not intended to be an exhaustive citation of the literature available it does provide a summary of the key developments, citing particularly the earlier reports of each development.

Telomerase reverse-transcriptase homozygous mutations in autosomal recessive dyskeratosis congenita and Hoyeraal-Hreidarsson syndrome

Dyskeratosis congenita (DC) is a multisystem bone marrow failure syndrome characterized by a triad of mucocutaneous abnormalities and an increased predisposition to malignancy. X-linked DC is due to mutations in DKC1, while heterozygous mutations in TERC (telomerase RNA component) and TERT (telomerase reverse transcriptase) have been found in autosomal dominant DC. Many patients with DC remain uncharacterized, particularly families displaying autosomal recessive (AR) inheritance. We have now identified novel homozygous TERT mutations in 2 unrelated consanguineous families, where the index cases presented with classical DC or the more severe variant, Hoyeraal-Hreidarsson (HH) syndrome. These TERT mutations resulted in reduced telomerase activity and extremely short telomeres. As these mutations are homozygous, these patients are predicted to have significantly reduced telomerase activity in vivo. Interestingly, in contrast to patients with heterozygous TERT mutations or hemizygous DKC1 mutations, these 2 homozygous TERT patients were observed to have higher-than-expected TERC levels compared with controls. Collectively, the findings from this study demonstrate that homozygous TERT mutations, resulting in a pure but severe telomerase deficiency, produce a phenotype of classical AR-DC and its severe variant, the HH syndrome.

Noncoding RNAs of the H/ACA family

The H/ACA RNAs are an abundant family of trans-acting, noncoding RNAs found in eukaryotes and archaea. More than 100 H/ACA RNAs are known to exist in humans. The function of the majority of the identified H/ACA RNAs is to guide sites-pecific pseudouridylation of ribosomal RNA. In eukaryotes, H/ACA RNAs also mediate the processing of pre-rRNA, provide the template for telomere synthesis, and guide pseudouridylation of other classes of target RNAs (e.g., small nuclear RNAs [snRNAs]). Thus, currently, the H/ACA RNAs are known to be integrally involved in the production of both ribosomes and spliceosomes, and in the maintenance of chromosome integrity. In addition, dozens of H/ACA RNAs have been identified for which no function has yet been determined. The H/ACA RNAs select and present substrate molecules via base pairing. All H/ACA RNAs contain conserved sequence elements (box H and box ACA) and assemble with a core set of four proteins to form functional ribonucleoprotein complexes (RNPs). Mutations in key RNA and protein components of H/ACA RNPs result in dyskeratosis congenita, a serious multisystem genetic disease. Impressive progress has been made very recently in understanding the biogenesis, trafficking, and function of H/ACA RNPs.

The box H/ACA RNP assembly factor Naf1p contains a domain homologous to Gar1p mediating its interaction with Cbf5p

Naf1 is an essential protein involved in the maturation of box H/ACA ribonucleoproteins, a group of particles required for ribosome biogenesis, modification of spliceosomal small nuclear RNAs and telomere synthesis. Naf1 participates in the assembly of the RNP at transcription sites and in the nuclear trafficking of the complex. The crystal structure of a domain of yeast Naf1p, Naf1Delta1p, reveals a striking structural homology with the core domain of archaeal Gar1, an essential protein component of the mature RNP; it suggests that Naf1p and Gar1p have a common binding site on the enzymatic protein component of the particle, Cbf5p. We propose that Naf1p is a competitive binder for Cbf5p, which is replaced by Gar1p during maturation of the H/ACA particle. The exchange of Naf1p by Gar1p might be prompted by external factors that alter the oligomerisation state of Naf1p and Gar1p. The structural homology with Gar1 suggests that the function of Naf1 involves preventing non-cognate RNAs from being loaded during transport of the particle by inducing a non-productive conformation of Cbf5.

Telomerase as a clinical target: Current strategies and potential applications

Chromosome ends are capped by telomeres, protective DNA-protein complexes that distinguish natural ends from random DNA breaks. Telomeres erode with each successive cell division, and such divisions cease once telomeres become critically short. This proliferation limit is important as a tumor suppressive mechanism, but also contributes to the degenerative conditions associated with cellular aging. In cell types that require continuous renewal, transient expression of telomerase delays proliferation arrest by the de novo synthesis of telomere repeats. Data from our work and others' has shown that deficient telomerase activity has a negative impact on normal human physiology. In the bone marrow failure syndrome dyskeratosis congenita, telomerase enzyme deficiency leads to the premature shortening of telomeres. Premature telomere shortening most grievously affects tissues that have a rapid turnover, such as the hematopoietic and epithelial compartments. In the most severe cases, compromised renewal of hematopoietic stem cells leads to bone marrow failure and premature death. Telomerase activation/replacement shows potential as a therapy for telomere maintenance deficiency syndromes, and in tissue engineering for the degenerative conditions that are associated with normal aging. Conversely, clinical researchers are developing telomerase inhibition therapies to treat tumors, which overcome the short-telomere barrier to unrestricted proliferation by over-expressing telomerase.

Biochemical Purification of Box H/ACA RNPs Involved in Pseudouridylation

Box H/ACA RNPs, each consisting of four common core proteins and a single unique RNA, are the most complex pseudouridylases yet discovered. The RNA component serves as a guide that directs a target uridine for modification. To study the functions and mechanisms of RNA pseudouridylation, it is desirable to isolate the intact box H/ACA RNP complexes. Purified RNPs will allow further identification and characterization of the RNA component in each RNP complex and permit a systematic analysis of the mechanism by which the enzymes convert uridines to pseudouridines in a site-specific manner. Over the years, a number of purification techniques have been developed, providing important tools for RNA pseudouridylation research. Here, we describe three of these techniques, including biotin-streptavidin affinity purification by use of biotinylated 5-fluorouridine (5FU)-containing RNA, tandem affinity purification (TAP) by TAP-tagging one of the four core proteins in the complex, and immunoprecipitation by use of antibodies against one of the four core proteins.

The U1 snRNA hairpin II as a RNA affinity tag for selecting snoRNP complexes

When isolating ribonucleoprotein (RNP) complexes by an affinity selection approach, tagging the RNA component can prove to be strategically important. This is especially true for purifying single types of snoRNPs, because in most cases the snoRNA is thought to be the only unique component. Here, we present a general strategy for selecting specific snoRNPs that features a high-affinity tag in the snoRNA and another in a snoRNP core protein. The RNA tag (called U1hpII) is a small (26 nt) stem-loop domain from human U1 snRNA. This structure binds with high affinity (K(D)=10(-11)M) to the RRM domain of the snRNP protein U1A. In our approach, the U1A protein contains a unique affinity tag and is coexpressed in vivo with the tagged snoRNA to yield snoRNP-U1A complexes with two unique protein tags-one in the bound U1A protein and the other in the snoRNP core protein. This scheme has been used effectively to select C/D and H/ACA snoRNPs, including both processing and modifying snoRNPs, and the snoRNA and core proteins are highly enriched. Depending on selection stringency other proteins are isolated as well, including an RNA helicase involved in snoRNP release from pre-rRNA and additional proteins that function in ribosome biogenesis. Tagging the snoRNA component alone is also effective when U1A is expressed with a myc-Tev-protein A fusion sequence. Combined with reduced stringency, enrichment of the U14 snoRNP with this latter system revealed potential interactions with two other snoRNPs, including one processing snoRNP involved in the same cleavages of pre-rRNA.

Dynamic association and localization of human H/ACA RNP proteins

Mammalian H/ACA RNPs are essential for ribosome biogenesis, pre-mRNA splicing, and telomere maintenance. To form mature RNA-protein complexes, one H/ACA RNA associates with four core proteins. In the cell, this process is assisted by at least one nuclear assembly factor, NAF1. Here we report several unanticipated dynamic aspects of H/ACA RNP proteins. First, when overexpressed, NAF1 delocalizes to the cytoplasm. However, its nucleocytoplasmic shuttling properties remain unaffected. These observations demonstrate a subtle equilibrium between NAF1 expression levels and the availability of NAF1 nuclear binding sites. Second, although NAF1 is excluded from mature RNPs in nucleoli and Cajal bodies, NAF1 associates with mature H/ACA RNA in cell lysates. This association occurs post-lysis because it is observed even when NAF1 and the H/ACA RNA are expressed in separate cells. This documents a protein-RNP association in cell lysates that is absent from intact cells. Third, in similar experiments, all H/ACA core proteins, except NAP57, exchange with their exogenous counterparts, portraying an unexpected dynamic picture of H/ACA RNPs. Finally, the irreversible association of only NAP57 with H/ACA RNA and the conundrum that only NAP57 is mutated in X-linked dyskeratosis congenita (even though most core proteins are required for maintaining H/ACA RNAs) may be more than a coincidence.

Telomeres and telomerase in prostatic intraepithelial neoplasia and prostate cancer biology

Telomeres are terminal, repeated deoxyribonucleic acid (DNA) sequences that stabilize and protect the ends of the chromosomes. Mounting evidence indicates that by initiating chromosomal instability, short dysfunctional telomeres may be involved in prostate carcinogenesis. Although the exact cause of the telomere shortening observed in prostate cancer remains a mystery, telomere loss is known to occur during cell division and oxidative DNA damage, 2 byproducts of chronic inflammation, which is a common histologic finding in the prostate. In addition to prostate cancer causation, telomeres may also play a role in disease progression, and there are indications that tumor telomere content may prove useful as a prognostic marker. Once established, prostate cancer cells almost invariably activate the telomeric DNA polymerase enzyme telomerase, the detection of which may prove useful for diagnostic purposes. Interestingly, telomerase activity is suppressed in prostate cancer cells after androgen withdrawal, raising the possibility that androgen ablative therapies may re-instigate telomere loss, and consequent genetic instability, in surviving cancer cells, thus contributing to the emergence of an androgen-independent, lethal phenotype. A more thorough understanding of telomere biology as it relates to prostate cancer should provide new opportunities for disease prevention, diagnosis, prognostication, and treatment.

Telomerase RNA level limits telomere maintenance in X-linked dyskeratosis congenita

Dyskeratosis congenita (DC) patients suffer a progressive and ultimately fatal loss of hematopoietic renewal correlating with critically short telomeres. The predominant X-linked form of DC results from substitutions in dyskerin, a protein required both for ribosomal RNA (rRNA) pseudouridine modification and for cellular accumulation of telomerase RNA (TER). Accordingly, alternative models have posited that the exhaustion of cellular renewal in X-linked DC arises as a primary consequence of ribosome deficiency or telomerase deficiency. Here we test, for the first time, whether X-linked DC patient cells are compromised for telomerase function at telomeres. We show that telomerase activation in family-matched control cells allows telomere elongation and telomere length maintenance, while telomerase activation in X-linked DC patient cells fails to prevent telomere erosion with proliferation. Furthermore, we demonstrate by phenotypic rescue that telomere defects in X-linked DC patient cells arise solely from reduced accumulation of TER. We also show that X-linked DC patient cells averted from premature senescence support normal levels of rRNA pseudouridine modification and normal kinetics of rRNA precursor processing, in contrast with phenotypes reported for a proposed mouse model of the human disease. These findings support the significance of telomerase deficiency in the pathology of X-linked DC.

Analysis of the binding of the N-terminal conserved domain of yeast Cbf5p to a box H/ACA snoRNA

During ribosome biogenesis, the RNA precursor to mature rRNAs undergoes numerous post-transcriptional chemical modifications of bases, including conversions of uridines to pseudouridines. In archaea and eukaryotes, these conversions are performed by box H/ACA small ribonucleoprotein particles (box H/ACA RNPs), which contain a small guide RNA responsible for the selection of substrate uridines and four proteins, including the pseudouridine synthase, Cbf5p. So far, no in vitro reconstitution of eukaryotic box H/ACA RNPs from purified components has been achieved, principally due to difficulties in purifying recombinant eukaryotic Cbf5p. In this study, we present the purification of a truncated derivative of yeast Cbf5p (Cbf5(Delta)p) that retains the highly conserved TRUB and PUA domains. We have used band retardation assays to show that Cbf5(Delta)p on its own binds to box H/ACA small nucleolar (sno)RNAs. We demonstrate that the conserved H and ACA boxes enhance the affinity of the protein for the snoRNA. Furthermore, like its archaeal homologs, Cbf5(Delta)p can bind to a single stem-loop-box ACA RNA. Finally, we report the first enzymatic footprinting analysis of a Cbf5-RNA complex. Our results are compatible with the view that two molecules of Cbf5p interact with a binding platform constituted by the 5' end of the RNA, the single-stranded hinge domain containing the conserved H box, and the 3' end of the molecule, including the conserved ACA box.

The biogenesis and regulation of telomerase holoenzymes

Chromosome stability requires a dynamic balance of DNA loss and gain in each terminal tract of telomeric repeats. Repeat addition by a specialized reverse transcriptase, telomerase, has an important role in maintaining this equilibrium. Insights that have been gained into the cellular pathways for biogenesis and regulation of telomerase ribonucleoproteins raise new questions, particularly concerning the dynamic nature of this unique polymerase.

The role of telomeres and telomerase in the pathology of human cancer and aging

Cellular senescence, the state of permanent growth arrest, is the inevitable fate of replicating normal somatic cells. Postulated to underlie this finite replicative span is the physiology of telomeres, which constitute the ends of chromosomes. The repetitive sequences of these DNA-protein complexes progressively shorten with each mitosis. When the critical length is bridged, telomeres trigger DNA repair and cell cycle checkpoint mechanisms that result in chromosomal fusions, cell cycle arrest, senescence and/or apoptosis. Should senescence be bypassed at such time, continued cell divisions in the face of dysfunctional telomeres and activated DNA repair machinery can result in the genomic instability favourable for oncogenesis. The longevity and malignant progression of the thus transformed cell requires coincident telomerase expression or other means to negate the constitutional telomeric loss. Practically then, telomeres and telomerase may represent plausible prognostic and screening cancer markers. Furthermore, if the argument is extended, with assumptions that telomeric attrition is indeed the basis of cellular senescence and that accumulation of the latter equates to aging at the organismal level, then telomeres may well explain the increased incidence of cancer with human aging.

Cotranscriptional recognition of human intronic box H/ACA snoRNAs occurs in a splicing-independent manner

Processing from pre-mRNA introns is a widespread mechanism to generate human box C/D and H/ACA snoRNAs. Recent studies revealed that an optimal position relative to the 3' splice site is important for efficient processing of most box C/D snoRNAs and that assembly of box C/D snoRNPs is stimulated by splicing factors likely bound to the branch point region. Here we have investigated the processing of another major class of human intron-encoded RNAs, the box H/ACA snoRNAs. Analysis of 80 H/ACA RNA genes revealed that human H/ACA RNAs possess no preferential localization close to the 3' or 5' splice site. In vivo processing experiments confirmed that H/ACA intronic snoRNAs are processed in a position-independent manner, indicating that there is no synergy between H/ACA RNA processing and splicing. We also showed that recognition of intronic H/ACA snoRNAs and assembly of pre-snoRNPs is an early event that occurs during transcription elongation parallel with pre-mRNA splice site selection. Finally, we found that efficient processing and correct nucleolar localization of the human U64 H/ACA snoRNA requires RNA polymerase II-mediated synthesis of the U64 precursor. This suggests that polymerase II-associated factors direct the efficient assembly and determine the correct subnuclear trafficking of human H/ACA snoRNPs.

Stepwise RNP assembly at the site of H/ACA RNA transcription in human cells

Mammalian H/ACA RNPs are essential for ribosome biogenesis, premessenger RNA splicing, and telomere maintenance. These RNPs consist of four core proteins and one RNA, but it is not known how they assemble. By interrogating the site of H/ACA RNA transcription, we dissected their biogenesis in single cells and delineated the role of the non-core protein NAF1 in the process. NAF1 and all of the core proteins except GAR1 are recruited to the site of transcription. NAF1 binds one of the core proteins, NAP57, and shuttles between nucleus and cytoplasm. Both proteins are essential for stable H/ACA RNA accumulation. NAF1 and GAR1 bind NAP57 competitively, suggesting a sequential interaction. Our analyses indicate that NAF1 binds NAP57 and escorts it to the nascent H/ACA RNA and that GAR1 then replaces NAF1 to yield mature H/ACA RNPs in Cajal bodies and nucleoli.

Crystal structure of a Cbf5-Nop10-Gar1 complex and implications in RNA-guided pseudouridylation and dyskeratosis congenita

H/ACA RNA-protein complexes, comprised of four proteins and an H/ACA guide RNA, modify ribosomal and small nuclear RNAs. The H/ACA proteins are also essential components of telomerase in mammals. Cbf5 is the H/ACA protein that catalyzes isomerization of uridine to pseudouridine in target RNAs. Mutations in human Cbf5 (dyskerin) lead to dyskeratosis congenita. Here, we describe the 2.1 A crystal structure of a specific complex of three archaeal H/ACA proteins, Cbf5, Nop10, and Gar1. Cbf5 displays structural properties that are unique among known pseudouridine synthases and are consistent with its distinct function in RNA-guided pseudouridylation. We also describe the previously unknown structures of both Nop10 and Gar1 and the structural basis for their essential roles in pseudouridylation. By using information from related structures, we have modeled the entire ribonucleoprotein complex including both guide and substrate RNAs. We have also identified a dyskeratosis congenita mutation cluster site within a modeled dyskerin structure.

Structural study of the H/ACA snoRNP components Nop10p and the 3' hairpin of U65 snoRNA

The H/ACA small nucleolar ribonucleoprotein (snoRNP) complexes guide the modification of uridine to pseudouridine at conserved sites in rRNA. The H/ACA snoRNPs each comprise a target-site-specific snoRNA and four core proteins, Nop10p, Nhp2p, Gar1p, and the pseudouridine synthase, Cbf5p, in yeast. The secondary structure of the H/ACA snoRNAs includes two hairpins that each contain a large internal loop (the pseudouridylation pocket), one or both of which are partially complementary to the target RNA(s). We have determined the solution structure of an RNA hairpin derived from the human U65 box H/ACA snoRNA including the pseudouridylation pocket and adjacent stems, providing the first three-dimensional structural information on these H/ACA snoRNAs. We have also determined the structure of Nop10p and investigated its interaction with RNA using NMR spectroscopy. Nop10p contains a structurally well-defined N-terminal region composed of a beta-hairpin, and the rest of the protein lacks a globular structure. Chemical shift mapping of the interaction of RNA constructs of U65 box H/ACA 3' hairpin with Nop10p shows that the beta-hairpin binds weakly but specifically to RNA. The unstructured region of Nop10p likely interacts with Cbf5p.

The vertebrate E1/U17 small nucleolar ribonucleoprotein particle

Each of the many different box H/ACA ribonucleoprotein particles (RNPs) present in eukaryotes and archaea consists of four common core proteins and one specific H/ACA small RNA, which bears the sequence elements H (ANANNA) and ACA. Most of the H/ACA RNPs are small nucleolar RNPs (snoRNPs), which are localized in nucleoli, and are one of the two major classes of snoRNPs. Most H/ACA RNPs direct pseudouridine synthesis in pre-rRNA and other RNAs. One H/ACA small nucleolar RNA (snoRNA), vertebrate E1/U17 (snR30 in yeast), is required for pre-rRNA cleavage processing that generates mature 18S rRNA. E1 snoRNA is encoded in introns of protein-coding genes, and the evidence suggests that human E1 RNA undergoes uridine insertional RNA editing. The vertebrate E1 RNA consensus secondary structure shows several features that are absent in other box H/ACA snoRNAs. The available UV-induced RNA-protein crosslinking results suggest that the E1 snoRNP is asymmetrical in vertebrate cells, in contrast to other H/ACA snoRNPs. The vertebrate E1 snoRNP in cells is surprisingly complex: (i) E1 RNA contacts directly and specifically several proteins which do not appear to be any of the H/ACA RNP four core proteins; and (ii) multiple E1 RNA sites are needed for E1 snoRNP formation, E1 RNA stability, and E1 RNA-protein direct interactions.

Mutations in dyskeratosis congenita: their impact on telomere length and the diversity of clinical presentation

The two genes mutated in the bone marrow failure syndrome dyskeratosis congenita (DC) both encode components of the telomerase complex responsible for maintaining the ends of chromosomes in stem cells and in the germ line. In reviewing the mutation profile that is found in DC, we describe 9 novel mutations in the DKC1 gene and 3 novel TERC mutations responsible for the X-linked and autosomal dominant forms of the disease, respectively, but find that two thirds of the families do not have mutations in either of these genes. In a significant subset of these uncharacterized families, the index case presents with severe disease previously defined as the Hoyeraal Hreidarsson (HH) syndrome. The diverse clinical phenotype seen in patients with X-linked DC is not explained by the different amino acid substitutions: Presentation of the recurrent A353V substitution ranges from classic DC to the severe HH variant. However, we do see that patients with HH have significantly shorter telomeres than those with a relatively mild presentation. In the new families described with TERC mutations, there is further evidence of disease anticipation associated with shorter telomeres in the younger generations. This study highlights the considerable genetic and phenotypic diversity of DC.

The Cbf5-Nop10 complex is a molecular bracket that organizes box H/ACA RNPs

Box H/ACA ribonucleoprotein particles (RNPs) catalyze RNA pseudouridylation and direct processing of ribosomal RNA, and are essential architectural components of vertebrate telomerases. H/ACA RNPs comprise four proteins and a multihelical RNA. Two proteins, Cbf5 and Nop10, suffice for basal enzymatic activity in an archaeal in vitro system. We now report their cocrystal structure at 1.95-A resolution. We find that archaeal Cbf5 can assemble with yeast Nop10 and with human telomerase RNA, consistent with the high sequence identity of the RNP components between archaea and eukarya. Thus, the Cbf5-Nop10 architecture is phylogenetically conserved. The structure shows how Nop10 buttresses the active site of Cbf5, and it reveals two basic troughs that bidirectionally extend the active site cleft. Mutagenesis results implicate an adjacent basic patch in RNA binding. This tripartite RNA-binding surface may function as a molecular bracket that organizes the multihelical H/ACA and telomerase RNAs.

Dyskeratosis Congenita: A Disorder of Defective Telomere Maintenance?

Dyskeratosis congenita (DC) is a rare multisystem bone marrow failure syndrome that displays marked clinical and genetic heterogeneity. X-linked recessive, autosomal dominant and autosomal recessive forms of the disease are recognized. The gene that is mutated in the X-linked form of the disease is DKC1. The DKC1-encoded protein, dyskerin, is a component of small nucleolar ribonucleoprotein particles, which are important in ribosomal RNA processing, and of the telomerase complex. The autosomal dominant form of DC is due to mutations in the gene for the RNA component of telomerase (TERC). Because both dyskerin and TERC are components of the telomerase complex and all patients with DC have short telomeres, the principal pathology of DC appears to relate to telomerase dysfunction, although defects in ribosomal processing via dyskerin's involvement in pseudouridylation cannot be completely ruled out. The gene or genes involved in autosomal recessive DC remain elusive, although genes whose products are required for telomere maintenance remain strong candidates. The study of DC highlights the importance of telomerase in humans and how its deficiency results in multiple abnormalities, including premature aging, bone marrow failure, and cancer.

Pseudouridylation of yeast U2 snRNA is catalyzed by either an RNA-guided or RNA-independent mechanism

Yeast U2 small nuclear RNA (snRNA) contains three pseudouridines (Psi35, Psi42, and Psi44). Pus7p and Pus1p catalyze the formation of Psi35 and Psi44, respectively, but the mechanism of Psi42 formation remains unclear. Using a U2 substrate containing a single (32)P radiolabel at position 42, we screened a GST-ORF library for pseudouridylase activity. Surprisingly, we found a Psi42-specific pseudouridylase activity that coincided with Nhp2p, a protein component of a Box H/ACA sno/scaRNP (small nucleolar/Cajal body-specific ribonucleoprotein). When isolated by tandem affinity purification (TAP), the other protein components of the H/ACA sno/scaRNP also copurified with the pseudouridylase activity. Micrococcal nuclease-treated TAP preparations were devoid of pseudouridylase activity; however, activity was restored upon addition of RNAs from TAP preparations. Pseudouridylation reconstitution using RNAs from a Box H/ACA RNA library identified snR81, a snoRNA known to guide rRNA pseudouridylation, as the Psi42-specific guide RNA. Using the snR81-deletion strain, Nhp2p- or Cbf5p-conditional depletion strain, and a cbf5 mutation strain, we further demonstrated that the pseudouridylase activity is dependent on snR81 snoRNP in vivo. Our data indicate that snRNA pseudouridylation can be catalyzed by both RNA-dependent and RNA-independent mechanisms.

Reconstitution of archaeal H/ACA small ribonucleoprotein complexes active in pseudouridylation

Pseudouridine (Ψ) are frequently modified residues in RNA. In Eukarya, their formation is catalyzed by enzymes or by ribonucleoprotein complexes (RNPs) containing H/ACA snoRNAs. H/ACA sRNA and putative ORFs for H/ACA sRNP proteins (L7Ae, aCBF5, aNOP10 and aGAR1) were found in Archaea. Here, by using Pyrococcus abyssi recombinant proteins and an in vitro transcribed P.abyssi H/ACA sRNA, we obtained the first complete in vitro reconstitution of an active H/ACA RNP. Both L7Ae and the aCBF5 RNA:Ψ synthase bind directly the sRNA; aCBF5 also interacts directly and independently with aNOP10 and aGAR1. Presence of aCBF5, aNOP10 and a U residue at the pseudouridylation site in the target RNA are required for RNA target recruitment. In agreement, we found that the aCBF5–aNOP10 pair is the minimal set of proteins needed for the formation of a particle active for pseudouridylation. However, particles more efficient in targeted pseudouridylation can be formed with the addition of proteins L7Ae and/or aGAR1. Although necessary for optimal activity, the conserved ACA motif in the sRNA was found to be not essential.

Cotranscriptional recruitment of the pseudouridylsynthetase Cbf5p and of the RNA binding protein Naf1p during H/ACA snoRNP assembly

H/ACA small nucleolar ribonucleoprotein particles (snoRNPs) are essential for the maturation and pseudouridylation of the precursor of rRNAs and other stable RNAs. Although the RNA and protein components of these RNPs have been identified, the mechanisms by which they are assembled in vivo are poorly understood. Here we show that the RNA binding protein Naf1p, which is required for H/ACA snoRNPs stability, associates with RNA polymerase II-associated proteins Spt16p, Tfg1p, and Sub1p and with H/ACA snoRNP proteins. Chromatin immunoprecipitation experiments show that Naf1p and the pseudouridylsynthetase Cbf5p cross-link specifically with the chromatin of H/ACA small nucleolar RNA (snoRNA) genes. Naf1p and Cbf5p cross-link predominantly with the 3' end of these genes, in a pattern similar to that observed for transcription elongation factor Spt16p. Cross-linking of Naf1p to H/ACA snoRNA genes requires active transcription and intact H/ACA snoRNA sequences but does not require the RNA polymerase II CTD kinase Ctk1p. These results suggest that Naf1p and Cbf5p are recruited in a cotranscriptional manner during H/ACA snoRNP assembly, possibly by binding to the nascent H/ACA snoRNA transcript during elongation or termination of transcription of H/ACA snoRNA genes.

RNA-guided RNA modification: functional organization of the archaeal H/ACA RNP

In eukaryotes and archaea, uridines in various RNAs are converted to pseudouridines by RNA-guided RNA modification complexes termed H/ACA RNPs. Guide RNAs within the complexes base-pair with target RNAs to direct modification of specific ribonucleotides. Cbf5, a protein component of the complex, likely catalyzes the modification. However, little is known about the organization of H/ACA RNPs and the roles of the multiple proteins thought to comprise the complexes. We have reconstituted functional archaeal H/ACA RNPs from recombinant components, defined the components necessary and sufficient for function, and determined the direct RNA-protein and protein-protein interactions that occur between the components. The results provide substantial insight into the functional organization of this RNP. The functional complex requires a guide RNA and each of four proteins: Cbf5, Gar1, L7Ae, and Nop10. Two proteins interact directly with the guide RNA: L7Ae and Cbf5. L7Ae does not interact with other H/ACA RNP proteins in the absence of the RNA. We have defined two novel functions for Cbf5. Cbf5 is the protein that specifically recognizes and binds H/ACA guide RNAs. In addition, Cbf5 recruits the two other essential proteins, Gar1 and Nop10, to the pseudouridylation guide complex.

The many facets of H/ACA ribonucleoproteins

The H/ACA ribonucleoproteins (RNPs) are known as one of the two major classes of small nucleolar RNPs. They predominantly guide the site-directed pseudouridylation of target RNAs, such as ribosomal and spliceosomal small nuclear RNAs. In addition, they process ribosomal RNA and stabilize vertebrate telomerase RNA. Taken together, the function of H/ACA RNPs is essential for ribosome biogenesis, pre-mRNA splicing, and telomere maintenance. Every cell contains 100-200 different species of H/ACA RNPs, each consisting of the same four core proteins and one function-specifying H/ACA RNA. Most of these RNPs reside in nucleoli and Cajal bodies and mediate the isomerization of specific uridines to pseudouridines. Catalysis of the reaction is mediated by the putative pseudouridylase NAP57 (dyskerin, Cbf5p). Unexpectedly, mutations in this housekeeping enzyme are the major determinants of the inherited bone marrow failure syndrome dyskeratosis congenita. This review details the many diverse functions of H/ACA RNPs, some yet to be uncovered, with an emphasis on the role of the RNP proteins. The multiple functions of H/ACA RNPs appear to be reflected in the complex phenotype of dyskeratosis congenita.

Cbf5p, the putative pseudouridine synthase of H/ACA-type snoRNPs, can form a complex with Gar1p and Nop10p in absence of Nhp2p and box H/ACA snoRNAs

Box C/D and box H/ACA small ribonucleoprotein particles (sRNPs) are found from archaea to humans, and some of these play key roles during the biogenesis of ribosomes or components of the splicing apparatus. The protein composition of the core of both types of particles is well established and the assembly pathway of box C/D sRNPs has been extensively investigated both in archaeal and eukaryotic systems. In contrast, knowledge concerning the mode of assembly and final structure of box H/ACA sRNPs is much more limited. In the present study, we have investigated the protein/protein interactions taking place between the four protein components of yeast box H/ACA small nucleolar RNPs (snoRNPs), Cbf5p, Gar1p, Nhp2p, and Nop10p. We provide evidence that Cbf5p, Gar1p, and Nop10p can form a complex devoid of Nhp2p and small nucleolar RNA (snoRNA) components of the particles and that Cbf5p and Nop10p can directly bind to each other. We also show that the absence of any component necessary for assembly of box H/ACA snoRNPs inhibits accumulation of Cbf5p, Gar1p, or Nop10p, whereas Nhp2p levels are little affected.

A new alpha-helical extension promotes RNA binding by the dsRBD of Rnt1p RNAse III

Rnt1 endoribonuclease, the yeast homolog of RNAse III, plays an important role in the maturation of a diverse set of RNAs. The enzymatic activity requires a conserved catalytic domain, while RNA binding requires the double-stranded RNA-binding domain (dsRBD) at the C-terminus of the protein. While bacterial RNAse III enzymes cleave double-stranded RNA, Rnt1p specifically cleaves RNAs that possess short irregular stem-loops containing 12-14 base pairs interrupted by internal loops and bulges and capped by conserved AGNN tetraloops. Consistent with this substrate specificity, the isolated Rnt1p dsRBD and the 30-40 amino acids that follow bind to AGNN-containing stem-loops preferentially in vitro. In order to understand how Rnt1p recognizes its cognate processing sites, we have defined its minimal RNA-binding domain and determined its structure by solution NMR spectroscopy and X-ray crystallography. We observe a new carboxy-terminal helix following a canonical dsRBD structure. Removal of this helix reduces binding to Rnt1p substrates. The results suggest that this helix allows the Rnt1p dsRBD to bind to short RNA stem-loops by modulating the conformation of helix alpha1, a key RNA-recognition element of the dsRBD.

Mouse dyskerin mutations affect accumulation of telomerase RNA and small nucleolar RNA, telomerase activity, and ribosomal RNA processing

Dyskerin is a nucleolar protein present in small nucleolar ribonucleoprotein particles that modify specific uridine residues of rRNA by converting them to pseudouridine. Dyskerin is also a component of the telomerase complex. Point mutations in the human gene encoding dyskerin cause the skin and bone marrow failure syndrome dyskeratosis congenita (DC). To test the extent to which disruption of pseudouridylation or telomerase activity may contribute to the pathogenesis of DC, we introduced two dyskerin mutations into murine embryonic stem cells. The A353V mutation is the most frequent mutation in patients with X-linked DC, whereas the G402E mutation was identified in a single family. The A353V, but not the G402E, mutation led to severe destabilization of telomerase RNA, a reduction in telomerase activity, and a significant continuous loss of telomere length with increasing numbers of cell divisions during in vitro culture. Both mutations caused a defect in overall pseudouridylation and a small but detectable decrease in the rate of pre-rRNA processing. In addition, both mutant embryonic stem cell lines showed a decrease in the accumulation of a subset of H/ACA small nucleolar RNAs, correlating with a significant decrease in site-specific pseudouridylation efficiency. Interestingly, the H/ACA snoRNAs decreased in the G402E mutant cell line differed from those affected in A353V mutant cells. Hence, our findings show that point mutations in dyskerin may affect both the telomerase and pseudouridylation pathways and the extent to which these functions are altered can vary for different mutations.

The Euplotes telomerase subunit p43 stimulates enzymatic activity and processivity in vitro

Telomerase is a reverse transcriptase that synthesizes telomeric DNA repeats at the ends of eukaryotic chromosomes. Although it is minimally composed of a conserved catalytic protein subunit (TERT) and an RNA component, additional accessory factors present in the holoenzyme play crucial roles in the biogenesis and function of the enzyme complex. Telomerase from the ciliate Tetrahymena can be reconstituted in active form in vitro. Using this system, we show that p43, a telomerase-specific La-motif protein from the ciliate Euplotes, stimulates activity and increases repeat addition processivity of telomerase. Activity enhancement by p43 requires its incorporation into a TERT.RNA.p43 ternary complex but is independent of other dissociable protein factors functioning in telomerase complex assembly. Stimulation is enhanced at elevated temperatures, supporting a role for p43 in structural stabilization of a critical region of the RNA subunit. To our knowledge, this represents the first demonstration that an authentic telomerase accessory protein can directly affect the enzymatic activity of the core enzyme in vitro.

Human box H/ACA pseudouridylation guide RNA machinery

Pseudouridine, the most abundant modified nucleoside in RNA, is synthesized by posttranscriptional isomerization of uridines. In eukaryotic RNAs, site-specific synthesis of pseudouridines is directed primarily by box H/ACA guide RNAs. In this study, we have identified 61 novel putative pseudouridylation guide RNAs by construction and characterization of a cDNA library of human box H/ACA RNAs. The majority of the new box H/ACA RNAs are predicted to direct pseudouridine synthesis in rRNAs and spliceosomal small nuclear RNAs. We can attribute RNA-directed modification to 79 of the 97 pseudouridylation sites present in the human 18S, 5.8S, and 28S rRNAs and to 11 of the 21 pseudouridines reported for the U1, U2, U4, U5, and U6 spliceosomal RNAs. We have also identified 12 novel box H/ACA RNAs which lack apparent target pseudouridines in rRNAs and small nuclear RNAs. These putative guide RNAs likely function in the pseudouridylation of some other types of cellular RNAs, suggesting that RNA-guided pseudouridylation is more general than assumed before. The genomic organization of the new box H/ACA RNA genes indicates that in human cells, all box H/ACA pseudouridylation guide RNAs are processed from introns of pre-mRNA transcripts which either encode a protein product or lack protein-coding capacity.

Holoenzyme proteins required for the physiological assembly and activity of telomerase

Many proteins have been implicated in the physiological function of telomerase, but specific roles of telomerase-associated proteins other than telomerase reverse transcriptase (TERT) remain ambiguous. To gain a more comprehensive understanding of catalytically active enzyme composition, we performed affinity purification of epitope-tagged, endogenously assembled Tetrahymena telomerase. We identified and cloned genes encoding four telomerase proteins in addition to TERT. We demonstrate that both of the two new proteins characterized in detail, p65 and p45, have essential roles in the maintenance of telomere length as part of a ciliate telomerase holoenzyme. The p65 subunit contains an La motif characteristic of a family of direct RNA-binding proteins. We find that p65 in cell extract is associated specifically with telomerase RNA, and that genetic depletion of p65 reduces telomerase RNA accumulation in vivo. These findings demonstrate that telomerase holoenzyme proteins other than TERT play critical roles in RNP biogenesis and function.

Architecture and assembly of mammalian H/ACA small nucleolar and telomerase ribonucleoproteins

Mammalian H/ACA small nucleolar RNAs and telomerase RNA share common sequence and secondary structure motifs that form ribonucleoprotein particles (RNPs) with the same four core proteins, NAP57 (also dyskerin or in yeast Cbf5p), GAR1, NHP2, and NOP10. The assembly and molecular interactions of the components of H/ACA RNPs are unknown. Using in vitro transcription/translation in combination with immunoprecipitation of core proteins, UV-crosslinking, and electrophoretic mobility shift assays, we demonstrate the following. NOP10 associates with NAP57 as a prerequisite for NHP2 binding. Although NHP2 on its own binds RNA nonspecifically, this NAP57-NOP10-NHP2 core trimer specifically recognizes H/ACA RNAs. GAR1 associates independently with NAP57 near the pseudouridylase core of mature H/ACA RNPs. In contrast to other RNPs whose assembly is initiated by protein-RNA interactions, the four H/ACA core proteins form a protein-only particle that associates with H/ACA RNAs. Nonetheless, functional H/ACA snoRNPs assembled in cytosolic extracts are stable and do not exchange their RNA components, suggesting that new particle formation requires de novo synthesis.

U17/snR30 is a ubiquitous snoRNA with two conserved sequence motifs essential for 18S rRNA production

Saccharomyces cerevisiae snR30 is an essential box H/ACA small nucleolar RNA (snoRNA) required for the processing of 18S rRNA. Here, we show that the previously characterized human, reptilian, amphibian, and fish U17 snoRNAs represent the vertebrate homologues of yeast snR30. We also demonstrate that U17/snR30 is present in the fission yeast Schizosaccharomyces pombe and the unicellular ciliated protozoan Tetrahymena thermophila. Evolutionary comparison revealed that the 3'-terminal hairpins of U17/snR30 snoRNAs contain two highly conserved sequence motifs, the m1 (AUAUUCCUA) and m2 (AAACCAU) elements. Mutation analysis of yeast snR30 demonstrated that the m1 and m2 elements are essential for early cleavages of the 35S pre-rRNA and, consequently, for the production of mature 18S rRNA. The m1 and m2 motifs occupy the opposite strands of an internal loop structure, and they are located invariantly 7 nucleotides upstream from the ACA box of U17/snR30 snoRNAs. U17/snR30 is the first identified box H/ACA snoRNA that possesses an evolutionarily conserved role in the nucleolytic processing of eukaryotic pre-rRNA.

Interaction of Saccharomyces Cdc13p with Pol1p, Imp4p, Sir4p and Zds2p is involved in telomere replication, telomere maintenance and cell growth control

Telomeres are the physical ends of eukaryotic chromosomes. They are important for maintaining the integrity of chromosomes and this function is mediated through a number of protein factors. In Saccharomyces cerevisiae, Cdc13p binds to telomeres and affects telomere maintenance, telomere position effects and cell cycle progression through G(2)/M phase. We identified four genes encoding Pol1p, Sir4p, Zds2p and Imp4p that interact with amino acids 1-252 of Cdc13p using a yeast two-hybrid screening system. Interactions of these four proteins with Cdc13p were through direct protein-protein interactions as judged by in vitro pull-down assays. Direct protein-protein interactions were also observed between Pol1p-Imp4p, Pol1p-Sir4p and Sir4p-Zds2p, whereas no interaction was detected between Imp4p-Sir4p and Zds2p-Imp4p, suggesting that protein interactions were specific in the complex. Pol1p was shown to interact with Cdc13p. Here we show that Zds2p and Imp4p also form a stable complex with Cdc13p in yeast cells, because Zds2p and Imp4p co-immunoprecipitate with Cdc13p, whereas Sir4p does not. The function of the N-terminal 1-252 region of Cdc13p was also analyzed. Expressing Cdc13(252-924)p, which lacks amino acids 1-252 of Cdc13p, causes defects in progressive cell growth and eventually arrested in the G(2)/M phase of the cell cycle. These growth defects were not caused by progressive shortening of telomeres because telomeres in these cells were long. Point mutants in the amino acids 1-252 region of Cdc13p that reduced the interaction between Cdc13p and its binding proteins resulted in varying level of defects in cell growth and telomeres. These results indicate that the interactions between Cdc13(1-252)p and its binding proteins are important for the function of Cdc13p in telomere regulation and cell growth. Together, our results provide evidence for the formation of a Cdc13p-mediated telosome complex through its N-terminal region that is involved in telomere maintenance, telomere length regulation and cell growth control.

YNMG tetraloop formation by a dyskeratosis congenita mutation in human telomerase RNA

Autosomal dominant dyskeratosis congenita (DKC) has been linked to mutations in the RNA component of telomerase, the ribonucleoprotein responsible for telomere maintenance. Recent studies have investigated the role of the GC (107-108) --> AG mutation in the conserved P3 helix in the pseudoknot domain of human telomerase RNA. The mutation was found to significantly destabilize the pseudoknot conformation, resulting in a shift in the thermodynamic equilibrium to favor formation of a P2b hairpin intermediate. In the wild-type sequence, the hairpin intermediate was found to form a novel sequence of pyrimidine base pairs in a continuous stem capped by a structured pentaloop. The DKC mutant hairpin was observed to be slightly more stable than the wild-type hairpin, further shifting the pseudoknot-hairpin equilibrium to favor the mutant P2b hairpin. Here we examined the solution structure of the DKC mutant hairpin to identify the reason for this additional stability. We found that the mutant hairpin forms the same stem structure as wild-type and that the additional stabilization observed using optical melting can be explained by the formation of a YNMG-type tetraloop structure, with the last nucleotide of the pentaloop bulged out into the major groove. Our results provide a structural explanation for the increased stability of the mutant hairpin and further our understanding of the effect of this mutation on the structure and stability of the dominant conformation of the pseudoknot domain in this type of DKC.

Telomerase RNA accumulates in Cajal bodies in human cancer cells

Telomerase synthesizes telomeric DNA repeats at the ends of eukaryotic chromosomes. The RNA component of the enzyme (hTR) provides the template for telomere synthesis, which is catalyzed by telomerase reverse transcriptase (hTERT). Little is known regarding the subcellular localization of hTR and hTERT and the pathway by which telomerase is assembled. Here we report the first glimpse of the detailed subcellular localization of endogenous hTR in human cells, which we obtained by fluorescence in situ hybridization (FISH). Our studies have revealed a distinctive hTR localization pattern in cancer cells. We have found that hTR accumulates within intranuclear foci called Cajal bodies in all typical tumor-derived cell lines examined (in which telomerase is active), but not in primary or ALT cells (where little or no hTERT is present). Accumulation of hTR in the Cajal bodies of primary cells is induced when hTERT is ectopically expressed. Moreover, we report that hTERT is also found in Cajal bodies. Our data suggest that Cajal bodies are involved in the assembly and/or function of human telomerase.

Telomere maintenance and disease

The proliferative capacity of human cells is regulated by telomerase, an enzyme uniquely specialised for telomeric DNA synthesis. The critical role of telomerase activation in tumour progression and tumour maintenance has been well established in studies of cancer and of oncogenic transformation in cell culture. New evidence suggests that telomerase activation has an important role in normal somatic cells, and that failure to activate sufficient telomerase also promotes disease. We review the evidence for premature telomere attrition in proliferative deficiencies of the human haemopoietic system, and discuss the potential use of telomerase activation in telomere-restorative gene therapy.

Structural and sequence evolution of U17 small nucleolar RNA (snoRNA) and its phylogenetic congruence in chelonians

Vertebrate U17 RNA is an intron-encoded H/CA box containing snoRNA, which has been intensively studied in the last decade, though its precise role in ribosome biogenesis is not yet clear. A consensus secondary structure for the U17 RNA molecule has been derived from the comparative sequence and structural evolution analysis of U17 snoRNA among vertebrates. Its phylogenetic congruence above class level has been tested and preliminary data on chelonians suggest that also in this order, U17 snoRNA evolved congruently with phylogeny. We herein extend our analysis to other components of this reptile group. According to the sequence data that have also emerged from chelonians, the U17 RNA molecule can be divided into two main domains: the 5'-variable domain, which presents the sequence motifs capable of base-pairing with the 18S rRNA target and spanning STEM1, -2, and -3, and the 3'-conserved domain, consisting of STEM4. In vertebrates, the latter RNA region shows a high conservation both in secondary structure and in the presence of the three sequence motifs 5'-AUUCCUA-3', 5'-U(G/U)ACU-3', and 5'-AACCC-3'. We tested the phylogenetic congruence of U17 evolution with chelonian relationships: Our results are significantly similar to those emerging from mtDNA and morphological systematics. Some discrepancies (e.g., the position of Platysternon) need to be addressed in greater depth.

Biochemical aspects of telomerase function

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