Architecture of human telomerase RNA

Abstract

Telomerase is a unique reverse transcriptase that catalyzes the addition of telomere DNA repeats onto the 3′ ends of linear chromosomes and plays a critical role in maintaining genome stability. Unlike other reverse transcriptases, telomerase is unique in that it is a ribonucleoprotein complex, where the RNA component [telomerase RNA (TR)] not only provides the template for the synthesis of telomere DNA repeats but also plays essential roles in catalysis, accumulation, TR 3′-end processing, localization, and holoenzyme assembly. Biochemical studies have identified TR elements essential for catalysis that share remarkably conserved secondary structures across different species as well as species-specific domains for other functions, paving the way for high-resolution structure determination of TRs. Over the past decade, structures of key elements from the core, conserved regions 4 and 5, and small Cajal body specific RNA domains of human TR have emerged, providing significant insights into the roles of these RNA elements in telomerase function. Structures of all helical elements of the core domain have been recently reported, providing the basis for a high-resolution model of the complete core domain. We review this progress to determine the overall architecture of human telomerase RNA.Box H/ACA RNA | NMR | pseudoknot | telomerase reverse transcriptase T elomerase is a large, multisubunit ribonucleoprotein (RNP) that replicates the 3′ end of linear chromosomes by processive synthesis of telomere DNA repeats. Telomeres, the physical ends of linear chromosomes, generally comprise dsDNA with a short repeating species-specific sequence ending in a 3′ single-stranded overhang of variable length plus associated telomere binding proteins, called shelterin in humans (1, 2). Telomeres protect the integrity of linear chromosomes by allowing the cellular DNA repair machinery to distinguish them from double-strand breaks, thus playing critical roles in maintaining genome stability in eukaryotes (1, 2). Shortening of telomeres below a critical length because of inherent incomplete replication of DNA ends ultimately leads to telomere fusions and cell senescence (3-6). The 3′ ends of telomeres are replicated by telomerase, a unique reverse transcriptase discovered almost three decades ago (7), which catalyzes the addition of telomere DNA repeats onto the ends of linear chromosomes using an embedded RNA as the template (8, 9). Although telomerase has a low or undetectable level of activity in most somatic cells, it is active in some germline, epithelial, and hematopoietic cells, and it is highly active in the majority (∼90%) of cancer cell lines (10-12). Telomerase deficiency because of mutations in human telomerase RNA (hTR) has also been linked to several inherited human diseases, such as dyskeratosis congenita, aplastic anemia, myelodysplasia, and idiopathic pulmonary fibrosis (13-26).The telomerase holoenzyme includes a unique reverse transcriptase [telomerase reverse transcriptase (TERT)], an essential RNA (TR), and several...

Telomerase and retrotransposons: Reverse transcriptases that shaped genomes

“…Impressive progress toward achieving this goal was made by the determination of the atomic resolution structure of TERT from Tribolium castaneum (19). At the colloquium and in an accompanying perspective, Juli Feigon summarized advances on the structure of human telomerase RNA (TER) and the RNP (20,21). The noncoding TER not only supplies the template for reverse transcription but also stimulates the polymerization by TERT through mechanisms that remain poorly understood (Fig.…”

Telomerase RNA stem terminus element affects template boundary element function, telomere sequence, and shelterin binding

“…Examination of the TER1-1066-1080 sequence ( Fig. 1 A and B) revealed that it is similar to the telomerase RNA TWJ consensus sequence and structure, which is found in >90% of vertebrate and yeast telomerase RNAs (11) as well as the human P6.1 helix (13,18). Based on its sequence, secondary structure, and critical role in telomere maintenance, we conclude that the TER1-1066-1080 stem loop and TWJ is the highly conserved STE in S. pombe (9,13).…”

“…The TWJ and TER1-1067-1078 regions are enlarged to show detail. Nucleotides and base pairs that are identical to the TWJ consensus (11) or the human P6.1 sequence (13,18) are in bold. SM, Sm binding site (6,19); TBE, template boundary element (6, 7); Template, templating sequence; TWJ, three way junction.…”

“…When the STE and hence the TBE are not fully functional, telomeric DNA contains a high number of aberrant repeats, and these reduce shelterin binding. Because a human partial loss-offunction STE allele that is associated with aplastic anemia results in short telomeres and very low telomerase activity (13), it would not be surprising if the human STE also affects TBE function and the sequence of telomeric DNA.…”

“…Most mammalian STEs, however, contain a terminal stem loop called the P6.1 helix (13), which partially overlaps the TWJ (11,13,14). As in…”