Telomerase: an RNP enzyme synthesizes DNA

Abstract

Telomerase is a eukaryotic ribonucleoprotein (RNP) whose specialized reverse transcriptase action performs de novo synthesis of one strand of telomeric DNA. The resulting telomerase-mediated elongation of telomeres, which are the protective end-caps for eukaryotic chromosomes, counterbalances the inevitable attrition from incomplete DNA replication and nuclease action. The telomerase strategy to maintain telomeres is deeply conserved among eukaryotes, yet the RNA component of telomerase, which carries the built-in template for telomeric DNA repeat synthesis, has evolutionarily diverse size and sequence. Telomerase shows a distribution of labor between RNA and protein in aspects of the polymerization reaction. This article first describes the underlying conservation of a core set of structural features of telomerase RNAs important for the fundamental polymerase activity of telomerase. These include a pseudoknot-plus-template domain and at least one other RNA structural motif separate from the template-containing domain. The principles driving the diversity of telomerase RNAs are then explored. Much of the diversification of telomerase RNAs has come from apparent gain-of-function elaborations, through inferred evolutionary acquisitions of various RNA motifs used for telomerase RNP biogenesis, cellular trafficking of enzyme components, and regulation of telomerase action at telomeres. Telomerase offers broadly applicable insights into the interplay of protein and RNA functions in the context of an RNP enzyme.

Figure 1.

The telomerase catalytic cycle. Stages in the catalytic cycle are illustrated using T. thermophila sequences of the TER template (red) and telomeric-repeat DNA (black). The active site (yellow) is within the TERT RT domain (filled with the darkest shade of blue). Some TERT-TER contacts that influence template use are stable across all states of the catalytic cycle (shown for T. thermophila as the template 5′-flanking region bound to the TERT TRBD domain in an intermediate shade of blue). Other TERT-TER and TERT-DNA contacts may be specific to particular configurations of template and product relative to the active site (shown here as changing contact between the template 3′-flanking region and the TERT TEN domain in the lightest shade of blue, which also contacts single-stranded DNA). Copying across the template with nucleotide addition processivity is accompanied by changes in the length of hybrid between template RNA and product DNA, depicted in states (A–C). Product released from the template can be held in association with the active enzyme by other interactions, as depicted in state (D). Realignment of the product 3′ end at the template 3′ end, as depicted in the transition from state (D) to state (A), allows for repeat addition processivity.

Figure 2.

Conserved TERT domains. TERT domain evolutionary conservation and protein-nucleic acid interactions are summarized, with the same color scheme of TERT and TER domains used in other figures.

Figure 3.

Conserved TER motifs within phylogenetically divergent secondary structures. The thickest region of line represents the template, whereas dashed lines represent sites of sequence variability within the phylogenetic group. PK indicates the pseudoknot. The color scheme of TER motifs matches that in the other figures.