The protein subunit of telomerase displays patterns of dynamic evolution and conservation across different metazoan taxa

Abstract

Background: Most animals employ telomerase, which consists of a catalytic subunit known as the telomerase reverse transcriptase (TERT) and an RNA template, to maintain telomere ends. Given the importance of TERT and telomere biology in core metazoan life history traits, like ageing and the control of somatic cell proliferation, we hypothesised that TERT would have patterns of sequence and regulatory evolution reflecting the diverse life histories across the Animal Kingdom.

Results: We performed a complete investigation of the evolutionary history of TERT across animals. We show that although TERT is almost ubiquitous across Metazoa, it has undergone substantial sequence evolution within canonical motifs. Beyond the known canonical motifs, we also identify and compare regions that are highly variable between lineages, but show conservation within phyla. Recent data have highlighted the importance of alternative splice forms of TERT in non-canonical functions and although animals may share some conserved introns, we find that the selection of exons for alternative splicing appears to be highly variable, and regulation by alternative splicing appears to be a very dynamic feature of TERT evolution. We show that even within a closely related group of triclad flatworms, where alternative splicing of TERT was previously correlated with reproductive strategy, we observe highly diverse splicing patterns.

Conclusions: Our work establishes that the evolutionary history and structural evolution of TERT involves previously unappreciated levels of change and the emergence of lineage specific motifs. The sequence conservation we describe within phyla suggests that these new motifs likely serve essential biological functions of TERT, which along with changes in splicing, underpin diverse functions of TERT important for animal life histories.

Keywords: Alternative splicing; Evolution; Metazoa; Planarian; TERT; Telomerase.

Fig. 1

Evolution of TERT across the Animal kingdom. Phylogenetic tree of TERT sequences from representative metazoan phyla, early-branching metazoans and unicellular relatives of metazoans constructed using the maximum-likelihood method. Bootstrap support percentages from 1000 replicates are labelled at the nodes. Scale bar denotes substitutions per site. Branches that contained less than 30% boostrap replicates were collapsed. Full list of species used and accession numbers can be obtained from Additional file 1: Table S2. Figure inset shows a schematic diagram of the canonical telomerase enzyme bound to the telomeric DNA substrate and the telomerase RNA template. The domain structure of TERT is also shown: the TEN domain with the GQ motif, the TRBD domain with the CP, QFP and T motifs, the RT domain with the 1, 2, A, B′, C, D and E motifs, the N-terminal linker and the CTE region

Fig. 2

Taxa-specific modifications of TERT domains. TERT canonical motifs from representative metazoan phyla, early-branching metazoans and unicellular relatives of metazoans were annotated in colour codes: TEN domain GQ motif (purple); TRBD domain CP (dark green), QFP (blue) and T (light green) motifs; reverse transcriptase domain motifs 1 (brown), 2 (yellow), A (dark blue), B′ (sky blue), C (black), D (orange) and E (pink). a Deuterostome TERT canonical motifs. b Protostome, early-branching metazoans and unicellular eukaryotes canonical motifs. Motif annotations were drawn according to scale based on multiple sequence alignments performed using MAFFT

Fig. 3

TERT exon-intron structure is conserved across the Animal kingdom. Schematic diagram depicting the intron positions mapped on the TERT orthologs relative to intron positions in human TERT (hTERT). a Deuterostome TERT intron positions. b Protostome, early-branching metazoans and unicellular eukaryotes intron positions. Purple triangles represent conserved introns (relative to hTERT intron positions) and green triangles represent species-specific introns

Fig. 4

TERT in highly regenerative triclad planarians. a The 11 flatworm species used in this study. Abbreviations represent: Pple (Procerodes plebeja), Dlac (Dendrocoelum lacteum), Pnig (Polycelis nigra), Pfel (Polycelis felina), Djap (Dugesia japonica), Dryu (Dugesia ryukyuensis), Dben (Dugesia Benazzi), Dtah (Dugesia tahitientis), Smed (Schmidtea mediterranea), Slug (Schmidtea lugubris) and Gtig (Girardia tigrina). b Gel image showing full length RT-PCR results of TERT transcripts and isoforms from 11 planarian species (NTC represents no cDNA template control for the PCR). c Domain structure of planarian TERTs; mollusc TERTs and hTERT were used for comparison. Multiple sequence alignments of the (d) N-terminal linker and (e) C-terminal extensions from planarians and the trematodes. Boxes also denote planarian-specific motifs with their respective descriptions written inside. f TERT gene structure of hTERT, S. mediterranea, G. tigrina and the liver fluke Opisthorchis viverrini (Trematoda Class) are shown. Purple triangles indicate conserved intron positions (relative to hTERT intron positions) and green triangles indicate Platyhelminthes-specific introns. Genomic sequence data is not available for other planarian species. Therefore, putative TERT exon-intron boundaries were annotated based on Smed_TERT data and confirmed by AS variant analyses of each species