Abstract
Telomerase RNA (TR) carries the template for synthesis of telomere DNA and provides a scaffold for telomerase assembly. Fungal TRs are long and have been compared to higher eukaryotes, where they show considerable diversity within phylogenetically close groups. TRs of several Saccharomycetaceae were recently identified, however, many of these remained uncharacterised in the template region. Here we show that this is mainly due to high variability in telomere sequence. We predicted the telomere sequences using Tandem Repeats Finder and then we identified corresponding putative template regions in TR candidates. Remarkably long telomere units and the corresponding putative TRs were found in Tetrapisispora species. Notably, variable lengths of the annealing sequence of the template region (1–10 nt) were found. Consequently, species with the same telomere sequence may not harbour identical TR templates. Thus, TR sequence alone can be used to predict a template region and telomere sequence, but not to determine these exactly. A conserved feature of telomere sequences, tracts of adjacent Gs, led us to test the propensity of individual telomere sequences to form G4. The results show highly diverse values of G4-propensity, indicating the lack of ubiquitous conservation of this feature across Saccharomycetaceae.
Highlights
Yeast telomeric repeat sequence (TG1–3 where more than 80% is TGTGGG)was first described in Saccharomyces cerevisiae in two back-to-back papers in Nature in 198421,22
In Saccharomyces cerevisiae, the real template is formed by 5′-CAccacacccacacACA32, while the predicted template region obtained in our approach is narrower
Tandem Repeats Finder analysis (TRFi) is a reference free alignment-based algorithm which can be set by several parameters, including score for matches, mismatches, and gaps[42], more details about our setup are described in Methods
Summary
Yeast telomeric repeat sequence (TG1–3 where more than 80% is TGTGGG)was first described in Saccharomyces cerevisiae in two back-to-back papers in Nature in 198421,22. The length of the whole telomerase RNA is highly variable across organisms, ranging from 150 to more than 2000 nucleotides[29,30], with the human sequence of 451 nt[31] or baker’s yeast of 1158 nt[32,33] as examples The reasons for this diversity are unknown, it may be related to structural rather than encoding functions of TRs. In the case of yeasts, several structural features were identified, such as the Ku binding hairpin, the Est[1] binding site, the pseudoknot, the three-way junction and the SM binding s ite[34] (Fig. 1). We compare propensities of the identified telomere candidates to form a guanine quadruplex (G4; G-quadruplex), and we discuss the significance of motif length in relation to telomerase processivity
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