Telomerase RNA contains a template for synthesizing telomeric DNA and has been proposed to act as a flexible scaffold for holoenzyme protein subunits in the RNP. In Saccharomyces cerevisiae, the telomerase RNA, TLC1, is bound by the Sm₇ protein complex, which is required for stabilization of the predominant, non-polyadenylated (poly(A)-) TLC1 isoform. However, it remains unclear (1) whether Sm₇ retains this function when its binding site is repositioned within TLC1, as has been shown for other TLC1-binding telomerase subunits, and (2) how Sm₇ stabilizes poly(A)- TLC1. Here, we first show that Sm₇ can stabilize poly(A)- TLC1 even when its binding site is repositioned via circular permutation to several different positions within TLC1, further supporting the conclusion that the telomerase holoenzyme is organizationally flexible. Next, we show that when an Sm site is inserted 5' of its native position and the native site is mutated, Sm₇ stabilizes shorter forms of poly(A)- TLC1 in a manner corresponding to how far upstream the new site was inserted, providing strong evidence that Sm₇ binding to TLC1 controls where the mature poly(A)- 3' is formed by directing a 3'-to-5' processing mechanism. In summary, our results show that Sm₇ and the 3' end of yeast telomerase RNA comprise an organizationally flexible module within the telomerase RNP and provide insights into the mechanistic role of Sm₇ in telomerase RNA biogenesis.

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Journal Article

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Hass EP, Zappulla DC

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