Correction to: Nature Structural & Molecular Biology https://doi.org/10.1038/s41594-020-0390-z, published online 10 March 2020.

In the version of this article initially published, in the caption for Extended Data Fig. 4i, Extended Data Fig. 1j was incorrectly cited as “Supplementary fig. 1j”. The error has been corrected in the HTML and PDF versions of the article.

In the supplementary information originally posted for this article, Supplementary Table 1 did not accurately reflect the oligonucleotide sequences used in this study. The last three sequences in the table, “polyA LNA CO”, “polyA PO” and “polyA EO” were not used in this study. These have been replaced with the sequences that were used: “TERC LNA CO”, TERC PO” and “TERC EO”. The errors have been corrected online.