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Proc Natl Acad Sci U S A. 2020 Jan 7;117(1):415-425. doi: 10.1073/pnas.1917190117. Epub 2019 Dec 23.

B2 and ALU retrotransposons are self-cleaving ribozymes whose activity is enhanced by EZH2.

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Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, MA 02115.
Department of Molecular Biology, Massachusetts General Hospital, Boston, MA 02114.
Department of Genetics, The Blavatnik Institute, Harvard Medical School, Boston, MA 02114.
Department of Molecular Biology, Massachusetts General Hospital, Boston, MA 02114;


Transposable elements make up half of the mammalian genome. One of the most abundant is the short interspersed nuclear element (SINE). Among their million copies, B2 accounts for ∼350,000 in the mouse genome and has garnered special interest because of emerging roles in epigenetic regulation. Our recent work demonstrated that B2 RNA binds stress genes to retard transcription elongation. Although epigenetically silenced, B2s become massively up-regulated during thermal and other types of stress. Specifically, an interaction between B2 RNA and the Polycomb protein, EZH2, results in cleavage of B2 RNA, release of B2 RNA from chromatin, and activation of thermal stress genes. Although an established RNA-binding protein and histone methyltransferase, EZH2 is not known to be a nuclease. Here, we provide evidence for the surprising conclusion that B2 is a self-cleaving ribozyme. Ribozyme activity depends on Mg+2 and monovalent cations but is resistant to protease treatment. However, contact with EZH2 accelerates cleavage rate by >100-fold, suggesting that EZH2 promotes a cleavage-competent RNA conformation. B2 modification-interference analysis demonstrates that phosphorothioate changes at A and C nucleotides can substitute for EZH2. B2 nucleotides 45 to 55 and 100 to 101 are essential for activity. Finally, another family of SINEs, the human ALU element, also produces a self-cleaving RNA and is cleaved during T-cell activation as well as thermal and endoplasmic reticulum (ER) stress. Thus, B2/ALU SINEs may be classified as "epigenetic ribozymes" that function as transcriptional switches during stress. Given their high copy numbers, B2 and ALU may represent the predominant ribozyme activity in mammalian cells.


ALU; B2; EZH2; SINE; ribozyme

[Available on 2020-06-23]

Conflict of interest statement

Competing interest statement: J.T.L. is a scientific cofounder of Translate Bio and Fulcrum Therapeutics, and also serves as an Adviser to Skyhawk Therapeutics.

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