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

More than 98% of the mammalian genome is noncoding and approximately half is made up of transposable elements. One of the most abundant is the short interspersed nuclear elements (SINE). Among the million copies of SINEs, B2 accounts for ~350,000 in the mouse genome and have garnered special interest because of emerging roles in gene regulation. Our recent work demonstrated that B2 RNA normally binds stress genes to retard transcription elongation. Though epigenetically silenced, B2s become massively upregulated during thermal stress. Specifically, an interaction between B2 RNA and the Polycomb protein, EZH2, results in cleavage of B2 RNA, release of B2 RNA from RNA Polymerase II, and activation of the 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 RNA. Contact with EZH2 accelerates cleavage rate by >100-fold, suggesting that EZH2 may assist cleavage as an RNA chaperone. Modification-interference analysis demonstrate that phosphorothioate changes at A and C nucleotides can substitute for EZH2’s function. B2 mutagenesis indicate that nucleotides around positions 45-55 and 100-101 are critical for cleavage reaction. Finally, we demonstrate that another family of SINEs, the ALU elements produce also a self-cleaving RNA. ALUs are intrinsically more auto-reactive than B2s. We propose that the B2/ALU SINEs are a new class of ribozymes whose activity is accelerated by EZH2. Sample from HeLa cells was subjected to heat shock and subsequently RNA was extracted and submitted to short-RNA-seq.