MORC3 represses a tandem repeat enhancer to regulate interferon

To counteract viral antagonism, MORC3 represses a noncanonical pathway of type-I-interferon (IFN) such that viral inhibition of MORC3 triggers (>10,000-fold) IFN induction. How MORC3 represses this pathway, and why IFN induction upon MORC3 loss is so potent without canonical IRF3/7 transcription factors, is unknown.

Here, we show that MORC3 restricts chromatin accessibility at tandem repeat elements harboring up to 61 homotypic transcription factor motifs. One such element becomes a potent enhancer of IFNB1 upon MORC3 loss. Its motif cluster contains 45 PU.1 binding sites and is necessary and sufficient for MORC3-mediated repression and enhancer activity upon MORC3 loss.