Mod(mdg4) variants repress telomeric retrotransposon Het-A by blocking subtelomeric enhancers

Telomeres in Drosophila are composed of sequential non-LTR retrotransposons: Het-A, TART, TAHRE. Although their expression is highly dynamic, molecular mechanism governing these retrotransposons is poorly understood. Here, we show that specific splice variants of Mod(mdg4) act as repressors of Het-A by blocking subtelomeric enhancers in ovarian somatic cells. We found that a variant, Mod(mdg4)-N, can repress Het-A expression most efficiently among variants. Mod(mdg4)-N mutant flies show elevated Het-A expression and female sterility. Further investigation revealed that Mod(mdg4)-N-binding subtelomeric sequences exhibit enhancer-blocking activity, and recruitment of RNA polymerase II (Pol II) on subtelomeres by Mod(mdg4)-N is essential for this enhancer-blocking. Moreover, Mod(mdg4)-N functions to form chromatin boundaries of higher-order chromatin conformation but this mechanism is independent with its Pol II recruitment activity observed at telomeres/subtelomeres. This study provides an unexpected link between enhancer-blocking and telomere regulation, and two different molecular mechanisms exhibited by an insulator protein to orchestrate precise gene expression. Overall design: Micro-C XL, RNA levels and Pol II occupancy in OSC depleted of Mod(mdg4).Localization of Ty1-tagged Mod(mdg4) isoforms. RNA levels in Mod(mdg4)-N mutant ovaries.