RNAi-dependent Polycomb repression controls transposable elements in Tetrahymena [small RNA-seq]. RNAi-dependent Polycomb repression controls transposable elements in Tetrahymena [small RNA-seq]

RNA interference (RNAi) and Polycomb repression play evolutionarily conserved and often coordinated roles in transcriptional silencing. Here we show that in the protozoan Tetrahymena thermophila, germ line-specific internally eliminated sequences (IES)—many related to transposable elements (TE)—are transcriptionally activated in mutants deficient in the RNAi-dependent Polycomb repression pathway. Mobilization of recently duplicated TE also dramatically increases in these mutants. Importantly, transcriptional silencing and activation of TE-related sequences are accompanied by switching between noncoding RNA (ncRNA) and mRNA production, which can be affected by co-transcriptional processing as well as RNAi and Polycomb repression. We posit that interplay between RNAi and Polycomb repression is a widespread phenomenon, whose ancestral role is epigenetic silencing of TE. Overall design: Small RNA-seq profiling of Tetrahymena thermophila cells (wild-type, DCL1KO, EZL1KO, and PDD1KO) at late conjugation (10 hours post-mixing).