Transcriptional repression facilitates RNA:DNA hybrid accumulation at DNA double-strand breaks.

RNA:DNA hybrids accumulate at DNA double-strand breaks (DSBs) and were shown to regulate homologous recombination repair. The mechanism responsible for the formation of these non-canonical RNA:DNA structures remains unclear although they were proposed to arise consequently to RNA polymerase II or III loading followed by DSB-induced de novo transcription at the break site. Here, we found no evidence of RNA polymerase recruitment at DSBs. Rather, strand-specific R-loop mapping revealed that RNA:DNA hybrids are mainly generated at DSBs occurring in transcribing loci, from the hybridization of pre-existing RNA to the 3' overhang left by DNA end resection. We further identified the H3K4me3 reader spindlin 1 and the transcriptional regulator PAF1 as factors promoting RNA:DNA hybrid accumulation at DSBs, through their role in mediating transcriptional repression in cis to DSBs. Altogether, we provide evidence that RNA:DNA hybrids accumulate at DSBs occurring in transcribing loci as a result of DSB-induced transcriptional shut down.