Topoisomerase I prevents transcription-replication conflicts at transcription termination sites.

R-loops have both positive and negative impacts on chromosome functions. To identify toxic R-loops, we mapped RNA:DNA hybrids, markers of replication fork stalling and DNA double-strand breaks along the human genome. This analysis indicates that transient replication fork pausing occurs at the transcription termination sites of highly expressed genes enriched in R-loops and prevents head-on conflicts with transcription, in a topoisomerase I-dependent manner.