Endogenous DNA damage at sites of terminated transcripts.

DNA damage promotes mutations that fuel cancer, ageing and neurodegenerative diseases, but surprisingly, the causes and types of damage remain largely unknown. There are three identified mechanisms that damage DNA during transcription: collision of RNA polymerase (RNAP) with the DNA-replication machinery head-on and co-directionally, and R-loop-induced DNA breakage. Here we identify novel DNA damage reaction intermediates and uncover a fourth transcription-related source of DNA damage: endogenous DNA damage at sites of terminated transcripts. We engineered proteins to capture single-stranded DNA (ssDNA) ends with 3' polarity in bacterial and human cells. In Escherichia coli, spontaneous 3'-ssDNA-end foci were unexpectedly frequent, at one or more per cell division, and arose via two identifiable pathways, both of which were dependent on DNA replication. A pathway associated with double-strand breaks was suppressed by overexpression of replicative DNA polymerase (pol) III, suggesting competition between pol III and DNA damage-promoting proteins. Mapping of recurrent 3'-ssDNA-ends identified distinct 3'-ssDNA-end-hotspots, mostly unrelated to double-strand breaks, next to the 5'-CCTTTTTT transcription-terminator-like sequence. These 3'-ssDNA-termini coincide with RNA 3'-termini identified by DirectRNA sequencing or simultaneous 5' and 3' end RNA sequencing (SEnd-seq) and were prevented by a mutant RNAP that reads through terminators. Our findings reveal that transcription termination or pausing can promote DNA damage and subsequent genomic instability.