Transcription near arrested DNA replication forks triggers ribosomal DNA copy number changes.

DNA copy number changes via chromosomal rearrangements or the production of extrachromosomal circular DNA. Here, we demonstrate that the histone deacetylase Sir2 maintains the copy number of budding yeast ribosomal RNA gene [ribosomal DNA (rDNA)] by suppressing end resection of DNA double-strand breaks (DSBs) formed upon DNA replication fork arrest in the rDNA and their subsequent homologous recombination (HR)-mediated rDNA copy number changes during DSB repair. Sir2 represses transcription from the regulatory promoter E-pro located near the fork arresting site. When Sir2 is absent, this transcription is stimulated but terminated by arrested replication forks. This transcription-replication collision induces DSB formation, DSB end resection and the Mre11-Rad50-Xrs2 complex-dependent DSB repair that is prone to chromosomal rDNA copy number changes and the production of extrachromosomal rDNA circles. Therefore, repression of transcription near arrested replication forks is critical for the maintenance of rDNA stability by directing DSB repair into the HR-independent, rearrangement-free pathway.