The BRCA1 BRCT promotes antisense RNA production and double-stranded RNA formation to suppress ribosomal R-loops.

R-loops, or RNA:DNA hybrids, can induce DNA damage, which requires DNA repair factors including breast cancer type 1 susceptibility protein (BRCA1) to restore genomic integrity. To date, several pathogenic mutations have been found within the tandem BRCA1 carboxyl-terminal (BRCT) domains that mediate BRCA1 interactions with proteins and DNA in response to DNA damage. Here, we describe a nonrepair role of BRCA1 BRCT in suppressing ribosomal R-loops via two mechanisms. Through its RNA binding and annealing activities, BRCA1 BRCT facilitates the formation of double-stranded RNA between ribosomal RNA (rRNA) and antisense-rRNA (as-rRNA), hereby minimizing rRNA hybridization to ribosomal DNA to form R-loops. BRCA1 BRCT also promotes RNA polymerase I-dependent transcription of as-rRNA to enhance double-stranded rRNA (ds-rRNA) formation. In addition, BRCA1 BRCT-mediated as-rRNA production restricts rRNA maturation in unperturbed cells. Hence, impairing as-rRNA transcription and ds-rRNA formation due to BRCA1 BRCT deficiency deregulates rRNA processing and increases ribosomal R-loops and DNA breaks. Our results link ribosomal biogenesis dysfunction to BRCA1-associated genomic instability.