Aberrant transcription‐associated RNA : DNA hybrid (R‐loop) formation often causes catastrophic conflicts during replication, resulting in DNA double‐strand breaks and genomic instability. Preventing such conflicts requires hybrid dissolution by helicases and/or RN ase H. Little is known about how such helicases are regulated. Herein, we identify DDX 5, an RGG / RG motif‐containing DEAD ‐box family RNA helicase, as crucial player in R‐loop resolution. In vitro , recombinant DDX 5 resolves R‐loops in an ATP ‐dependent manner, leading to R‐loop degradation by the XRN 2 exoribonuclease. DDX 5‐deficient cells accumulate R‐loops at loci with propensity to form such structures based on RNA : DNA immunoprecipitation ( DRIP )‐ qPCR , causing spontaneous DNA double‐strand breaks and hypersensitivity to replication stress. DDX 5 associates with XRN 2 and resolves R‐loops at transcriptional termination regions downstream of poly(A) sites, to facilitate RNA polymerase II release associated with transcriptional termination. Protein arginine methyltransferase 5 ( PRMT 5) binds and methylates DDX 5 at its RGG / RG motif. This motif is required for DDX 5 interaction with XRN 2 and repression of cellular R‐loops, but not essential for DDX 5 helicase enzymatic activity. PRMT 5‐deficient cells accumulate R‐loops, resulting in increased formation of γH2 AX foci. Our findings exemplify a mechanism by which an RNA helicase is modulated by arginine methylation to resolve R‐loops, and its potential role in regulating transcription.

Cockayne syndrome group B (CSB) protein has been implicated in the repair of a variety of DNA lesions that induce replication stress. However, little is known about its role at stalled replication forks. Here, we report that CSB is recruited to stalled forks in a manner dependent upon its T1031 phosphorylation by CDK. While dispensable for MRE11 association with stalled forks in wild-type cells, CSB is required for further accumulation of MRE11 at stalled forks in BRCA1/2-deficient cells. CSB promotes MRE11-mediated fork degradation in BRCA1/2-deficient cells. CSB possesses an intrinsic ATP-dependent fork reversal activity in vitro, which is activated upon removal of its N-terminal region that is known to autoinhibit CSB’s ATPase domain. CSB functions similarly to fork reversal factors SMARCAL1, ZRANB3 and HLTF to regulate slowdown in fork progression upon exposure to replication stress, indicative of a role of CSB in fork reversal in vivo. Furthermore, CSB not only acts epistatically with MRE11 to facilitate fork restart but also promotes RAD52-mediated break-induced replication repair of double-strand breaks arising from cleavage of stalled forks by MUS81 in BRCA1/2-deficient cells. Loss of CSB exacerbates chemosensitivity in BRCA1/2-deficient cells, underscoring an important role of CSB in the treatment of cancer lacking functional BRCA1/2.

R-loops are three-stranded structures consisting of a DNA/RNA hybrid and a displaced DNA strand. The regulatory factors required to process this fundamental genetic structure near double-strand DNA breaks (DSBs) are not well understood. We previously reported that cellular depletion of the ATP-dependent DEAD box RNA helicase DDX5 increases R-loops genome-wide causing genomic instability. In this study, we define a pivotal role for DDX5 in clearing R-loops at or near DSBs enabling proper DNA repair to avoid aberrations such as chromosomal deletions. Remarkably, using the non-homologous end joining reporter gene (EJ5-GFP), we show that DDX5-deficient U2OS cells exhibited asymmetric end deletions on the side of the DSBs where there is overlap with a transcribed gene. Cross-linking and immunoprecipitation showed that DDX5 bound RNA transcripts near DSBs and required its helicase domain and the presence of DDX5 near DSBs was also shown by chromatin immunoprecipitation. DDX5 was excluded from DSBs in a transcription- and ATM activation-dependent manner. Using DNA/RNA immunoprecipitation, we show DDX5-deficient cells had increased R-loops near DSBs. Finally, DDX5 deficiency led to delayed exonuclease 1 and replication protein A recruitment to laser irradiation-induced DNA damage sites, resulting in homologous recombination repair defects. Our findings define a role for DDX5 in facilitating the clearance of RNA transcripts overlapping DSBs to ensure proper DNA repair.