Polynucleotide Phosphorylase Stops Replication Restart by Degrading the RNA within R-loops during Replication-Transcription Conflicts

DNA replication and transcription machineries function simultaneously, on the same template, leading to conflicts between the two. During conflicts, the nascent mRNAs can hybridize to their template strands, generating RNA:DNA hybrids, which reside within nucleic acid structures known as R-loops. Accumulation of R-loops can lead to severe replication fork stalling, eventually leading to cell death. In bacteria, there are well-known replication restart mechanisms that should rescue these stalled replication forks, and yet, during conflicts, this process is inhibited. Additionally, in vitro work has shown that RNA:DNA hybrids are substrates for replication restart. Here, we discovered that the highly conserved exonuclease Polynucleotide Phosphorylase (PNPase in Bacillus subtilis) prevents replication restart from RNA:DNA hybrids, specifically at regions of severe conflicts where R-loops accumulate. Our in vitro data show that PNPase binds RNA:DNA hybrids and digests the RNA in these structures. Consistently, we found that in vivo, PNPase binds to conflict regions and reduces R-loop levels. To our knowledge, the only other class of enzymes known to digest the RNA from hybrids are RNases H. Our findings identify PNPase as a new enzyme that can perform a similar function. Importantly, our data show that PNPase inhibits replication restart from R-loops and its absence allows for replication restart from conflict regions. We also observed that PNPase activity reduces mutations, suggesting that replication restart from RNA:DNA hybrids is highly mutagenic. We propose that PNPase acts as a safeguard against mutagenic replication restart from RNA:DNA hybrids within R-loops by digesting the RNA which could re-initiate replication.