We propose that both PACT and ADAR1 act as essential barriers against PKR, creating a threshold of tolerable levels to endogenous dsRNA in cells without activating PKR-mediated translation shutdown and cell death.

Simultaneous deletion of PACT and ADAR1 results in synthetic lethality, which can be fully rescued in PKR-deficient cells. This suggests PACT cooperates with ADAR1 to suppress PKR activation from self-dsRNAs in uninfected cells.

Our Alphafold 3 modeling analysis of PACT reveals the formation of a dimer that surrounds dsRNA, blocking access to both sides of the dsRNA and thereby preventing PKR activation.

We find that cells deficient for PACT hyperactivate PKR in response to several different RNA viruses, raising the question of why cells need to limit PKR activity? We show that it is critical to prevent PKR activation from self-dsRNA.

Our findings uncover the mechanisms behind replication fork collapse during replication catastrophe and show that APOBEC3B drives cell sensitivity caused by ATR and PARP inhibitors.

Additionally, we show that APE1-mediated DNA cleavage is the critical step for PARP1 trapping and hyperactivation in cells, regardless of how abasic sites are formed, likely through the generation of 5'-dRP on DNA.

PARP1 activation at the forks mediated by ATR inhibition recruits DNA repair factors such as XRCC1.

We find that ATR inhibition depletes RPA, exposing single-stranded DNA at replication forks to APOBEC3B. This triggers a cascade involving UNG2 creating abasic sites and APE1, which subsequently cleaves abasic sites, causing fork collapse and PARP1 activation.