Sugimura et al. reveal that one DNA-fixing protein helps create this delay by fending off another DNA repair enzyme.
Damage that severs both strands of a DNA molecule creates what's called a double-stranded break, or DSB. When the cell is copying its DNA, DSBs can cause trouble for replication forks, the spots where the double helix unzips so enzymes can copy the two strands. If a replication fork runs into a double-stranded break, researchers think that DNA duplication usually slows or stops. Previous work has revealed that the protein PARP-1 helps correct single-stranded DNA breaks. But whether it helps heal the DSBs that arise during replication was unclear.
Sugimura et al. tested whether PARP-1 inhibits the movement of replication forks along the DNA molecule. In vitro studies suggested that it did, but in vivo evidence was lacking. In human cells, replication fork speed doubled after addition of RNAi that targeted PARP-1, the scientists found. A PARP-1 inhibitor had a similar effect.
Cells deploy two mechanisms for mending DSBs—non-homologous end-joining (NHEJ) and homologous recombination (HR). The team found that the replication forks decelerate in cells that can't perform NHEJ, but not in cells where HR is defective. That result suggests that replication fork slowing occurs because HR is at work repairing the break. PARP-1 helps delay the fork's progress, the researchers determined, by allowing HR proteins access to the DNA but obstructing the protein Ku70, which is necessary for NHEJ and can block HR.