RECQ1 is required for cellular resistance to replication stress and catalyzes strand exchange on stalled replication fork structures.

RECQ1 is the most abundant of the five human RecQ helicases, but little is known about its biological significance. Recent studies indicate that RECQ1 is associated with origins of replication, suggesting a possible role in DNA replication. However, the functional role of RECQ1 at damaged or stalled replication forks is still unknown. Here, for the first time, we show that RECQ1 promotes strand exchange on synthetic stalled replication fork-mimicking structures and comparatively analyze RECQ1 with the other human RecQ helicases. RECQ1 actively unwinds the leading strand of the fork, similar to WRN, while RECQ4 and RECQ5β can only unwind the lagging strand of the replication fork. Human replication protein A modulates the strand exchange activity of RECQ1 and shifts the equilibrium more to the unwinding mode, an effect also observed for WRN. Stable depletion of RECQ1 affects cell proliferation and renders human cells sensitive to various DNA damaging agents that directly or indirectly block DNA replication fork progression. Consequently, loss of RECQ1 activates DNA damage response signaling, leads to hyper-phosphorylation of RPA32 and activation of CHK1, indicating replication stress. Furthermore, depletion of RECQ1 leads to chromosomal condensation defects and accumulation of under-condensed chromosomes. Collectively, our observations provide a new insight into the role of RECQ1 in replication fork stabilization and its role in the DNA damage response to maintain genomic stability.