The block of DNA polymerase delta strand displacement activity by an abasic site can be rescued by the concerted action of DNA polymerase beta and flap endonuclease 1.

Abasic (AP) sites are very frequent and dangerous DNA lesions. Their ability to block the advancement of a replication fork has been always viewed as a consequence of their inhibitory effect on the DNA synthetic activity of replicative DNA polymerases (DNA pols). Here we show that AP sites can also affect the strand displacement activity of the lagging strand DNA pol d, thus preventing proper Okazaki fragment maturation. This block can be overcome through a polymerase switch, involving the combined physical and functional interaction of DNA pol ss and Flap- endonuclease 1. Our data identify a previously unnoticed deleterious effect of the AP site lesion on normal cell metabolism and suggest the existence of a novel repair pathway which might be important in preventing replication fork stalling.