FIG. 7.

Possible mechanisms for DSB formation from closely spaced lesions. When there is coordination of BER enzymes (WT), closely opposed base lesions can be repaired one by one. Once the first lesion is processed by glycosylases followed by AP endonucleases, strand displacement and 5′-flap removal are quickly finished to seal the nick generated during repair before starting nicking of the second lesion, thus avoiding the formation of DSBs. When there is a defect in the coordination of BER enzymes (pol32 rad27-p mutant), the strand displacement and/or 5′-dRP end flap removal might be slowed, resulting in increased time and space for the generation of a second nick 5′ of the closely opposed lesion, thereby leading to DSB formation. In this case, widely separated intermediate SSBs (e.g., >30 nt apart) might, in essence, be brought together to form DSBs. Alkylated bases are shown as filled red circles. 5′-dRP is shown as filled blue circles. Arrowheads on DNA strands correspond to 3′ ends. Red dashed lines represent repair-associated DNA synthesis, and red solid lines represent newly synthesized DNA. Note that this figure shows only one orientation of the two closely opposed SSBs, where DNA synthesis and strand displacement intermediates face each other. DSBs would not be generated if DNA synthesis and strand displacement at closely opposed SSBs proceeded in the opposite orientation.