FIGURE 3 Pol V can bypass an abasic site in a DNA substrate. (a) Primer 2 (P2) is added to convert the single-stranded DNA upstream of the lesion to duplex DNA. Primer 1 (P1), labeled with ³²P (orange ball), is added to allow observation of extension over the template lesion. (b) PAGE analysis of the reaction products. Only in the presence of RecA, UmuC, and UmuD′ is extension of primer 1 observed (lanes 5 and 6).