FIGURE 4.

Repeat expansion involves concerted actions of pol β and pol δ. Hairpin retention/removal assays were performed in reactions containing a purified polymerase system, as indicated, in the presence of PCNA and RFC. D402A is a mutant pol δ defective in 3′-5′ proofreading activity. Products were digested with BsrBI and analyzed by Southern blot analysis using a ³²P-labeled probe (purple or blue bar) at the indicated location. The blue and red types/lines represent CAG and CTG repeats, respectively. A, Southern blot analyses showing hairpin retention and removal by pol β and/or pol δ. B, DNA sequence of the shortened band shown in A. C, proposed substrate structure that leads to the shortened DNA synthesis product. The arrow in B indicates the location where the 25-nt green fragment in C (also see Fig. 2C) is deleted in the shortened product.