FIG. 1.

Experimental system. (A) The recombination substrate consists of 1,137-bp direct repeats [700-bp GFP and 437-bp poly(A) signal sequences] separated by 225 bp and SV40 promoter-driven neo. The XhoI frameshift mutation (black bar) inactivates the upstream GFP. HR includes gene conversion of the XhoI site without an associated crossover or deletion of one GFP copy by crossover or single-strand annealing. PCR primers targeted to T3 and T7 promoter sequences amplify GFP in both types of recombination products. (B) RT-PCR analysis of recombination substrate in RKO derivatives. RT-PCR products from RKO derivatives (with or without XhoI) are indicated; control (nontransfected) RKO cells are shown in lane C, and size markers (M) are ΦX-174/HaeIII. (C) Western analysis of p53 and p21 in RKO36 cells with or without 8-Gy X-ray exposure, with β-actin loading controls below.