pLrec plasmid DNA was introduced into Chinese hamster cell lines defective in DNA repair (Pa13, Pb4, xrs6) and the parental CHO-K1 cell line. Clones with stable integrated plasmid were isolated and integrity of the incorporated DNA was checked by Southern blotting and PCR. Intact pLrec plasmid was found in 11% of the isolated CHO-K1 clones. In contrast, intact plasmid copies were found in 68.8%, 50%, 35.7% clones of Pa13, Pb4 and xrs6 cell lines, respectively. We conclude that certain DNA repair defects may facilitate intact plasmid integration. The higher frequency of integration of the intact vector into the genome of cell lines defective in DNA repair as compared to the parental cell line points to two possibilities, not mutually exclusive: (1) these cells possess a mechanism that facilitates the process of plasmid incorporation and hence, plasmid DNA is incorporated into the genome before extrachromosomal recombination takes place; (2) the vector is inserted into a less recombination prone site in the genome
