Low-level DNA exchanges in normal human and xeroderma pigmentosum cells after UV irradiation.

We have used a T4 endonuclease V assay method for UV-induced pryrimidine dimers in cellular DNA in vivo to obtain evidence for recombinational DNA exhanges after UV irradiation of normal human and Xeroderma pigmentosum (XP) cells. Our data indicate that the endonuclease-sensitive sites in excision-defective XP cells are removed very slowly from the irradiated parental strands and appear concomitantly in daughter strands newly synthesized during post-UV incubation. In the defective XP cells, the extent of appearance of sensitive sites in daughter strands synthesized during a period of 24 h after 10 J/m2 appears to be small, probably less than 15% of the initial number of sensitive sites detected in cellular parental strands. Demonstration of such exchanges between normal-density parental and 5-bromodeoxyuridine-labeled daughter strands by alkaline CsCl isopycnic centrifugation was unsuccessful. Further, the extent is much lower in normal human cells because of their efficient excision repair of the dimers before and after exchanges than in the defective XP cells.