Human tumor strains defective in the repair of alkylated DNA fail to regenerate rapidly-sedimenting nucleoids after N-methyl-N'-nitro-N-nitrosoguanidine treatment.

Upon treatment with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), human cell strains characterized as either proficient or defective both in repair of alkylation-damaged DNA and in supporting the growth of MNNG-treated adenovirus (Mer+ and Mer- phenotypes (1,2)), all underwent a rapid relaxation of nucleoid DNA, as judged by sedimentation in 15--30% neutral sucrose gradients. DNA in the repair-proficient Mer+ strains (normal fibroblast and tumor) was restored to the rapidly-sedimenting (control) form within 2--4 h after the removal of MNNG. In contrast, nucleoid DNA of the repair-deficient Mer- tumor strains remained slowly-sedimenting even after 48 h of incubation. The delayed recovery of Mer- nucleoid DNA was specific for MNNG damage, since after u.v. irradiation, to which Mer+ and Mer- strains are equally resistant (2), all cell lines tested underwent DNA relaxation within the first hour after irradiation (3 J/m2) and regenerated rapidly-sedimenting nucleoids within 4--6 h of repair incubation.