Repair of deoxyribonucleic acid in ultraviolet light-sensitive and -resistant Dictyostelium discoideum strains

Some responses of the cellular slime mold Dictyostelium discoideum to ultraviolet light (UV) irradiation were investigated by analyzing two aspects of deoxyribonucleic acid (DNA) excision repair in the vegetative cells: (i) the fate of thymine-containing dimers and (ii) the production and rejoining of single-strand breaks. Experiments were done with the parental, radiation-resistant NC-4 strain and with the radiation-sensitive gammas-13 strain. The majority (greater than 85%) of the thymine-containing dimers produced in both strains by an energy fluence of 100/Jm2 were removed from the acid-insoluble DNA fraction within the first 3 to 4 h of reincubation in the dark. Moreover, as measured by alkaline sucrose gradients, single-strand breaks appeared in the DNA of both NC-4 and gammas-13 irradiated cells very rapidly and at low temperatures. This was presumed to be a result of the incision (nicking) step of excision repair as performed by UV-specific endonuclease(s). In NC-4 the time required for dimer excision correlated with the sealing of breaks as well as with the UV-induced division delays. In gammas-13 the single-strand breaks were closed at a slower rate than in NC-4. However, this was not accompanied by more extensive division delays.