Nuclear DNA synthesis is blocked by UV irradiation in dictyostelium discoideum

Abstract

We have used a thymidine auxotroph of the simple eukaryote, Dictyostelium discoideum and alkaline sucrose gradients of isolated nuclei to study alterations in DNA synthesis following irradiation of replicating haploid cells with 254 nm UV light. Three responses were charcterized using pulse-chase protocols: (1) Lags in DNA synthesis as measured by the amount of label incorporated were 4, 9, and 20 h after 10, 50, and 200 J/m2. (2) The DNA synthesized during a 15-min pulse immediately after irradiation was of lower single strand molecular weight: 7, 3.5, and 3 × 106 dalton after 0, 50, and 200 J/m2. (3) The time required for maturation of the nascent DNA to full-sized strands of 2 × 108 dalton was 45−50 min for unirradiated cells, 3 h after 10 J/m2. The DNA of the irrafiated cells did not mature uniformly during these delays; instead, a period of no increase in size was followed by a rapid, nearly control rate of maturation. We conclude: (a) at least some UV lesions block elongation of relicons; (b) the elongation of the replicons and their subsequent joining to yield mature high molecular weight DNA occurs after most of the lesions are repaired; (c) the timing of the different aspects of recovery suggest that initiation of replication is also inhibited.