SURVIVAL, SPORE FORMATION AND EXCISION REPAIR OF UV-IRRADIATED DEVELOPING CELLS OF DICTYOSTELIUM DISCOIDEUM NC-4

Abstract— The synchronously developing aggregates of the cellular slime mold, Dictyostelium discoideum NC-4, were disaggregated into individual cells and irradiated with 254 nm UV light at preaggregation (0h), late interphase (6h), late aggregation (12 h), and preculmination (18 h). When assayed for replica-tive ability (colony formation), the developing cells at 0, 6, 12, and 18h showed the same sensitivity as vegetative cells; the 10% survival dose (D₁₀) was 160 J/m². The spores were more sensitive, with D₁₀ of 70 J/m². Excision repair of the nuclear DNA of the developing cells was studied by alkaline sucrose gradients. UV-induced single-strand breakage and rejoining of the DNA occurred to the same final extent in the cells from the 0, 6, 12 and 18 h stages of development, but a longer time was required for the completion of rejoining at the later stages (for example, at 54 J/m², 6.6 h for preculmination cells, 3.3 h for preaggregation cells). When the cells irradiated at various stages were required to redevelop, as measured by the relative numbers of spores produced, their sensitivity for completing this development increased the later the stage from which they were taken. The D₁₀s for spore production were 200, 130, 100 and 70 J/m² for cells at the 0, 6, 12 and 18 h stages, respectively. The fractional viability among the spores that appeared after this treatment was the same independent of the stage at which the cells were irradiated; the D₁₀ for this viability was 160 J/m², the same as if the cells had been plated immediately with no intervening developmental sequence. We conclude that DNA excision repair as related to replicative ability is retained at all stages of development; however, development seems independent of replicative ability and depends upon DNA and/or non-DNA damage in a more complex way.