Nuclear and Mitochondrial DNA Synthesis in Gamma Ray-Resistant and -Sensitive Slime Mold Amebas

Uptake of [ methyl- 3H]thymidine label from Escherichia coli 15T - into the DNA of Dictyostelium discoideum has been measured in control and [ 60Co]-gamma-irradiated cells of the resistant strain NC-4 (D 10, colony-forming survival = 300 krad) and two sensitive daughter strains, γs-18 (D 10 = 75 krad) and γs-13 (D 10 = 4 krad). Nuclear (n) and mitochondrial (m) DNA were resolved by isopycnic CsCl gradients. The uptake of label into n-DNA during the immediate postirradiation period was selectively inhibited by irradiation, compared with uptake into m-DNA. For all three strains, the gamma ray dose to reduce the uptake into n-DNA to 37% of the control during the first hour after irradiation was 3 krad, whereas for uptake into m-DNA it was 75 krad. After the initial dose- and strain-dependent lag, uptake into n-DNA resumed. γs-18 showed longer lags in n-DNA synthesis and cell division than did NC-4. γs-13 resumed n-DNA synthesis and cell division after slightly shorter lags than for NC-4. The early postlag uptake into n-DNA in this strain was almost at the control rate and was accompanied by division until the cell number had nearly doubled. The rate of label uptake then declined, division stopped, and gradual cell lysis ensued. The postdelay response of γs-13 was almost independent of dose in the range of 10–100 krad. The response of γs-18 in these and earlier experiments is consistent with the view-point that it is sensitive because of a decreased rate of repair of DNA damage. However, the basis for the sensitivity of γs-13 seems to be more complex. This strain undergoes a premature but short-lived burst of n-DNA synthesis and division for what appears to be about one round of replication. Replication then ceases, even at very low doses, leading to greatly reduced probability of survival.