ALTERATION OF IRRADIATED SHUTTLE VECTOR PROCESSING BY EXPOSURE OF HUMAN LYMPHOBLAST HOST-CELLS TO SINGLE OR SPLIT GAMMA-RAY DOSES

The repair of damaged DNA by mammalian cells exposed to single or spli t doses of radiation was probed with shuttle vector pZ189. Human lymph oblast hosts who received a single 120 cGy dose 2h before transfection with 2500 cGy-damaged pZ189 yielded a two-fold higher frequency of pr ogeny plasmids with mutations in their supF-tRNA target genes than did unirradiated host cells. Delaying transfection for 12 h, however, red uced the mutation frequency by half versus unirradiated controls. Plas mid survival was also affected by the time between host cell irradiati on and transfection. Splitting doses of 50-500 cGy into two equal frac tions separated by 4 h lowered mutation frequency and increased plasmi d survival compared with equivalent acute doses; increasing the interv al between dose fractions to 8 h, however, lowered plasmid survival co mpared with acute doses. Sequence analyses of the target gene in mutan t plasmids revealed increased multiple-base substitution mutations amo ng progenies recovered from irradiated hosts, indicating enhanced exci sion repair. These findings support modulation of mammalian cell DNA r epair by ionizing radiation, disclose the transient nature of the effe ct of radiation on DNA repair, and demonstrate a quantitative differen ce in the effectiveness of single and split doses.