A LOW, ADAPTIVE DOSE OF GAMMA-RAYS REDUCED THE NUMBER AND ALTERED THESPECTRUM OF S1(-) MUTANTS IN HUMAN-HAMSTER HYBRID A(L) CELLS

We examined the effects of a low, adaptive dose of Cs-137-gamma-irradi ation (0.04 Gy) on the number and kinds of mutants induced in A(L) hum an-hamster hybrid cells by a later challenge dose of 4 Gy. The yield o f S1(-) mutants was significantly less (by 53%) after exposure to both the adaptive and challenge doses compared to the challenge dose alone . The yield of hprt(-) mutants was similarly decreased. Incubation wit h cycloheximide (CX) or 3-aminobenzamide largely negated the decrease in mutant yield. The adaptive dose did not perturb the cell cycle, was not cytotoxic, and did not of itself increase the mutant yield above background. The adaptive dose did, however, alter the spectrum of S1(- ) mutants from populations exposed only to the adaptive dose, as well as affecting the spectrum of S1(-) mutants generated by the challenge dose. The major change in both cases was a significant increase in the proportion of complex mutations compared to small mutations and simpl e deletions.