Induction of c-mutations in extracellular phage lambda by gamma-rays.

Mutagenic action of 60Co gamma-rays on extracellular phages lambda red1 and lambda red113 after irradiation in 4% nutrient broth in the absence or in the presence of 0.1 M cysteamine or in dried samples was studied. The yield of c mutations was almost independent of the repair genotype of the host cells (uvrA6, polA1, recA13, lexA102, uvrE502, uvrD3 or xthA9), of the phage Red function and of the conditions of gamma-irradiation and was 1 . 10(-12) per base pair and 1 rad. When the SOS-repair system of the host cells was induced by moderate UV irradiation, the yield of c-mutations was drastically enhanced in phage irradiated in broth, but not in phage irradiated in the dried state. These data allow us to suppose that the direct action of gamma-rays induces, in phage DNA, premutational lesions that are fixed into mutations by replication. On the other hand after gamma-irradiation in broth, when indirect radiation effects are only partially suppressed, about 85% of premutational lesions are converted into mutations by means of the inducible, errorprone SOS-repair system.