Abstract

About ten years ago (Demerec, 1946), it was reported that the yield of bacteriophage-resistant mutants induced by ultraviolet light in Escherichia coli increases as the irradiated population is allowed to multiply, until ten or twelve divisions are accomplished. Since that time, it has been observed repeatedly that maximal yields of many kinds of induced bacterial mutants, including auxotrophs (Davis, 1948), prototrophs (Davis, 1950; Demerec and Cahn, 1953) and streptomycin-independent variants (Labrum, 1953), are obtained only under conditions permitting more or less extensive periods of cell division after exposure to ultraviolet light. This “delayed appearance” of induced bacterial mutants has become one of the hardy perennials among the unsolved problems of microbial genetics.