Induction of lambda prophage and of mutations to streptomycin resistance in separate small fractions of a lysogenic derivative of Escherichia coli B/r by very low doses of ultraviolet light.

The number of induced mutations to streptomycin resistance is compared at doses of ultraviolet (UV) light between 0.2 and 6.4 J/m2 in a Uvr- (excision-deficient) derivative of E. coli B/r, strain WU, and in its lambda lysogen, strain WU(lambda). At UV doses up to about 1 J/m2, which converts about 5% of the lysogenic population into enfective centers, no difference is observed in the number of mutations to streptomycin resistance produced by the two strains. It is concluded that the capacity to produce UV-induced mutations is not coupled with lysis due to the induction of lambda prophage at low doses of UV radiation. At UV doses above 1 J/m2, the number of mutatants detected in the lysogenic strain decreases appreciably compared to the number detected in the nonlysogen, and is only about 10% as high at UV doses of 3 J/m2 and higher, doses which cause maximal induction of prophage. The results are compatible with the operation of a common "all-or-none" induction signal resulting in expression of UV-inducible functions at high UV doses, but not at low doses.