Multiple, independent components of ultraviolet radiation mutagenesis in Escherichia coli K-12 uvrB5

Reversion systems involving the lacZ53(amber) and leuB19)missense) mutations were developed to study the mutant frequency response of Escherichia coli K-12 uvrB5 (SR250) to ultraviolet radiation (254 nm). A one-hit mutant frequency response was discernible at ultraviolet radiation fluences below approximately 0.5 J m-2. At higher fluences the overall mutant frequency response could be resolved into one-hit and two-hit components. A new interpretation of the published data on E. coli K-12 indicates that SR250 is not unique in this respect. In addition, the Lac reversion system showed enhanced mutagenesis after ultraviolet radiation fluences of approximately 1 to 3 J m-2, whereas the Leu reversion system did not. We conclude that the complex ultraviolet radiation mutant frequency response curves for E. coli K-12 uvrB5 were the result of three independent mutagenic processes for Lac reversion and two for Leu reversion.