Sequence specificity of mutagenesis in the cI gene of bacteriophage lambda

Studies of DNA base sequence alterations have shown that for every agent the mutagenic process is specific with respect to the types of base changes induced and the location of the changes in the DNA. Analysis of the types of mutations produced by mutagenic agents can provide insight into the mechanism of mutation and can suggest which DNA lesions may be involved in the actual mutagenic event. We have developed a system for the analysis of chemically induced base sequence alterations in the cI repressor gene of bacteriophage lambda using DNA sequencing techniques. To illustrate the utility of this type of analysis, we present the results obtained with ultraviolet light (UV). Irradiation of target DNA with UV alone, or UV followed by photoreactivating light (which removes dimers), produces mostly transitions at pyrimidine-pyrimidine sites. Conversely, irradiation with 313 nm light plus acetophenone (which produces only thymine dimers) produces mostly transversions at low efficiency. This and other evidence suggests that the actual premutagenic UV lesion in E. coli may not be pyrimidine-pyrimidine dimers, but rather pyr(6-4)pyo photoproducts.