We have tested the two-event model for UV mutagenesis producing class 2 suppressor mutations at glutamine tRNA genes in Escherichia coli. In the model used, the induction/indexing lesion is any type of pyrimidine dimer and the premutational photoproduct at the target site is a cytosine-containing dimer. Specific mutation-frequency responses were analyzed under conditions in which the ratio of thymine-thymine dimers to cytosine-containing dimers was modified by using 313-nm light and 0.0%, 0.1%, or 0.2% acetophenone. Changes observed in the production of class 2 suppressor mutations were consistent with the model and suggested that the G X C leads to A X T transitions responsible for class 2 suppressor mutations are targeted by cytosine-containing pyrimidine dimers at the mutational sites.