Frameshift mutagenesis by ultraviolet light effects of broth and caffeine in the post-irradiation plating medium

Ultraviolet-induced back mutation yields were studied in the frameshift strain of Salmonella typhimurium, LT2 hisC3076. The numbers and frequencies per 10⁸ survivors of small and large revertant colonies were found to be affected significantly by plating density, but it was possible to detect a considerable enhancement of mutation frequency when broth (2.5%, v/v) was present in the post-irradiation plating medium. Caffeine also significantly enhanced the yields of UV-induced frameshift mutations, but not of γ-induced frameshifts, indicating that the UV-induced pre-mutational lesions which lead to frameshift mutations may be treated in a similar way by the excision-repair system to those which lead to base-pair substitutions.