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 10(8) 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 gamma-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.