Influence of the UV-activated SOS response on the gamma-radiation-induced mutation spectrum in the lacI gene

Previous studies of our group have shown that intracellular or extracellular gamma-irradiation of the lacI gene results in different mutational spectra. One cause for these differences might be the error-prone SOS response, which is activated in the intracellular situation by gamma-irradiation but not in the extracellular situation. Since UV-radiation is a well-established strong inducer of the SOS response, we used bacterial host cells, pretreated with UV-light to study the influence of the SOS response on the gamma-radiation-induced mutation spectrum in the lacI gene in the extracellular situation. If the SOS response was activated, mutations on A:T base pairs and frameshift mutations accounted for 16% and 12% of all mutations, respectively, but they were hardly detected in the absence of an induced SOS response. G:C to T:A transversions increased from 14% to 24% in the presence of an activated SOS response. We can therefore conclude from this study, that SOS-induction of host cells by UV-light influences the extracellular mutation spectrum in the lacI gene, with respect to mutations on A:T base pairs, G:C to T:A transversions and frameshift mutations. This conclusion is supported by the fact that the previously obtained intracellular gamma-radiation-induced mutation spectrum in the lacI gene, in which the SOS response is also involved, shows great similarities with the extracellular mutation spectrum in the presence of an activated SOS response in this study.