UV-induced mutation fixation in Bacillus subtilis.

The influence was studied of a temporary specific inhibition of post-radiation macromolecular synthesis and of preliminary UV irradiation on the kinetics of accumulation of fixed mutations, that is mutations insensitive to MFD, in UV-irradiated B. subtilis cells. From experimental results it is deduced that the entry of a pre-mutagenic lesion into a round of replication, initiated before irradiation, is not a fixing event in UV mutagenesis. For performance of fixation, the proceeding of replication, initiated after irradiation, and protein synthesis are necessary. In irradiated cells incubated in medium with lowered concentration of nitrogen sources, the antimutagenic activity of a uvr+-dependent repair system competes with the process of fixation for pre-mutagenic lesions and reduces the efficiency of mutagenesis. The most efficient fixation and mutagenesis occur at high concentrations of nitrogen sources in post-radiation medium, when the manifestation of antimutagenic activity appears to be blocked. The possible nature of a process leading to mutation fixation is discussed.