[Ecologic ultraviolet--a real mutagenic factor for vegetative cells of Bacillus subtilis]

The frequency of leu----Leu+ reversions represented mainly by suppressor mutations is increased in Bacillus subtilis uvr+ and uvr-1 cells after exposure to natural sunlight. Dependence of mutation yield on the time of exposure is linear (one hit kinetics) in case of the uvr-1 strain. In the uvr+ cells the yield of mutations is also linear, but only at short times of exposure, the curve bending and levelling off the plateau after 10-min cell illumination. It has been established in the experiments with optical filters that the mutagenic effect is related to wavelengths which correspond to the UVB zone of ecological UV. The mutagenesis caused by sunlight can be modified (weakened) by some post-irradiation treatments of bacteria, which also led to a decrease of mutations frequencies in B. subtilis uvr+ and uvr-1 cells after exposure to 254-nm UV. The data indicate that: 1) mutagenic influence of sunlight can be overcome only by the joint action of activities of the two cellular repair systems--photoreactivation and excision repair, 2) the real mutagenic effect of sunlight on such a non-photoreactivating organism as B. subtilis would not be enhanced with the increase of the UVB flow in sunlight spectrum.