Genotoxicity of ultraviolet light and sunlight in the bacterium Caulobacter crescentus: Wavelength-dependence

The ultraviolet (UV) component of sunlight can damage DNA. Although most solar UV is absorbed by the ozone layer, wavelengths > 300 nm (UVA and UVB bands) can reach the Earth's surface. It is essential to understand the genotoxic effects of UV light, particularly in natural environments. Caulobacter crescentus, a bacterium widely employed as a model for cell cycle studies, was selected for this study. Strains proficient and deficient in DNA repair (uvrA-) were used to concurrently investigate three genotoxic endpoints: cytotoxicity, SOS induction, and gene mutation, using colony-formation, the SOS chromotest, and Rif^R mutagenesis, respectively. Our findings underscore the distinct impacts of individual UV bands and the full spectrum of sunlight itself in C. crescentus. UVC light was highly genotoxic, especially for the repair-deficient strain. A UVB dose equivalent to 20 min sunlight exposure also affected the cells. UVA exposure caused a significant response only at high doses, likely due to activation of photorepair. Exposure to solar irradiation resulted in reduced levels of SOS induction, possibly due to decreased cell survival. However, mutagenicity is increased, particularly in uvrA- deficient cells.