Wavelength dependence of inactivation and mutagenesis in haploid yeast cells of different sensitivities

Solar UV radiation and microbial life in the atmosphere

Many microorganisms are alive while suspended in the atmosphere, and some seem to be metabolically active during their time there. One of the most important factors threatening their life and activity is solar ultraviolet (UV) radiation. Quantitative understanding of the spatial and temporal survival patterns in the atmosphere, and of the ultimate deposition of microbes to the surface, is limited by a number factors some of which are discussed here. These include consideration of appropriate spectral sensitivity functions for biological damage (e.g. inactivation), and the estimation of UV radiation impingent on a microorganism suspended in the atmosphere. We show that for several bacteria (E. coli, S. typhimurium, and P. acnes) the inactivation rates correlate well with irradiances weighted by the DNA damage spectrum in the UV-B spectral range, but when these organisms show significant UV-A (or visible) sensitivities, the correlations become clearly non-linear. The existence of these correlations enables the use of a single spectrum (here DNA damage) as a proxy for sensitivity spectra of other biological effects, but with some caution when the correlations are strongly non-linear. The radiative quantity relevant to the UV exposure of a suspended particle is the fluence rate at an altitude above ground, while down-welling irradiance at ground-level is the quantity most commonly measured or estimated in satellite-derived climatologies. Using a radiative transfer model that computes both quantities, we developed a simple parameterization to exploit the much larger irradiance data bases to estimate fluence rates, and present the first fluence-rate based climatology of DNA-damaging UV radiation in the atmosphere. The estimation of fluence rates in the presence of clouds remains a particularly challenging problem. Here we note that both reductions and enhancements in the UV radiation field are possible, depending mainly on cloud optical geometry and prevailing solar zenith angles. These complex effects need to be included in model simulations of the atmospheric life cycle of the organisms.

Inactivation and mutation induction in Saccharomyces cerevisiae exposed to simulated sunlight: evaluation of action spectra

The effectiveness of polychromatic light irradiation was investigated for haploid yeast cells. Inactivation and mutation induction were measured in both a RAD-wildtype strain and an excision-repair defective strain. The behaviour of vegetative "wet" cells was compared to that of dehydrated cells. The aim of the study was to assess the interaction of UVC with other wavelengths in cells of different states of humidity. The irradiation procedure was therefore carried out using a solar simulator either with full spectrum or with a UVC-blocking filter (modified sunlight) added. The results were analysed on the basis of separately determined action spectra. The summation of the efficiency of individual wavelengths was compared to the values obtained from polychromatic irradiation. It is shown that the effects caused by the whole-spectrum irradiation in wet cells can be predicted sufficiently from the calculation, while dried wildtype cells exhibit higher mutation rates. Thus it can be assumed that drying-specific damage leads to lethal and mutagenic lesions which are processed in different ways, causing a synergistic behaviour in mutation induction. Irradiation of vegetative cells with modified sunlight (UVC-) results in less inactivation and lower mutation rates than were calculated. From these results it can be concluded that this antagonistic behaviour is caused by the interaction of near-UV photoproducts.

Effects of near-ultraviolet light on mutations, intragenic and intergenic recombinations in Saccharomyces cerevisiae

The effects of far (254 nm) and near (290-350 nm) ultraviolet (UV) light on mutations, intragenic and intergenic recombinations were compared in diploid strains of Saccharomyces cerevisiae. At equivalent survival levels there was not much difference in the induction of nonsense and missense mutations between far- and near-UV radiations. However, frameshift mutations were induced more frequently by near-UV than by far-UV radiation. Near-UV radiation induced intragenic recombination (gene conversion) as efficiently as far-UV radiation and the induced levels were similar in both radiations at equitoxic doses. A strikingly higher frequency was observed for the intergenic recombination induced by near-UV radiation than by far-UV radiation when compared at equivalent survival levels. Photoreactivation reduced the frequency only slightly in far-UV induced intergenic recombination and not at all in near-UV induction. These results indicate that near-UV damage involves strand breakage in addition to pyrimidine dimers and other lesions induced, whereas far-UV damage consists largely of photoreactivable lesions, pyrimidine dimers, and near-UV induced damage is more efficient for the induction of crossing-over.