UV photobiochemistry of anhydrobiotic organisms at extremely low temperatures

Spores of Bacillus subtilis (TKJ 3412), cells of Deinococcus radiodurans R₁ (wild type) and conidia of Aspergillus ochraceus (strain 3174) have been UV irradiated (254 nm) in the dry state (3% relative humidity, argon) or in aqueous suspension at room temperature, at ?55°C to ?70°C and at ?165°C to ?170°C. The following effects have been analyzed: decrease in viability, occurrence of DNA strand breaks (pulsed-field gel electrophoresis) and production of DNA-protein cross-links (membrane filter method). The loss in viability is usually more pronounced at around ?70°C than at room temperature, but it is lowest around ?170°C. The kind of prevailing DNA damage varies from organism to organism. The amount of UV induced DNA-protein cross-link products steadily decreases with the temperature and is lowest at ?170°C. The decrease in highly polymeric DNA by double strand breaks follows no universal pattern. The observed hypersensitivity of the three very different species at ?70°C can therefore not be simply explained on the basis of the number of DNA lesions analyzed in the course of this work. We suggest that also the changing state of cellular water below and above about ?130°C significantly contributes to the change in photosensitivity.