Repair of polychromatic radiation induced DNA damage in the archaeon Halobacterium salinarum NRC-1

Abstract

Solar ultraviolet (UV) radiation has mutagenic and often lethal effects on all living organisms. However, there are extremophiles such as the halophilic model archaeon Halobacterium (Hbt.) salinarum NRC-1 that developed high resistance against this kind of stress, which is primarily due to efficient DNA repair systems. These systems include the light-dependent repair mechanism photoreactivation, which allows the direct reversal of pyrimidine dimer formation and the light-independent nucleotide-excision repair (NER) mechanism. Hbt. salinarum NRC-1 thrives in high-salt environments such as solar salterns or ancient halite. With regards to the recent discovery of liquid water in form of high-saline brines on Mars, investigations on the viability and adaptability of Hbt. salinarum NRC-1 to space- and Mars relevant parameters such as non-ionizing UV radiation becomes more interesting for astrobiological research. While it is already well known that these extremophiles are capable of repairing DNA damage after exposure to UV radiation, the main focus of this work lies in the investigation of whether Hbt. salinarum NRC-1 is also able to actively repair UV radiation induced DNA damage during irradiation. Enzymatic activity is generally reduced at lower temperatures. Therefore we first examined the survivability of Hbt. salinarum NRC-1 following exposure to simulated solar radiation at 4°C and 37°C. As a means of comparison, we determined the amount of general DNA damage induced by UV radiation using RAPD-PCR and qPCR. Furthermore, we analyzed the regulation of repair genes, such as the photolyase, during irradiation by qRT-PCR. Our results demonstrate that the survival of Hbt. salinarum NRC-1 is better when exposed to solar radiation at 37°C compared to 4°C. This correlates with less UV radiation induced DNA damage when irradiated at these temperatures. However, we did not observe an upregulation of the phr2 gene under these experimental conditions. To test the hypothesis that another protection mechanism is active at higher temperatures such as quenching of reactive oxygen species (ROS) by bacterioruberin, we analyzed the production of ROS during exposure. Nonetheless, this hypothesis could not be confirmed in these experiments, which is why further investigations are necessary to provide greater insight into how Hbt. salinarum NRC-1 manages to survive in its extreme environments and how they were able to adapt to hostile conditions. This would help understanding the basic conditions and limitations for the evolution and distribution of life