Repair of DNA double-strand breaks induced by ionizing radiation damage correlates with upregulation of homologous recombination genes in Sulfolobus solfataricus

The mechanisms used by members of the archaeal branch of life to repair DNA damage are not well understood. DNA damage responses have been of particular interest in hyperthermophilic archaea, since these microbes live under environmental conditions that constantly elevate the potential for DNA damage. The work described here focuses on the response of four Sulfolobus solfataricus strains to ionizing radiation (IR) damage. Cellular survival of three wild-type strains and a defined deletion mutant strain was examined following exposure to various IR doses. Using pulsed-field gel electrophoresis, we determined chromosomal DNA double-strand break persistence and repair rates. Among the strains, variable responses were observed, the most surprising of which occurred with the defined deletion mutant strain. This strain displayed higher chromosomal repair rates than the parent strain and was also found to have increased resistance to IR. Using quantitative real-time PCR, we found that transcript levels of homologous recombination-related genes were strongly upregulated following damage in all the strains. The mutant strain again had an enhanced response and dramatically upregulated expression of recombination genes above levels observed for the parent strain, suggesting that increased levels of recombinational repair could account for its increased radiation resistance phenotype. Our results demonstrate a transcriptional response to IR in S. solfataricus for the first time and describe a defined deletion mutant strain that may give the first insight into a damage-based archaeal control element.