**5. Recombination in archaea acts in DNA repair**

In the previous sections it was proposed that genome repair processes emerged in the RNA world and that, after going through several evolutionary stages, such repair processes were present in organisms with DNA genomes. The archaea are single-celled microorganisms whose genome is DNA. These organisms are regarded as descendants of a form of life that arose subsequent to organisms with RNA genomes but prior to eukaryotes [49].

The evolution of the eukaryotic cell appears to trace back to the establishment of a symbiotic relationship between a host anaerobic archaeal cell and an internalized bacterium capable of aerobic metabolism [50]. The eukaryotic cell emerged at least 1.5 billion years ago [51]. Eukaryotic genes of archaeal origin appear to have a more central role in basic cellular functions than genes of eubacterial origin [49]. Thus the manner in which present day archaea deal with genome damage may throw light on how genome repair processes that arose in the RNA world became adapted for repair in both the archaeal and the eukaryote DNA world.

Recent findings show that cells of archaeal species, particularly *Sulfolobus solfataricus* and *Sulfolobus acidocaldarius,* under stressful environmental conditions that cause DNA damage, aggregate and transfer DNA from one cell to another through direct contact [52, 53]. Exposure of *S. solfataricus* to UV irradiation strongly induces type IV pili formation which facilitates cellular aggregation [54, 55]. This induced cellular aggregation mediates intercellular chromosome marker exchange with high frequency. UV irradiated cultures were found to have recombination rates exceeding those of uninduced cultures by up to three orders of magnitude. The UV-inducible DNA transfer process and subsequent homologus recombination are considered to represent a repair mechanism for maintaining chromosome integrity [54, 56, 57]. Also in *S. solfataricus*, exposure to bleomycin or mitomycin C, agents that cause double-strand breaks and other damages, induces cellular aggregation [54]. In *S. acidoclaldarius,* genes that facilitate DNA transfer are upregulated by DNA damaging UV irradiation [52]. DNA damage can be lethal to a cell unless repaired. DNA transfer between neighboring archaeal cells appears to be an adaptation for aiding survival of nearby (and likely genetically related) damaged cells by facilitating recombinational repair.

The repair capabilities of archaea suggest that ancestral organisms arising early in the DNA world underwent processes that allowed DNA damage in one cell to be repaired by transfer of DNA sequence information from a neighboring cell in order to facilitate recombinational repair.
