UV stimulation of chromosomal marker exchange in Sulfolobus acidocaldarius: Implications for DNA repair, conjugation and homologous recombination at extremely high temperatures

The hyperthermophilic archaeon Sulfolobus acidocaldarius exchanges and reco mbines chromosomal markers by a conjugational mechanism, and the overall yi eld of recombinants is greatly increased by previous exposure to UV light. This stimulation was studied in an effort to clarify its mechanism and that of marker exchange itself. A variety of experiments failed to identify a s ignificant effect of UV irradiation on the frequency of cell pairing, indic ating that subsequent steps are primarily affected, i.e., transfer of DNA b etween cells or homologous recombination. The UV-induced stimulation decaye d rather quickly in parental cells during preincubation at 75 degrees, and the rate or decay depended on the incubation temperature. Preincubation at 75 degrees decreased the yield of recombinants neither from unirradiated pa rental cells nor from parental suspensions subsequently irradiated. We inte rpret these results as evidence that marker exchange is stimulated by recom binogenic DNA lesions formed as intermediates in the process of repairing U V photoproducts in the S. acidocaldarius chromosome.