Plasmid topology varies transiently in hyperthermophilic archaea during thermal stress. As in mesophilic bacteria,
DNA linking number (Lk) increases during heat shock and decreases during cold shock. Despite this correspondence, plasmid
DNA topology and
proteins presumably involved in
DNA topological control in each case are different. Plasmid
DNA in hyperthermophilic archaea is found in a topological form from relaxed to positively supercoiled in contrast to the negatively supercoiled state typical of bacteria, eukaryotes and mesophilic archaea. We have analysed the regulation of
DNA topological changes during thermal stress in Sulfolobus islandicus (kingdom Crenarchaeota), which harbours two plasmids,
pRN1 and pRN2. In parallel with plasmid topological variations, we analysed levels of
reverse gyrase,
topoisomerase VI (
Topo VI) and the small
DNA-binding protein Sis7, as well as topoisomerase activities in
crude extracts during heat shock from 80 degrees C to 85-87 degrees C, and cold shock from 80 degrees C to 65 degrees C. Quantitative changes in
reverse gyrase,
Topo VI and Sis7 were not significant. In support of this, inhibition of
protein synthesis in S. islandicus during shocks did not alter plasmid topological dynamics, suggesting that an increase in topoisomerase levels is not needed for control of
DNA topology during thermal stress. A
reverse gyrase activity was detected in
crude extracts, which was strongly dependent on the assay temperature. It was inhibited at 65 degrees C, but was greatly enhanced at 85 degrees C. However, the intrinsic
reverse gyrase activity did not vary with heat or cold shock. These results suggest that the control of
DNA topology during stress in Sulfolobus relies primarily on the physical effect of temperature on topoisomerase activities and on the geometry of
DNA itself. Additionally, we have detected an enhanced thermoresistance of
reverse gyrase activities in cultures subject to prolonged heat shock (but not cold shock). This acquired thermotolerance at the enzymatic level is abolished when cultures are treated with
puromycin, suggesting a requirement for
protein synthesis.