Plasmid pGS5 from the hyperthermophilic archaeon Archaeoglobus profundus is negatively supercoiled

We present evidence that, in contrast to plasmids from other hyperthermophilic archaea, which are in the relaxed to positively supercoiled state, plasmid pGS5 (2.8 kb) from Archaeoglobus profundus is negatively supercoiled. This might be due to the presence of a gyrase introducing negative supercoils, since gyrase genes are present in the genome of its close relative A. fulgidus, and suggests that gyrase activity predominates over reverse gyrase whenever the two topoisomerases coexist in cells. Previous topological analyses of native plasmids from hyper-thermophiles belonging to the two major phylogenetic lineages within the domain Archaea revealed a relaxed to positively supercoiled state of DNA (1, 2, 10). This was in contrast to the negatively supercoiled state of DNA in all mesophiles, including mesophilic archaea (1). The strains investigated in those studies were members of the orders Sulfolobales and Thermococcales, representing the two major archaeal king-doms, Crenarchaeota and Euryarchaeota, respectively. All of them possessed reverse gyrase, a topoisomerase specific fo