Abstract
The halophilic archaebacterium Halobacterium strain GRB harbours a multicopy plasmid of 1.7 kb which is negatively supercoiled. After addition of novobiocin to culture medium all 1.7 kb plasmid molecules become positively supercoiled. Positive supercoiling occurs at the same dose of novobiocin inhibiting the eubacterial DNA gyrase in vitro. Novobiocin also induces positive supercoiling of pHV2, a 6.3 kb plasmid from Halobacterium volcanii. These results indicate the existence of a mechanism producing positive superturns in halobacteria. The 1.7 kb plasmid from Halobacterium GRB could be used to produce high amounts of pure positively supercoiled DNA for biophysical and biochemical studies.