Positively supercoiled DNA in a virus-like particle of an archaebacterium

Abstract

The topological state of DNA is of importance in a variety of essential biological events1–4. Covalently closed circular DNA duplexes isolated from eukaryotic cells and their viruses, as well as from eubacteria and their bacteriophages are, without exception, negatively supercoiled. Little is known about the topological state of the DNA in archaebacteria, a group of organisms distinct from both ekaryotes and eubacteria5, but recently an ATP-dependent DNA topoisomerase has been isolated from the archaebacterium Sulfolobus acidocaldarius. This enzyme, termed ‘reverse gyrase’, converts relaxed or negatively supercoiled DNA into positively supercoiled forms in vitro6–9. It is not a gyrase-like type II topoisomerase, as initially reported9, but an ATP-dependent type I enzyme7,8. The function of this enzyme in vivo has not been established, largely because of the lack of data on the topological state of Sulfolobus DNA. Here we show that DNA exists in a positively superhelical state in the genome of the virus-like particle SSV-1, present in Sulfolobus.