A supercoil-specific endonuclease from Salmonella typhimurium cleaves both negatively and positively supercoiled DNA.

An enzyme which specifically cleaves supercoiled DNA to linear form through nicked circular form as intermediate was isolated from rifampicin-resistant mutant of Salmonella typhimurium, rif 39. The enzyme activity was stimulated by Mg+2, whereas Ca+2 had no effect. It does not require ATP for its activity. No activity could be detected with relaxed or single stranded circular DNAs. The molecular weight of the enzyme is approximately 34 kDa. The most characteristic feature of this enzyme is that it cleaves both positively and negatively supercoiled DNAs.