SV40 nucleoprotein complex activity unwinds superhelical turns in SV40 DNA

Abstract

THE replication of closed circular superhelical DNA of simian virus 40 (SV40) has been shown to proceed through replicative intermediates that are composed of newly synthesised linear polynucleotide chains hydrogen bonded to closed circular template (parental) strands^1–3. To replicate these molecules in a semiconservative fashion a swivel, about the phosphodiester backbone of the template strand of DNA, required during polynucleotide chain propagation^2,3. Such a swivel could be obtained by a nuclease which introduces a single-stranded break in the template strand, thereby allowing free rotation about the phosphodiester bond opposite the nick and relaxing the superhelical turns in the DNA molecule. This type of single-stranded break must, however, be transient and rapidly ligased closed in the cell because breaks in the template strands of replicative intermediates have not been detected^1–3. Alternatively, an activity has been described in E. coli^4,5 and mouse cells⁶, that can relax superhelical turns in closed circular DNAs without introducing either single or double-stranded chain scissions in the DNA product of this reaction. This ‘untwisting activity’ is also a potential candidate for the swivel required during SV40 DNA replication.