Studies on the Regulation and Specificity of the DNA-untwisting Enzyme

The DNA-untwisting enzyme catalyzes the breakage and reclosure of DNA strands (Champoux and Dulbecco 1972; Champoux 1976). Our present understanding of the mechanism of the overall reaction from studies with the purified rat liver enzyme (Champoux and McConaughy 1976) is illustrated schematically in Figure 1. The enzyme nicks the DNA and becomes attached covalently to the broken strand (Champoux 1977a). Since the free end of the broken strand can be phosphorylated by polynucleotide kinase, the enzyme must be attached at the 3′ end (Champoux 1978a). Presumably the linkage between the enzyme and the DNA conserves the energy which is required for the closure phase of the nicking-closing cycle.