VACCINIA TOPOISOMERASE BINDS CIRCUMFERENTIALLY TO DNA

Vaccinia DNA topoisomerase, a member of the eukraryotic type I enzyme family, binds duplex DNA and forms a covalent adduct at sites containi ng a conserved sequence element 5'-CCCTT down arrow in the scissile st rand. The protein-DNA interface entails essential contacts with four p hosphate moieties within the CCCTT motif, including the scissile phosp hate, and three phosphates within the GGGAA sequence on the noncleaved strand, Critical protein-phosphate contacts are arrayed across the mi nor groove of the DNA helix, Base-specific contacts with the pentamer element are within the major groove and are situated on the opposite f ace of the helix. Thus, vaccinia topoisomerase binds circumferentially to its target site in duplex DNA, This binding mode suggests that the eukaryotic enzyme adopts a toroidal shape in the DNA-bound state, Con formational isomerization of the bound protein provides a plausible me chanism for DNA relaxation.