Melanoplus sanguinipes entomopoxvirus (MsEPV) encodes a 328 amino acid polypeptide related to the type I topoisomerases of six other genera of vertebrate and insect poxviruses. The gene encoding MsEPV topoisomerase was expressed in bacteria, and the recombinant protein was purified by ion- exchange chromatography and glycerol gradient sedimentation. MsEPV topoisomerase, a monomeric protein, catalyzed the relaxation of supercoiled plasmid DNA at ~0.6 supercoils/s. Like other poxvirus topoisomerases, the MsEPV enzyme formed a covalent adduct with duplex DNA at the target sequence CCCTT ↓. The kinetic and equilibrium parameters of the DNA transesterification reaction of MsEPV topoisomerase were k(cl) = 0.3 s-1 and K(cl) = 0.25. The introduction of a 5'-bridging phosphorothiolate at the scissile phosphate increased the cleavage equilibrium constant from 0.25 to ≥30. Similar phosphorothiolate effects were observed with vaccinia topoisomerase. Kinetic analysis of single-turnover cleavage and religation reactions established that the altered equilibrium was the result of a ~10- 4 decrement in the rate of topoisomerase-catalyzed attack of 5'-SH DNA on the DNA-(3'-phosphotyrosyl)enzyme intermediate. 5'-bridging phosphorothiolates at the scissile phosphate and other positions within the CCCTT element had no significant effect on k(cl).
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