An engineered mutant of vaccinia virus DNA topoisomerase I is sensitive to the anti-cancer drug camptothecin.

Although highly homologous to the other eukaryotic type I DNA topoisomerases, vaccinia virus DNA topoisomerase I is distinct in its resistance to the anti-cancer drug camptothecin. After comparison of available sequences of sensitive and resistant type I topoisomerases, the aspartic acid at position 221 of vaccinia virus topoisomerase I is mutated to a valine. The resulting mutant protein is partially active. In contrast to the wild type enzyme, the relaxation of supercoiled DNA is inhibited by camptothecin. Its cleavage reaction with DNA is enhanced by camptothecin due to inhibition of religation of DNA. This demonstrates that even though the size of vaccinia virus is only about one-third that of the other camptothecin-sensitive topoisomerases, it has a potential interaction site for camptothecin.