Crystal structure of the amino-terminal fragment of vaccinia virus DNA topoisomerase I at 1.6 å resolution

Background Vaccinia virus, a cytoplasmically-replicating poxvirus, encodes a type I DNA topoisomerase that is biochemically similar to eukaryotic-like DNA topoisomerases I, and which has been widely studied as a model topoisomerase. It is the smallest topoisomerase known and is unusual in that it is resistant to the potent chemotherapeutic agent camptothecin.Results The crystal structure of a 9kDa amino terminal fragment of vaccinia virus DNA topoisomerase I has been determined at 1.6 å resolution. The fragment forms a five-stranded, anti-parallel β -sheet with two short α-helices and connecting loops. Residues that are conserved between all eukaryotic-like type I topoisomerases are not clustered in particular regions of the structure.Conclusions This is the first atomic structure of any region of a eukaryotic-like DNA topoisomerase I. It has provided insights into the structural bases of the phenotypes of some single-site mutants of the intact topoisomerase. The structure has enabled us to study the interactions within a well-folded protein fragment. and the camptothecin resistance of the viral topoisomerase.