DNA topoisomerase V: a new fold of mysterious origin

Although all other topoisomerases have a broad phylogenetic distribution, DNA topoisomerase V, the major component of the ThermoFidelase sequencing kit, is presently only known in a single species--the archaeon Methanopyrus kandleri. Resolution of the structure of this enzyme by Taneja and co-workers now reveals that this atypical topoisomerase has no structural similarity with other proteins. So, where did it come from? It is my contention that Topo V, and many other orphan proteins, could have a viral origin.