Characterisation of two topoisomerase 1 genes in the pufferfish (Fugu rubripes).

Eukaryotic DNA topoisomerase I manipulates the higher order structures of DNA. Only one functional topoisomerase 1 (top1) gene has previously been identified in any individual eukaryotic species. Here we report the identification and characterisation of two top1 genes in the pufferfish, Fugu rubripes. This shows that the copy number of top1, like that of other topoisomerases, may vary between eukaryotes. Both Fugu genes have 21 exons; a gene structure similar to that of human TOP1. Despite this conservation of structure, and some non-coding elements, both genes are less than a tenth of the size of the human gene. Sequence and phylogenetic analyses have shown that this duplication is ancient and also affects other species in the fish lineage.