Identification of the thymidylate synthase within the genome of white spot syndrome virus

Thymidylate synthase (TS) (EC 2.1.1.45) is essential for the de novo synthesis of dTMP in prokaryotic and eukaryotic organisms. Within the white spot syndrome virus (WSSV) genome, an open reading frame (WSV067) that encodes a 289 amino acid polypeptide showed significant homology to all known TSs from species including mammals, plants, fungi, protozoa, bacteria and DNA viruses. In this study, WSV067 was expressed in Escherichia coli, and the purified recombinant protein showed TS activity in dUMP-folate-binding assays using ultraviolet difference spectroscopy. RT-PCR and Western blot analyses showed that WSV067 was a genuine and early gene. Phylogenetic analysis revealed that WSSV-TS was more closely related to the TSs of eukaryotes than to those from prokaryotes.