Constitutive expression of thymidylate synthase from LCDV-C induces a transformed phenotype in fish cells.

Thymidylate synthase (TS), an essential enzyme in DNA synthesis and repair, plays a key role in the events of cell cycle regulation and tumor formation. Here, an investigation was presented about subcellular location and biological function of viral TS from lymphocystis disease virus from China (LCDV-C) in fish cells. Fluorescence microscopy revealed that LCDV-C TS was predominantly localized in the cytoplasm in fish cells. Cell cycle analysis demonstrated that LCDV-C TS promoted cell cycle progression into S and G2/M phase in the constitutive expressed cells. As a result, the cells have a faster growth rate compared with the control cells as revealed by cell growth curves. For foci assay, the TS-expressed cells gave rise to foci 4-5 weeks after incubation. Microscopic examination of the TS-induced foci revealed multilayered growth and crisscross morphology characteristic of transformed cells. Moreover, LCDV-C TS predisposed the transfected cells to acquire an anchorage-independent phenotype and could grow in 0.3% soft agar. So the data reveal LCDV-C TS is sufficient to induce a transformed phenotype in fish cells in vitro and exhibits its potential ability in cell transformation. To our knowledge, it is the first report on viral TS sequences associated with transforming activity.