Murine cytomegalovirus stimulates cellular thymidylate synthase gene expression in quiescent cells and requires the enzyme for replication

Herpesviruses accomplish DNA replication either by expressing their own deo xyribonucleotide biosynthetic genes or by stimulating the expression of the corresponding cellular genes. Cytomegalovirus (CMV) has adopted the latter strategy to allow efficient replication in quiescent cells. In the present report, we show that murine CMV (MCMV) infection of quiescent fibroblasts induces both mRNA and protein corresponding to the cellular thymidylate syn thase (TS) gene, which encodes the enzyme that catalyzes the de novo synthe sis of thymidylic acid. The increase in TS gene expression was due to an in crease in gene transcription, since the activity of a reporter gene driven by the mouse TS promoter was induced following MCMV infection. Mutagenesis of the potential E2F responsive element immediately upstream from the TS es sential promoter region abolished the virus-mediated stimulation of the TS promoter, suggesting that the transactivating activity of MCMV infection wa s E2F dependent. Cotransfection experiments revealed that expression of the ,viral immediate-early 1 protein was sufficient to mediate the increase in TS promoter activity. Finally, MCR TV replication and viral DNA synthesis w ere found to be inhibited by ZD1694, a quinazoline-based folate analog that inhibits TS activity. These results demonstrate that upregulation of cellu lar TS expression is required for efficient MCMV replication in quiescent c ells.