Expression of an altered ribonucleotide reductase activity associated withthe replication of murine cytomegalovirus in quiescent fibroblasts

Ribonucleotide reductase (RNR) is an essential enzyme for the de novo synth esis of both cellular and viral DNA and catalyzes the conversion of ribonuc leoside diphosphates into the corresponding deoxyribonucleoside diphosphate s. The enzyme consists of two nonidentical subunits, termed R1 and R2, whos e expression is very low in resting cells and maximal in S-phase cells. Her e we show that murine cytomegalovirus (MCMV) replication depends on ribonuc leotide reduction since it is prevented by the RNR inhibitor hydroxyurea. M CMV infection of quiescent fibroblasts markedly induces both mRNA and prote in corresponding to the cellular R2 subunit, whereas expression of the cell ular RI subunit does not appear to be up-regulated. The increase in R2 gene expression is due to an increase in gene transcription, since the activity of a reporter gene driven by the mouse R2 promoter is induced following vi rus infection. Cotransfection experiments revealed that expression of the v iral immediate-early 1 protein was sufficient to mediate the increase in R2 promoter activity. It was found that the viral gene M45, encoding a putati ve homologue of the R1 subunit, is expressed 24 and 48 h after infection. M eanwhile, we observed an expansion of the deoxyribonucleoside triphosphate pool between 24 and 48 h after infection; however, neither CDP reduction no r viral replication was inhibited by treatment with 10 mM thymidine. These findings indicate the induction of an RNR activity with an altered alloster ic regulation compared to the mouse RNR following MCMV infection and sugges t that the virus R1 homologue may complex with the induced cellular R2 prot ein to reconstitute a new RNR activity.