The herpes simplex virus type 2 gene which encodes the large subunit of ribonucleotide reductase has unusual regulatory properties

Expression of herpes simplex virus type 2 (HSV-2) encoded ribonucleotide reductase (RR) is required for growth of the virus in non-dividing cells. The functional enzyme is composed of a large (RRA) and small (RRB) subunit and the enzyme is expressed as a delayed early activity. The promoter of RRA contains a cis-acting motif (TAATGARAT) which resembles those found in immediate early (IE) genes suggesting RRA is an IE gene. When primate cells were infected with HSV-2, low levels of RRA transcripts were expressed in the presence of cycloheximide indicating RRA is not a true IE gene. Conditions which allow for efficient RRA RNA expression in the presence of cycloheximide were identified in human cells. A phorbol ester, 12-O-tetradecanoyl phorbol-13-acetate (TPA), and hydroxyurea increased the level of RRA RNA expression in the presence of cycloheximide. Hydroxyurea and TPA also stimulated RRA promoter activity in transient assays suggesting these agents induced factors which transactivated the RRA promoter. Expression of an intact c-myc gene transactivated the RRA promoter more than 30-fold in transient assays. Although expression of an intact retinoblastoma gene (Rb) had a slight stimulatory effect on the RRA promoter, mutant Rb proteins also stimulated the RRA promoter. These studies demonstrated that inducible factors in permissive cells increase the steady state levels of RRA RNA in the presence of cycloheximide.