THE MURINE CYTOMEGALOVIRUS IMMEDIATE-EARLY 1-PROTEIN STIMULATES NF-KAPPA-B ACTIVITY BY TRANSACTIVATING THE NF-KAPPA-B P105 P50 PROMOTER/

The transcription of murine cytomegalovirus (MCMV) immediate-early (IE ) genes is regulated by a large and complex enhancer containing severa l consensus binding sites for the ubiquitous transcription factor NF-k appa B. To verify whether MCMV, like the human CMV, can activate NF-ka ppa-B-dependent transcription, we transfected murine embryo fibroblast s cells with a construct containing three copies of the NF-kappa B ele ment in front of the homologous minimal MCMV IE1-3 promoter. Upon MCMV infection the reporter gene activity was transactivated to about thre e-fold above the basal level. The specificity of this transactivation was demonstrated by the lack of any significant effect on the activity of DNA constructs containing either a mutated NF-kappa B trimer or an ATF/CRE trimer. Gel shift assays with a NF-kappa B probe revealed tha t MCMV infection activated DNA binding proteins showing NF-kappa B cha racteristics. The DNA-binding activity remained elevated during the co urse of infection and was associated to an increase in the steady-stat e mRNA levels for the NF-kappa B subunit p105/p50. Since the promoter of the p105/p50 gene was transactivated by MCMV infection during the p eriod in which the IE proteins are expressed, the role of the two majo r IE transcriptional regulatory proteins was examined. In cotransfecti on experiments, the IE1 protein transactivated the p105/p50 promoter, whereas the IE3 was ineffective in increasing the transcription of the reporter gene. Taken as a whole, these results demonstrate that MCMV, like its human counterpart, regulates the cellular NF-kappa B activit y needed for the initial induction of the IE genes and the progression of the viral replicative cycle.