To study the mechanism of a novel herpes simplex virus (HSV) activity that stimulates expression of reporter genes containing beta interferon (IFN-β)- coding sequences, we have established permanent DNA-transfected cell lines that each contain two distinct hybrid genes encoding mRNA species with different half-lives. These reporter genes comprised either the human IFN-β- or bacterial chloramphenicol acetyltransferase (CAT)-coding and 3' untranslated regions placed under the transcriptional control of the powerful major immediate-early promoter-enhancer region (IE94) from simian cytomegalovirus. Most of the dual-transfected cell lines yielded significant levels of steady-state IE94-CAT mRNA and abundant constitutive synthesis of CAT enzyme activity, whereas no accumulation of IE94-IFN mRNA could be detected. However, infection with HSV type 1 resulted in a 300-fold increase in IE94-IFN-specific mRNA transcripts, compared with no more than 3- to 5- fold stimulation of IE94-CAT-specific mRNA. In contrast, cycloheximide treatment increased stable mRNA levels and transcription initiation rates from both the IE94-IFN and IE94-CAT hybrid genes. Run-on transcription assays in isolated nuclei suggested that induction of IE94-IFN gene expression by HSV type 1 occurred predominantly at the posttranscriptional level. Enhancement of the unstable IFN mRNA species after HSV infection was also observed in cell lines containing a simian virus 40 enhancer-driven IFN gene (SV2-IFN). Similarly, in transient-transfection assays, both SV2-IFN and IE94-IFN gave only low basal mRNA synthesis, but superinfection with HSV again led to high-level accumulation of IFN mRNA. Finally, substitution of the SV2-IFN gene 3' region with poly(A) and splicing signals from the SV2-CAT gene cassette led to stabilization of the IFN mRNA even in the absence of HSV. Therefore, we conclude that HSV infection leads to selective accumulation of IFN-β mRNA by a posttranscriptional mechanism that is reporter gene specific and promoter independent.
ASJC Scopus subject areas
- Insect Science