Transcriptional activation with concurrent or nonconcurrent template replication has differential effects on transient expression from herpes simplex virus promoters.

We have used two methods to induce template replication in order to assess the effect on expression of marker genes controlled by herpes simplex virus type 1 (HSV-1) promoters. One method used the HSV-1 origin of DNA replication from the short repeat region of the viral genome (HSV-1 oris), and allowed simultaneous replication and transcriptional activation of the plasmid-borne template. The other, using the simian virus 40 origin of replication (SV40 ori) allowed plasmid template replication prior to activation of transcription by HSV-1 infection. The two regimes had markedly different effects upon the levels of reporter gene activity induced by HSV-1 superinfection. Replication of reporter plasmids using the SV40 ori yielded levels of reporter gene activity proportional to plasmid copy number when cells were superinfected with HSV-1. In contrast, our results indicated that sequences containing, or in close proximity to, the HSV-1 oris in the reporter plasmid had a significant inhibitory effect on expression from all viral promoters whether or not the plasmid was allowed to replicate. Still, the early (beta) promoter-controlled reporter enzyme activity declined at late times while that controlled by the strict late (gamma) promoter was significantly higher following HSV-1 oris-mediated template replication.