REPRESSION OF THE HERPES-SIMPLEX VIRUS-1 ALPHA-4 GENE BY ITS GENE-PRODUCT (ICP4) WITHIN THE CONTEXT OF THE VIRAL GENOME IS CONDITIONED BY THE DISTANCE AND STEREOAXIAL ALIGNMENT OF THE ICP4 DNA-BINDING SITE RELATIVE TO THE TATA BOX

Infected cell protein no. 4 (ICP4), the major regulatory protein encoded by the alpha 4 gene of herpes simplex virus 1, binds to a site (alpha 4-2) at the transcription initiation site of the alpha 4 gene. An earlier report described the construction of recombinant viruses that contained chimeric genes (alpha 4-tk) that consisted of the 5' untranscribed and transcribed noncoding domains of the alpha 4 gene fused to the coding sequences of the thymidine kinase gene and showed that disruption of the alpha 4-2 binding site by mutagenesis derepressed transcription of this gene (N. Michael and B. Roizman, Proc. Natl. Acad. Sci. USA 90:2286-2290, 1993). This experimental design was used to determine the effect of displacement of the alpha 4-2 binding site on the repression of alpha 4 gene transcription by ICP4. We report the following findings. (i) In the absence of the alpha 4-2 binding site, at 4 h after infection, alpha 4-tk RNA levels increased 10-fold relative to the corresponding RNA levels of a gene that contained the alpha 4-2 site at its natural location. Displacement of the alpha 4-2 binding site by approximately one, two, and three turns of the DNA helix, i.e., by 10, 21, and 30 nucleotides downstream of the original site, increased the concentration of alpha 4-tk RNA 2.4-, 3.5-, and 5.8-fold, respectively. (ii) Displacement of 16 nucleotides, i.e., approximately 1.5 helical turns, increased the accumulation of alpha 4-tk by 5.3-fold, i.e., more than predicted by displacement alone. (iii) At 8 h after infection in the absence of the binding site, the accumulation of alpha 4-tk RNA increased 13.6-fold. However, in cells infected with recombinants that carried displaced alpha 4-2 binding sites, RNA accumulation decreased relative to the levels seen at 4 h after infection. The insertion of DNA sequences in order to displace the alpha 4-2 binding site had no effect on accumulation of RNA in the presence of cyclohex-imide, i.e., in the absence of ICP4, or on maximum accumulation of alpha 4-tk RNA in the absence of the alpha 4-2 binding site. We conclude that (i) the inhibition of transcription by ICP4 is affected by both orientation and distance of the binding site from the TATA box, (ii) the model that best fits the available data is that ICP4 bound to DNA at an appropriate distance and orientation interacts with and precludes the transcription complex from transcribing the gene, and (iii) at 8 h after infection, additional factors such as protein binding upstream of the TATA box and/or posttranslational modification of ICP4 strengthen the repressive effect of ICP4 at the alpha 4-2 binding site.