The NH2 terminus of the herpes simplex virus type 1 regulatory protein ICPO contains a promoter-specific transcription activation domain

The transcriptional program of herpes simplex virus is regulated by the concerted action of three immediately (cli) proteins, ICP4, ICP27, and ICP0. The experiments described in this study examine the role of the acidic amino terminus (amino acids I to 103) of ICP0 in gene activation. When tethered to a DNA binding domain, this sequence activates transcription in the yeast Saccharomyces cerevisiae. Deletion of these amino acids affects the ability of ICP0 to activate ru-gene promoter reporters in transient expression assays, while it has little or no effect on a beta- and a gamma-gene reporter in the same assay. Viruses that express the deleted form of ICP0 (ICP0-NX) have a small-plaque phenotype on both Vero cells and the complementing cell line L7. Transient expression and immunofluorescence analyses demonstrate that ICP0-NX is a dominant negative form of ICP0. Immunoprecipitation of ICP0 from cells coinfected with viruses expressing ICP0-NX and ICP0 revealed that ICP0 oligomerizes in infected cells. These data, in conjunction,vith the finding that ICP0-N/X is dominant negative, provide both biochemical and genetic evidence that ICP0 functions as a multimer in infected cells.