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 th e concerted action of three immediately (cli) proteins, ICP4, ICP27, a nd ICP0. The experiments described in this study examine the role of t he acidic amino terminus (amino acids I to 103) of ICP0 in gene activa tion. When tethered to a DNA binding domain, this sequence activates t ranscription in the yeast Saccharomyces cerevisiae. Deletion of these amino acids affects the ability of ICP0 to activate ru-gene promoter r eporters in transient expression assays, while it has little or no eff ect on a beta- and a gamma-gene reporter in the same assay. Viruses th at express the deleted form of ICP0 (ICP0-NX) have a small-plaque phen otype on both Vero cells and the complementing cell line L7. Transient expression and immunofluorescence analyses demonstrate that ICP0-NX i s a dominant negative form of ICP0. Immunoprecipitation of ICP0 from c ells 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 biochemic al and genetic evidence that ICP0 functions as a multimer in infected cells.