THE HERPES-SIMPLEX VIRUS IMMEDIATE-EARLY PROTEIN ICP0 AFFECTS TRANSCRIPTION FROM THE VIRAL GENOME AND INFECTED-CELL SURVIVAL IN THE ABSENCEOF ICP4 AND ICP27

ICP4, ICP0, and ICP27 are the immediate-early (IE) regulatory proteins of herpes simplex virus that have the greatest effect on viral gene e xpression and growth. Comparative analysis of viral mutants defective in various subsets of these IE genes should help elucidate how these p roteins affect cellular and viral processes. This study focuses on the mutant d97, which is defective for the genes encoding ICP4, ICP0, and ICP27 and expresses the bacterial beta-galactosidase (beta-gal) gene from the ICP0 promoter. Together with the d92 virus (ICP4(-) ICP27(-)) and the ICP0-complementing cell line L7, d97 provided a unique opport unity to evaluate ICP0 function in the absence of the regulatory activ ities specified by ICP4 and ICP27. The pattern of protein synthesis in d97 infected cells was unique relative to other LE gene mutants in th at it was similar to that seen in the absence of prior viral protein s ynthesis, possibly approximating the effect of cellular factors and vi rion components alone. Inactivation of ICP0 in the absence of ICP4 pro duced a significant decrease in the levels of the early mRNAs ICP6 and thymidine kinase (tk). There was also a marginal reduction in the lev els of the IE ICP22 mRNA, and this was most notable at low multiplicit y of infection (MOI). In d97-infected L7 cells, the levels of the vira l mRNAs were mostly restored to those observed in infections with d92. Nuclear runoff transcription analysis demonstrated that the presence of ICP0 resulted in an increase in the transcription rates of the anal yzed genes. The transcription rates of the early genes were dramatical ly reduced in the absence of ICP0. At low MOI, the transcription rates of ICP6 and tk were comparable to the rate of transcription of a cell ular gene. Relevant to the potential use of d97 as a transfer vector, it was also determined that the absence of ICP0 reduced the cellular t oxicity of the virus compared to that of d92. The beta-gal transgene e xpressed from an IE promoter was detected for up to 14 days postinfect ion; however, the level of beta-gal expression declined dramatically a fter 1 day postinfection. In the presence of ICP0, the level of expres sion of beta-gal was increased; however the infected monolayer was des troyed by 3 days postinfection. Therefore, deletion of ICP0 in the abs ence of ICP4 and ICP27 reduces toxicity and lowers the level of expres sion of genes from the viral genome.