The pathogenic and cytotoxic effects of viruses are largely due to the expression of viral gene products. Therefore, the determinants of these outcomes are the mechanisms underlying the expression of viral genes. The hallmark of herpes simplex virus gene expression is the sequential and coordinately regulated expression of the approximately 80 viral genes. This regulation occurs largely through modulation of RNA polymerase II transcription. Two viral proteins, VP16 and ICP4, function to activate transcription of the five immediate early genes, and the remainder of the HSV genome, respectively. ICP4 appears to be unique in that it doesn't require the cellular coactivators used by many other activators for the activation of viral late genes. However, the cellular requirements for the activation of early genes by ICP4 are greater than those for late genes, suggesting a different mechanism of action. We hypothesize that ICP4 and late promoters, which consist simply of TATA box and Inr element, have evolved to efficiently function with the available polll machinery present late after infection, while ICP4 functions by a different mechanism with the less altered transcriptional machinery present at earlier times post infection to activate early genes. As infection proceeds a number of viral activities result in the altered abundance and activities of cellular proteins. Therefore, it is reasonable to propose that the abundance, subunit composition, and activities of polll trancription factor complexes change as infection proceeds, and that these changes contribute to the regulated cascade of viral gene expression. To address these hypotheses we will: 1) Determine the mechanism of transcription activation of early and late genes by ICP4. 2) Determine how the phosphorylation of Spl during infection affects its ability to activate transcription. 3) Use the combined results from microarray analysis and biochemical fractionation of virus infected cells to determine how the transcription machinery of the cell changes as a consequence of infection and how these changes may affect viral gene expression.