Expression of the approximately 70 herpes simplex virus type 1 (HSV-1) genes is regulated primarily at the level of mRNA transcription, and can be conveniently divided into three phases based on the temporal order of mRNA synthesis. Immediate-early genes are expressed soon after virus infection, and their gene products are required for the expression of early genes, many of whose gene products are involved in viral DNA replication. Expression of viral late genes requires viral DNA replication and functional immediate-early gene products. Understanding the regulation of HSV gene expression is necessary for the development and evaluation of successful vaccines. In addition, the development of HSV as a gene therapy vector requires an understanding of the regulation of foreign gene expression in HSV vectors. One aspect of this research project is designed to study the temporal expression of herpes simplex type 1 late genes, those genes expressed only after viral DNA replication. Most of the products of these genes are structural proteins of the virion and include at least 15 glycoproteins and 7 different capsid proteins. Extensive mutational analysis of the promoter regions of several glycoprotein genes has identified three separate sequence elements that are involved in expression. These elements are interchangeable from one promoter to another, and include a TATA element, an initiator-like region at the start of transcription, and a downstream element necessary for maximal expression. Currently, we are investigating the function of the initiator region and the downstream activation region by construction of hybrid viral promoters containing similar regions from other eukaryotic promoters. A second aspect of this work investigates the expression of foreign genes in herpes simplex vectors. Currently, we are investigating the use of the HIV LTR as an inducible promoter for foreign gene expression in an HSV vector. Foreign genes under the control of the LTR in an HSV vector are inducible by the HIV Tat regulatory protein which suggests that this promoter may be useful for regulated expression of therapeutic genes in HIV infected cells. The third aspect of this research project is designed to evaluate various HSV gene products as potential components of subunit vaccines. Expression plasmids have been made that express different HSV glycoproteins and these are being used for direct injection into mice to determine which proteins or combinations of proteins provide optimal protection against HSV infection.