26 Transcriptional Activation by Viral Immediate-Early Proteins: Variations on a Common Theme

Abstract

OVERVIEW
Viral immediate-early proteins are the first viral proteins to be synthesized following infection (or upon reactivation from latency) and are required for the transcription, or the enhancement of transcription, of many viral genes. More than 20 viral immediate-early proteins have been identified. Until recently, it has been generally assumed that these viral proteins function in a manner fundamentally different from that of typical cellular transcriptional activators. In this review, we summarize current evidence indicating that many viral transcriptional activators function through mechanisms that are analogous to those of cellular transcriptional activators: The viral proteins directly participate in the transcription complex to stimulate transcription. Variations on the theme of the typical sequence-specific cellular activators can account for the idiosyncratic features of viral activators, which have been difficult to reconcile with notions of cellular activators. Such characteristics of some viral activators include the lack of specific DNA-binding activity, promiscuous activation, and requirements for RNA elements.

INTRODUCTION
Typical Cellular Activators
Transcriptional initiation by RNA polymerase II is usually regulated by cellular factors that bind to the DNA located upstream of the gene, termed a promoter. Promoters consist of a variety of DNA sequence elements, or regions of about 8–20 bp that conform to one of a number of consensus motifs, which are specifically recognized and bound by DNA-binding proteins. On the basis of the functional characteristics of the proteins that bind to these sequence elements, the promoter can be divided into two distinct regions. The first region, located in the vicinity...