Transactivation of Adenovirus E2-early Promoter by E1A and E4 6/7 in the Context of Viral Chromosome

Abstract

Transcription from adenovirus E2-early promoter is controlled by a unique array of four cis-acting elements which include an atypical TBP site, two E2F sites present in an inverted orientation relative to each other, and an ATF site. In virus-infected cells, this promoter is transactivated by E1A and the E4 6/7 proteins. In addition, it is also stimulated by the DNA-binding protein (DBP) in transient transfection assays. Here we describe a genetic analysis of the E2 transcriptional regulation in the context of the viral chromosome. By using genetically engineered mutant adenoviruses we have determined the interrelationship between the different cis-acting elements of the E2-early promoter during basal transcription, the extent to which E1A and E4 6/7 contribute to the E2 promoter activation and the E2 promoter elements that respond to these transactivators. We show that at eight hours following infection, E1A can transactivate the promoter about 21-fold whereas E4 6/7 can induce the promoter by only fivefold. DBP does not induce the promoter in the chromosomal context. Our mutational analysis suggests that the unique architecture of the E2-early promoter necessitates the concerted interaction of all three host transcription factors with their cognate recognition elements to form a stable and functional transcription complex. E1A mediated transactivation is dependent on this stable basal transcription complex and transactivation may involve simultaneous interaction of E1A with each of the three transcription factors present in the multicomponent basal transcription complex. The E4 6/7 protein can transactivate the E2-early promoter in the absence of ATF presumably by promoting the DNA binding capacity of transcription factor E2F and thereby stabilizing the basal transcription complex. We discuss some of the possible protein-protein interactions that may take place at the level of the multicomponent transcriptional complex at the E2-early promoter during transcriptional activation and the discrepancies that arise when a promoter is analyzed in infection versustransfection assays.