The mechanism by which the adenovirus-encoded nuclear oncogene EIA activates transcription of several viral and host promoters is an important issue in the regulation of eucaryotic gene expression and virus-host cell interactions. Identification of cis-acting elements of the promoters and the cognate host transcription factors that are targets for EIA action is crucial for our understanding of the EIA-mediated control of coordinately regulated genes. The adenovirus EII early promoter has a complex architecture and contains two overlapping promoters with start sites at +1 (major promoter) and -26 (minor promoter). The major promoter responds strongly to virus-encoded trans activators EIA and EIV and contains four elements: a TAGA motif analogous to the TATA box, two EIIF sites present in an inverted orientation, and an ATF/CREB site. To determine precisely the roles played by these cis-acting elements in both basal and virus-induced transcription when the promoter is situated in its natural context, we investigated the phenotype of a series of linker scan promoter substitution mutants inserted into the viral chromosome. Promoter constructs harboring linker scan mutations in each element were rebuilt into a novel EIA- adenovirus vector, and transcriptional activity was monitored in viurs-infected cells. In the absence of virus-encoded trans activators, basal activity in vivo was dependent on all four cis-acting elements. Surprisingly, a promoter mutant with only one of the two EIIF sites intact could not promote transcription in vivo, suggesting that the two EIIF sites function cooperatively even in basal transcription. Promoters harboring mutations in either of these two EIIF sites also failed to bind to an infection-specific form of EIIF in gel shift assays and competed only very weakly for EIIF binding with the wild-type promoter fragment. The dramatic cooperativity shown by the two inverted EIIF sites of the EII promoter both in vivo and in vitro could reflect simultaneous contact of both sites by the transcription factor EIIF. Furthermore, promoter mutants with mutations in the TAGA motif, the two EIIF sites, and the single ATF site all failed to respond to virus-encoded trans activators. Whereas recent results demonstrate that EIIF activity can be modulated independently by EIV, leading to transactivation of this promoter, our results and those published previously strongly indicate that the three different transcription factors that bind to TAGA, EIIF, and ATF motifs of the EII early promoter are all targets for EIA regulation in vivo. Thus, strong transactivation of the EII early promoter through these multiple EIA-sensitive elements and independently by the recently discovered EIV pathway suggests that the EII early promoter is stringently regulated in virus-infected cells. Such a stringent regulation of this promoter is consistent with the vital roles played by the three gene products of this transcription unit in the viral lytic cycle.
ASJC Scopus subject areas
- Insect Science