ABSTRACT
Removal of core promoter elements like the TATA box converts several regulatory upstream regions of viral and cellular genes into classical enhancers, i.e., cis-regulatory elements capable of activating transcription over long distances in an orientation-independent manner. This is not the case with herpes simplex virus (HSV) immediate-early gene promoters, which are strongly induced by the viral transactivator VP16 (Vmw65, alphaTIF, ICP25) complexed with the cellular factors Oct-1 and HCF. Here we report that the VP16 complex can readily bring about strong activation from a promoter-proximal position but fails to induce transcription from a distal downstream enhancer position. This is in striking contrast to results obtained with GAL fusion proteins: in this context, the C-terminal "general" activation domain of VP16 activates transcription to high levels over long distances. Thus, this paradoxical behavior suggests that the VP16 activation domain is not accessible to the transcription machinery when the VP16-Oct-1-HCF complex is bound in a remote position. Only upon specific interactions in a promoter-proximal position, perhaps with the basal transcription factors, can transcription be strongly induced. In agreement with such a proposed mechanism, VP16 proteins to which a heterologous general activation domain has been added strongly activate transcription from a downstream position. The biological role of this unexpected and sophisticated mechanism is most probably a limitation of the VP16 activity to the associated immediate-early genes, without undesired long-range effects on other viral promoters within the tightly packed HSV genome.
