THE VP16 PARADOX - HERPES-SIMPLEX VIRUS VP16 CONTAINS A LONG-RANGE ACTIVATION DOMAIN BUT WITHIN THE NATURAL MULTIPROTEIN COMPLEX ACTIVATES ONLY FROM PROMOTER-PROXIMAL POSITIONS

Removal of core promoter elements like the TATA box converts several r egulatory upstream regions of viral and cellular genes into classical enhancers, i.e., cis-regulatory elements capable of activating transcr iption over long distances in an orientation independent manner, This is not the case with herpes simplex virus (HSV) immediate early gene p romoters, which are strongly induced by the viral transactivator VP16 (Vmw65, alpha TIF, ICP25) complexed with the cellular factors Oct-1 an d HCF, Here we report that the VP16 complex can readily bring about st rong activation from a promoter-proximal position but fails to induce transcription from a distal downstream enhancer position, This is in s triking contrast to results obtained with GAL fusion proteins: in this contest, the C-terminal ''general'' activation domain of VP16 activat es transcription to high levels over long distances, Thus, this parado xical behavior suggests that the VP16 activation domain is not accessi ble to the transcription machinery when the VP16-Oct-1-HCF complex is bound in a remote position, Only upon specific interactions in a promo ter-proximal position, perhaps with the basal transcription factors, c an transcription be strongly induced, In agreement with such a propose d mechanism, VP16 proteins to which a heterologous general activation domain has been added strongly activate transcription from a downstrea m position, The biological role of this unexpected and sophisticated m echanism is most probably a limitation of the VP16 activity to the ass ociated immediate-early genes, without undesired long-range effects on other viral promoters within the tightly packed HSV genome.