Synergy between HIV-1 Tat and adenovirus E1A is principally due to stabilization of transcriptional elongation.

We studied the combined effects of Tat and general trans-activators, such as E1A and phorbol esters, on human immunodeficiency virus-1 (HIV-1) gene expression. Interaction between these two types of trans-activators may be involved in the transition from transcriptional quiesence during viral latency to active gene expression during productive infection. E1A cooperated with Tat to produce a fourfold greater increase in accumulation of full-length, cytoplasmic HIV-1-directed RNA than is expected if they were acting additively to increase RNA accumulation. Similarly, phorbol 12-myristate 13-acetate (PMA) also cooperated with Tat to elevate HIV RNA levels synergistically. Analysis of transcription rates across the HIV-1-directed transcription unit indicated, unexpectedly, that synergy between Tat and E1A could not be accounted for by increased promoter proximal transcription rates that were merely additive. However, Tat and E1A produced a greater than additive increase in transcription rates in the 3' end of the gene. These findings imply that synergy between Tat and E1A (or other general transcriptional activators) is due principally to stabilization of transcriptional elongation. Furthermore, the observation that Tat elicits only a small increase in promoter proximal transcription in the presence of E1A suggests that the magnitude of the effect of Tat on initiation is decreased when the basal level of transcription is increased. These findings underscore the importance of the ability of Tat to stabilize elongation, as well as to stimulate initiation, in an HIV-1-directed transcription unit.