RNA-targeted activators, but not DNA-targeted activators, repress the synthesis of short transcripts at the human immunodeficiency virus type 1 long terminal repeat.

The human immunodeficiency virus type 1 (HIV-1) promoter directs the synthesis of two types of RNA molecules: full-length transcripts, whose synthesis is activated by the viral activator Tat, and short transcripts, whose synthesis is dependent on the inducer of short transcripts (IST), a bipartite DNA element located in large part downstream of the HIV-1 transcriptional start site. In the absence of Tat, short transcripts constitute the large majority of the RNA molecules synthesized from the HIV-1 promoter. In the presence of Tat, synthesis of the short transcripts is repressed and synthesis of the full-length transcripts is activated. Tat is unique among transcriptional activators in acting through an RNA target, the TAR element. However, Tat has been shown to activate transcription from a DNA target when fused to the appropriate DNA binding domain, raising the question of why Tat has been directed to the RNA. Here we have compared the abilities of Tat and other RNA- and DNA-bound activators to stimulate transcription from the HIV-1 promoter. We show that DNA-targeted activators, including DNA-targeted Tat, activate the synthesis of both short and long transcripts, while RNA-targeted Tat and another RNA-targeted activator activate the synthesis of full-length transcripts but specifically repress that of short transcripts. The unique ability of RNA-targeted activators to down-regulate short transcript synthesis suggests that Tat is directed to the RNA specifically for the purpose of repressing short transcripts.