Tat trans-activates the human immunodeficiency virus through a nascent RNA target

Expression of the human immunodeficiency virus type 1 (HIV-1) genome is greatly dependent on the viral trans-activator protein Tat. Tat functions through the TAR element, which is represented in both viral DNA and RNA. At present, there is no definitive evidence that determines whether Tat acts through a DNA or RNA form of TAR. We have used an intramolecular mutagenesis approach to change selectively the RNA secondary structure of TAR without affecting its primary sequence. We show that a specific RNA secondary structure for TAR is needed for biological activity. Furthermore, transcripts that only transiently form a native TAR RNA hairpin, which is not maintained in the mature mRNA, are completely trans-activated by Tat, suggesting that TAR is recognized as a nascent RNA.