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Abstract
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.