HIV-1 TAR RNA enhances the interaction between Tat and cyclin T1

Human immunodeficiency virus, type 1 (HIV-1), Tat activates elongation of R NA polymerase II transcription at the HIV-1 promoter through interaction wi th the cyclin T1 (CycT1) subunit of the positive transcription elongation f actor complex, P-TEFb, Binding of Tat to CycT1 induces cooperative binding of the beta -TEFb complex onto nascent HIV-1 TAR RNA. Here the specific int eraction between Tat protein, human cyclin T1, and HIV-1 TAR RNA was analyz ed by fluorescence resonance energy transfer, using fluorescein-labeled TAR RNA and a rhodamine-labeled Tat protein synthesized through solid-phase ch emistry. We find that CycT1 remodels the structure of Tat to enhance its af finity for TAR RNA and that TAR RNA further enhances the interaction betwee n Tat and CycT1, We conclude that TAR RNA nucleates the formation of the Ta t.beta -TEFb complex through an induced fit mechanism.