Nuclear transcriptional activator of viral gene expression, that is essential for viral transcription from the LTR promoter and replication. Acts as a sequence-specific molecular adapter, directing components of the cellular transcription machinery to the viral RNA to promote processive transcription elongation by the RNA polymerase II (RNA pol II) complex, thereby increasing the level of full-length transcripts. Tat binds to a hairpin structure at the 5'-end of all nascent viral mRNAs referred to as the transactivation responsive RNA element (TAR RNA) in a CCNT1-independent mode. Tat then recruits the CCNT1/cyclin-T1 component of the P-TEFb complex (CDK9 and CCNT1), which promotes RNA chain elongation. The CDK9 component of P-TEFb hyperphosphorylates the C-terminus of RNA Pol II that becomes stabilized and much more processive (Probable). Binds to bovine CCNT1/cyclin-T1. Also binds human, equine and mouse CCNT1. The sequence shown is that of isolate R29-127. Strongly transactivates the viral promoter. Strongly transactivates the viral promoter. Belongs to the lentiviruses Tat family.