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. In the absence of Tat, the RNA Pol II generates short or non-processive transcripts that terminate at approximately 60 bp from the initiation site. Tat associates with the CCNT1/cyclin-T1 component of the P-TEFb complex (CDK9 and CCNT1), which promotes RNA chain elongation. This binding increases Tat's affinity for a hairpin structure at the 5'-end of all nascent viral mRNAs referred to as the transactivation responsive RNA element (TAR RNA) and allows Tat/P-TEFb complex to bind cooperatively to TAR RNA. The CDK9 component of P-TEFb and other Tat-activated kinases hyperphosphorylate the C-terminus of RNA Pol II that becomes stabilized and much more processive (By similarity). Extracellular circulating Tat can be endocytosed by surrounding uninfected cells via the binding to several surface receptors. Endosomal low pH allows Tat to cross the endosome membrane to enter the cytosol and eventually further translocate into the nucleus, thereby inducing severe cell dysfunctions ranging from cell activation to cell death. Through (By similarity). Binds human CCNT1. Binds also to CCNT2, but the resulting complex is unable to bind to TAR RNA. May participate in the formation of a complex composed at least of Tat, P-TEFb, TAR RNA, RNA Pol II (By similarity). The Arg-rich RNA-binding region binds the TAR RNA. This region also mediates the nuclear localization (By similarity). The phosphorylation by CDK9 does not seem to be important for transactivation function. This isolate is from a Gambian case of 'neuro-AIDS'. Expressed in the late stage of the infection cycle, when unspliced viral RNAs are exported to the cytoplasm by the viral Rev protein. Belongs to the lentiviruses Tat family.