Note: this site last updated in 2006
An article from "The Molecules of HIV" (c) Dan Stowell
The TAR is found in the Long Terminal Repeat of HIV RNA/DNA. It's present in all mRNA transcripts and prevents the efficient use of the mRNA. tat binds to the RNA version of TAR and cancels its effect, allowing the mRNA to be efficiently processed into protein.
The TAR regions of HIV-1 and HIV-2 are different (HIV-1 TAR is pictured below) - but the conserved CUGGGA loop (at the top of the diagram), as well as the bulge just a little way down from that, has been shown to be essential for the trans-activation response.