UCSF

The HIV-1 Tat protein relieves a block to transcription elongation by recruiting positive transcription elongation factor b (P-TEFb) to an RNA element (TAR) on the viral mRNA, but why the HIV-1 promoter was inactive was unclear. Now it is shown that Tat binds to P-TEFb in its inactive 7SK snRNP form, and this complex is recruited to the HIV-1 promoter before transcription initiation, in a TAR-independent manner. The inhibitory 7SK snRNP complex is displaced once transcription begins, resulting in P-TEFb activation.