Abstract

The human immunodeficiency virus type I (HIV-1) nuclear protein Tat is a potent activator of viral gene transcription. Activation by Tat requires a cis-acting element, the transactivation response (TAR) site, located immediately downstream of the transcription start site. Several observations suggest that TAR functions as the nascent RNA product of the HIV long-terminal-repeat promoter (for a review, see ref. 6). Indeed, Tat protein and several cellular proteins bind directly to nascent TAR RNA in vitro. The significance of these in vitro interactions remains to be established. Here we report that Tat can activate transcription when bound to nascent RNA through the RNA-binding domain of another HIV-1 protein, Rev. Rev is a sequence-specific RNA-binding protein, which interacts with the viral RNA element RRE (refs 11-15). A Tat-Rev fusion protein efficiently activates transcription from an HIV-1 promoter derivative, in which TAR has been replaced by the RRE. We conclude that activation of transcription by Tat can occur by direct binding to nascent RNA, and that the sole function of TAR may be to provide a Tat-binding site. Our results further suggest that cellular proteins that bind specifically to TAR RNA or TAR DNA may not be essential for Tat-responsiveness.