The interaction between two regulatory proteins plays a crucial role in the control of several biological events, including gene transcription. In this report, we demonstrate that the interaction between the cellular sequence-specific single-stranded DNA binding protein Purα and the HIV type 1 (HIV-1) Tat protein is mediated by specific ribonucleic acids. The region of Tat that is important for its interaction with Purα includes the region demonstrated to bind Tat's viral RNA target, TAR. A 10-nucleotide GC-rich consensus sequence identified in RNAs associated with Purα derived from human U-87MG cells plays an important role in the Purα . Tat interaction as examined by an in vitro reconstitution assay. Furthermore, expression of the Purα - associated RNA in these cells enhances transcriptional activation of the HIV-1 promoter by Tat and Purα .