Functional Analysis of Human T-Cell Leukemia Virus Type I rex-Response Element: Direct RNA Binding of Rex Protein Correlates with In Vivo Activity

The human T-cell leukemia virus type I rex gene product plays a critical role in the expression of the retroviral structural proteins Gag and Env from incompletely spliced mRNAs. Rex protein acts through a cis element (rex-response element [RxRE]) which is located in the U3/R region of the 3 ' long terminal repeat and is present on all human T-cell leukemia virus type I-specific mRNAs. Two domains of the predicted secondary structure of the RxRE are crucially important for Rex action in vivo as measured by two assay systems. In vitro studies using highly purified recombinant Rex protein revealed a specific and direct interaction with radiolabeled RxRE sequences. The correlation between our in vivo results and the direct binding of Rex protein to mutant and wild-type RxRE sequences supports both the existence of the predicted secondary structure and the importance of this direct interaction with the cis-acting RNA sequence for Rex function in vivo. Human T-cell leukemia virus type I (HTLV-I), the etio-logic agent of adult T-cell leukemia and HTLV-I-associated myelopathy (8, 18-20, 22, 27), is provided with a complex genome organization. In addition to the retroviral structural genes gag, pol, and env, HTLV-I encodes at least two regulatory proteins (Tax and Rex) in the 3 ' region of th