Twenty-one base pair repeat elements influence the ability of a Gal4-Tax fusion protein to transactivate the HTLV-I long terminal repeat.

The Tax1 protein of the human T-cell leukemia virus (HTLV-I) is a 40-kDa positive transactivator of viral gene expression. Tax1 does not bind directly to DNA, but associates indirectly with DNA via cellular transcription factors. To further investigate the activation of HTLV-I transcription by Tax1, a chimeric protein containing Tax1 fused to the DNA binding domain of Gal4 was created (Gal4-Tax). HTLV-I long terminal repeat (LTR) reporter plasmids were constructed in which specific Tax1 responsive elements were replaced with Gal4 binding sites. Cotransfection of Gal4-Tax or Tax1 with HTLV-I LTR reporter constructs containing Gal4 binding sites demonstrated that Gal4 sequences were necessary but not sufficient for maximal activation of the promoter by Gal4-Tax. Sequences surrounding the Gal4 binding sites were important in determining the level of Gal4-Tax activation. Association of Gal4-Tax with promoters which contained six Gal4 binding sites, but which lacked flanking LTR sequences, were weakly transactivated by Gal4-Tax (sevenfold). In contrast, LTR-CAT reporter constructs containing three Gal4 binding sites flanked by two 21 base pair repeat elements demonstrated a ninefold greater response to Gal4-Tax. These results suggest that cellular transcription factors, which bind the 21 base pair repeat elements, influence the ability of Tax1 to function as a transactivator. Furthermore, this effect is not fully explained by the ability of these factors to physically direct Tax1 to the LTR.