IDENTIFICATION OF HTLV-I TAX TRANSACTIVATOR MUTANTS EXHIBITING NOVEL TRANSCRIPTIONAL PHENOTYPES

The type I human T-cell leukemia virus (HTLV-I) encodes a 40-kD nuclear trans-regulatory protein termed Tax that transcriptionally activates the HTLV-I long terminal repeat (LTR), as well as selecting cellular and heterologous viral promoters. Tax does not bind DNA specifically but, rather, acts in a more indirect manner. Tax activation of the HTLV-I LTR is mediated through constitutively expressed cellular factors that bind to cAMP response elements (CREs) present within the 21-bp enhancers of the LTR. In contrast, Tax trans-activation of the interleukin-2 receptor-α gene (IL-2Rα) and LTR of the type 1 human immunodeficiency virus (HIV-1) involves the induced nuclear expression of NF-κB. We now report the identification of missense mutations within the tax gene that functionally segregate these two pathways of trans-activation. Additionally, we demonstrate that the carboxyl terminus of the Tax protein, despite its acidic and predicted α-helical structure, is completely dispensable for trans-activation through either of these transcription factor pathways. Finally, we demonstrate that mutations within a putative zinc finger domain disrupt the nuclear localization of Tax and abolish trans-activation. These results demonstrate that Tax trans-activation of viral and cellular promoters involves at least two mechanisms of host transcription factor activation and suggest that this activation is likely mediated through distinct functional domains.