The control of HTLV-1 viral gene expression is complex and involves the regulation of mRNA synthesis by Tax1 and post-transcriptional processing by Rex1. The Tax1 protein functions as a transcriptional transactivator of the HTL V-1 LTR and cellular genes through cooperation and association with cellular transcription factors which bind to the LTR. In this article, we discuss (1) the functional domains of the Tax1 regulatory protein; (2) Tax1 regulation of viral and cellular gene expression; (3) interaction of Tax1 with cellular transcription factors; (4) the potential role of extracellular Tax1 protein as a regulator of cellular gene expression. Understanding of the molecular mechanism by which Tax1 transactivates viral and cellular genes will provide important insights into the interaction between the virus and the host cell during viral replication and transformation.
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