HUMAN T-CELL LEUKEMIA-VIRUS TYPE-I TAX PROTEIN TRANSACTIVATES RNA-POLYMERASE-III PROMOTER IN-VITRO AND IN-VIVO

Tax protein of the human T-cell lymphotropic virus type 1 (HTLV-I) is critical for viral replication and is a potent transcriptional activat or of viral and cellular polymerase II (pol II) genes, We report here that Tax is able to transactivate a classical pol III promoter, VA-I. In cotransfection experiments, Tax is shown to increase transcription of the VA-I promoter approximately 25-fold. Moreover, Tax is able to a ctivate VA-I transcription when added exogenously to an in vitro trans cription reaction, Using Tax affinity column chromatography, we demons trate that Tax is able to deplete a HeLa cell extract for components r equired for transcription of VA-I. The transcriptional activity of the Tax-depleted extract can be restored by the 0.6 M phosphocellulose fr action. Interestingly, a consensus binding site for cAMP-responsive el ement binding protein (CREB) is located upstream of the VA-I promoter, and deletion of this element results in the loss of Tax responsivenes s, When this CREB binding site is replaced by a Gal-4 binding site, th e VA-I promoter can be transactivated by a Gal4-Tax fusion protein, Ta ken together, these results suggest that Tax may activate pol III and pol II promoter through a similar mechanism involving the CREB activat ion pathway, It is also possible that Tax affects pol III transcriptio n by direct interaction with a component of the pol III transcriptiona l machinery.