2 DISTINCT REGIONS FORM A FUNCTIONAL ACTIVATION DOMAIN OF THE HTLV-1 TRANSACTIVATOR TAX1

Taxi of human T-cell leukemia virus type 1 (HTLV-1) is an enigmatic vi ral transactivator that regulates expression of the viral gene and als o several cellular genes normally controlled by various mitogenic sign als. However, previous studies have failed to define the functional do mains of Taxi for enhancer specificities and for transcriptional activ ation (95% of the protein portion was indispensable for the activation function). This complexity has hampered understanding of the molecula r basis of Taxi action. In this study, we analysed the activation func tion of a Taxi fused to the heterologous DNA-binding domain of the yea st transcription factor GAL4 and dissected the domain required for the activation function by using derivatives of a Taxi mutant with an ins ertion between amino acids (a.a.) 170 and 171. Analysis of the derivat ives of the mutant fusion protein having various partial overlaps enco mpassing the interrupted site suggested that two contiguous stretches, AD-I (2-255 a.a.) and AD-II (227-337 a.a.), should be both intact for the activation function of Taxi and that they form a functional activ ation domain.