DEFINITION OF A MINIMAL ACTIVATION DOMAIN IN HUMAN T-CELL LEUKEMIA-VIRUS TYPE-I TAX

Fourteen mutants were used to delineate a minimal activation domain in the Tax protein of human T-celI leukemia virus type I, In an assay us ing a Gal4-Tax (GalTx) fusion protein and a responsive promoter contai ning Gal4 consensus binding sites, we found that activation was ''sque lched'' by coexpression of wild-type Tax protein in trans. When Tax mu tants were tested for squelching, many competed effectively against Ga lTx. However, those containing changes in amino acids 289 to 322 faile d to inhibit activity. In particular, three mutants that were expresse d stably, with changes at amino acids 289, 296, and 320 respectively, did not squelch GalTx activity. On the other hand, mutants with indivi dual changes at amino acid 3, 9, 29, 41, 273, and 337 efficiently inhi bited GalTx function. Three other mutants failed to be stably expresse d. In separate experiments, when fused alone to the DNA-binding domain of Gal4, amino acids 289 to 322 of Tax conferred trans activation abi lity. This fusion protein was able to activate a core promoter. These findings suggest that amino acids 289 to 322 define a region that cont acts an essential transcription factor and that this region is a modul ar activation domain.