The gene coding for thetrans‐activating factor (Tax) of the human T‐cell leukemia virus, type I (HTLV‐I) was mutagenized in vitro using oligonucleotide‐directed mutagenesis and recombinant DNA techniques. All except one of the mutagenizedtax constructs failed totrans‐activate the HTLV‐I LTR in a eukaryotic test system. Moreover, negative Tax mutant Arg‐39→Gly was found to betrans‐dominant. This observation suggests that Tax contains distinct functional domains mediating different interactions of the protein in the process oftrans‐activation.
