Expression of the complete human T-cell leukemia virus type I pX coding sequence as a functional protein in...

Human T-cell leukemia virus type I (HTLV-I), a virus associated with adult T-cell leukemia, contains a long open reading frame (LOR) in the 3' end of its genome between the env region and the 3' long terminal repeat (LTR). This open reading frame encodes a 40-kDa protein (designated p40x) that has been implicated as a positive control element for transcription from the HTLV-I LTR in a phenomenon known as trans-activation. We now report the expression of the complete p40x coding sequence as a 40-kDa protein in Escherichia coli. The p40x protein produced in bacteria is shown, using the protoplast fusion technique, to possess biological activity by its ability to trans-activate a HTLV-I LTR-chloramphenicol acetyltransferase plasmid that is stably integrated into the genome of mouse L cells. This stimulatory activity could be detected within 2 hr after fusion, suggesting the possibility of a direct role for p40x in trans-activation of the HTLV-I LTR. The production of p40x in large quantities in E. coli, together with the rapid protoplast fusion assay for its biological activity, should facilitate the analysis of p40x mutants and the elucidation of the molecular mechanism of trans-activation.