The human T-cell leukemia viruses, HTLV-I and HTLV-II, contain a gene, termed x, with transcriptional regulatory function. The properties of the x proteins were analyzed by constructing mutant genes containing site-directed deletions and point mutations. The results demonstrate that the amino terminal 17 amino acids of the x protein constitute part of a functional domain that is critical for the transcriptional activating properties of the protein. Within this region, substitution of a leucine residue for a proline residue results in major changes in the trans-activation phenotype of the protein. The mutant HTLV-II x protein, though incapable of activating the HTLV-II long terminal repeat, will block trans-activation of the HTLV-II long terminal repeat by the wild-type protein. The altered phenotype of this mutant suggests a potential negative regulatory function of the x protein.

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