A transcriptional activator protein encoded by the x-lor region of the human T-cell leukemia virus.

Human T-cell leukemia viruses type I and II (HTLV-I and -II) exhibit several features characteristic of this retroviral family: the presence of an x-lor gene encoding a nuclear protein, transformation properties suggesting the involvement of a virus-associated trans-acting factor, and transcriptional trans-activation of the long terminal repeat (LTR) in infected cells. In the study described here the HTL x-lor products, in the absence of other viral proteins, were able to activate gene expression in trans directed by HTLV LTR. The regulation of the expression of particular genes in trans by HTLV x-lor products suggests that they play a role in viral replication and possibly in transformation of T lymphocytes.