Activation of enhancer sequences in type II human T-cell leukemia virus and bovine leukemia virus long terminal repeats by virus-associated trans-acting regulatory factors.

The ability of the sequences present in the long terminal repeats (LTRs) of human T-cell leukemia viruses type I and II (HTLV-I and HTLV-II) and of bovine leukemia virus to function as enhancer elements was investigated. Recombinant plasmids that contained the HTLV-I, HTLV-II, and bovine leukemia virus LTRs at a distance from a simian virus 40 promoter element located 5' to the bacterial gene encoding chloramphenicol acetyltransferase (EC 2.3.1.28) were constructed. We report that all three LTR sequences contain enhancer elements capable of increasing the level of gene expression directed from a distal heterologous promoter. The enhancer present in the HTLV-I LTR was active in uninfected cells of lymphoid and nonlymphoid origin. In contrast, the enhancer activity of the HTLV-II and bovine leukemia virus LTR sequences was evident only in virus-infected cells. This activity is likely due to virus-associated trans-acting transcriptional factors previously shown to be present in HTLV- and bovine leukemia virus-infected cells. The implication of these observations for virus replication and transforming activity are discussed.