Activation of the human T-cell leukemia virus type I enhancer is mediated by binding sites for Elf-1 and the pets factor.

Infection with human T-cell leukemia virus type I (HTLV-I) is associated with adult T-cell lymphoma/leukemia. This disease occurs in only a small minority of people infected with HTLV-I and manifests itself many years after infection. Therefore, it appears that a fine balance exists between HTLV-I and the host T-cell factors with which it interacts. HTLV-I encodes a transactivating protein, Tax, which activates viral transcription via cellular mechanisms which are incompletely understood. As viral gene expression is negligible during latency, it is doubtful that Tax controls the initial transition to the replicative state. Tax-independent cellular factors which control HTLV-I transcription, and presumably latency, have received little study. Recently, the product of the chicken proto-oncogene ets-1 has been shown to bind to the HTLV-I enhancer and modestly activate transcription in certain cell types (S. C. Gitlin, R. Bosselut, A. Gégonne, J. Ghysdael, and J. N. Brady, J. Virol. 65:5513-5523, 1991). However, the functional significance of the ets-binding site in the intact enhancer has not previously been shown. We now demonstrate that site-specific mutation of the purine-rich ets-binding site significantly diminishes inducible enhancer function, but not Tax response, in the human Jurkat T-cell line. Similarly, mutation of the peri-ets (pets) site, not previously noted in the HTLV-I enhancer, markedly inhibits inducible enhancer function but not Tax response. Further, we show that the predominant protein binding the purine-rich HTLV-I enhancer element in human T cells is not ets-1 but Elf-1, a member of the ets family which is very similar to the Drosophila morphogen E74. Regulation of HTLV-I through Elf-1/pets enhancer motifs resembles that seen with human immunodeficiency virus type 2 (D. M. Markovitz, M. Smith, J. Hilfinger, M. C. Hannibal, B. Petryniak, and G. J. Nabel, J. Virol. 66:5479-5484, 1992; J. M. Leiden, C.-W. Wang, B. Petryniak, M. Smith, D. M. Markovitz, G. J. Nabel, and C. B. Thompson, J. Virol. 66:5890-5897, 1992), another human pathogenic retrovirus with a relatively long incubation period.