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Identification of a region of a murine leukemia virus long terminal repeat with novel transcriptional regulatory activities

The 93-bp region downstream of the enhancer (DEN) in the long terminal repeat (LTR) of the mink cell focus-forming virus (MCF13) has been shown to be important for transcriptional activation and viral lymphomagenicity (J. C. Tupper, H. Chen, E. F. Hays, G. C. Bristol, and F. K. Yoshimura, J. Virol. 66:7080-7088, 1992). In this report, we have further explored the role of the DEN region in transcriptional activation. We observed that it has enhancer-like abilities as well as some unique LTR properties. Transcriptional activation by the DEN region involved interactions with enhancer sequences that were either synergistic or additive, depending on the cell type. The most intriguing property of the DEN region is its ability to induce transcription in activated T cells. This activity is unique for the LTR in that no other LTR region can do this. We also examined the role of the DEN region in retroviral lymphomagenesis. We cloned and sequenced proviral LTRs integrated upstream of the cellular c-myc gene from DNA obtained from thymic tumors induced by DEN region deletion mutant viruses in AKR mice. We determined that for transcriptional activation of the c-myc proto-oncogene, enhancer sequences can substitute for the DEN region. This study identifies the significance of non-enhancer sequences in the LTR for the oncogenesis of the MCF13 retrovirus.