We have identified nucleotide sequences that regulate transcription in both a cell-type-specific and general manner in the long terminal repeat of the MCF13 murine leukemia virus. Besides the enhancer element, we have observed that the region between the enhancer and promoter (DEN) has a profound effect on transcription in different cell types. This effect, however, was dependent on the copy number of enhancer repeats and was detectable in the presence of a single repeat. When two enhancer

more »

... ts were present, the effect of DEN on transcription was abrogated except in T cells. DEN also makes a significant contribution to the leukemogenic property of the MCF13 retrovirus. Its deletion from the MCF13 virus dramatically reduced the incidence of thymic lymphoma and increased the latency of disease in comparison with the wild-type virus. This effect was most marked when one rather than two enhancer repeats was present in the mutant viruses. We also observed that the removal of one repeat alone remarkably reduced leukemogenicity by the MCF13 virus. A newly identified protein-binding site (MLPal) located within DEN affects transcription only in T cells, and its deletion attenuates the ability of an MCF13 virus with a single enhancer repeat to induce thymic lymphoma. This observation suggests that the MLPal protein-binding site contributes to the effect of the DEN region on T-cell-specific transcription and viral leukemogenicity. This study identifies the importance of nonenhancer sequences in the long terminal repeat for the oncogenesis of the MCF13 retrovirus. * Corresponding author. from an AKR spontaneous lymphoma [39]) have demonstrated that the LVb and core sites are significant for transcriptional activity in T cells (1, 29, 47, 49) . In SL3-3 MLV, the protein-binding site that overlaps with both the LVb and core sites is referred to as SEF 1 (48). The LVb and core sites were also shown to determine disease specificity in studies examining the effects of mutated LTRs upon viral pathogenicity (17, 46). Other protein-binding sites which affected transcription in various cell types and increased the latency of leukemogenesis included the Mo-MLV NF1 and GRE sites (29, 35, 46, 47) . In addition, a GC-rich sequence downstream of the Mo-MLV enhancer has been shown to contribute to enhancer activity and to the type of leukemia induced by this virus (19, 27) . The notion that the activity of this downstream region is mediated by transactivating factors is supported by the observations that protein binding occurs to analogous sites in the Friend and MCF13 MLV LTRs (32, 58). MCF13 MLV is an example of a class I MCF virus that is able to induce thymic lymphoma (5, 20, 36, 51) . A nucleotide sequence comparison revealed that the MCF13 MLV LTR contains sequences highly homologous to the protein-binding sites identified in the Mo-MLV LTR. The LVb and GRE sites are identical for these two MLVs, and the LVa, LVc, core, and NF1 sites are highly homologous. Moreover, these sites are similarly located in the two LTRs. In addition, we have identified by gel mobility shift and DNA footprinting analyses a 25-bp protein-binding site, MLPal, starting 15 bp downstream of the MCF13 MLV enhancer repeat (58, 59). By a deletion analysis of the MCF MLV LTR, we demonstrated that a region in the enhancer that includes the LVb, core, and LVc sites was responsible for a large amount of transcriptional activity in both T cells and fibroblasts (23). 7080 on May 9, 2020 by guest