CBF, MYB, AND ETS BINDING-SITES ARE IMPORTANT FOR ACTIVITY OF THE CORE-I ELEMENT OF THE MURINE RETROVIRUS SL3-3 IN T-LYMPHOCYTES

Transcriptional enhancers within the long terminal repeats of murine l eukemia viruses are major determinants of the pathogenic properties of these viruses. Mutations were introduced into the adjacent binding si tes for three transcription factors within the enhancer of the T-cell- lymphomagenic virus SL3-3. The sites that were tested were, in 5'-to-3 ' order, a binding site for core binding factor (CBF) called core II, a binding site for c-Myb, a site that binds members of the Ets family of factors, and a second CBF binding site called core I. Mutation of e ach site individually reduced transcriptional activity in T lymphocyte s. However, mutation of the Myb and core I binding sites had larger ef fects than mutation of the Ets or core II site. The relative effects o n transcription in T cells paralleled the effects of the same mutation s on viral lymphomagenicity, consistent with the idea that the role of these sequences in viral lymphomagenicity is indeed to regulate trans cription in T cells. Mutations were also introduced simultaneously int o multiple sites in the SL3-3 enhancer. The inhibitory effects of thes e mutations indicated that the transcription factor in T cells that re cognizes the core I element of SL3-3, presumably CBF, needed to synerg ize with one or more factors bound at the upstream sites to function. This was tested further by generating a multimer construct that contai ned five tandem core I elements linked to a basal long terminal repeat promoter. This construct was inactive in T cells. However, transcript ional activity was detected with a multimer construct in which the tra nscription factor binding sites upstream of the core were also present . These results are consistent with the hypothesis that CBF requires h eterologous transcription factors bound at nearby sites to function in T cells.