Functional interaction between transcriptional elements in the long terminal repeat of reticuloendotheliosis virus: cooperative DNA binding of promoter- and enhancer-specific factors.

Transcription from reticuloenodotheliosis virus strain T (REV-T), an avian retrovirus unrelated to avian leukosis and sarcoma viruses, is modulated by sequences in at least five functional domains. A promoter containing a TATA and multiple CCAAT motifs in U3 of the long terminal repeat was absolutely required for transcription. Transcriptional efficiency was greatly augmented by an enhancer immediately upstream, which contained a 22-base-pair repeated sequence. Transcription was further influenced by a negative-acting domain in the 5' region of U3 and two downstream domains in the transcribed non-protein-coding region. One of these latter domains contained a consensus enhancer core sequence and positively affected transcription in both mammalian and avian cells; the other acted negatively in a dog cell line. Transcription from REV-T in vivo required cellular factors which could be competed for specifically by the promoter or enhancer domain. The downstream domains competed with reporter genes containing these domains, but not directly with the U3 sequences. The promoter, enhancer, and the positive-acting downstream domains formed multiple complexes with distinct classes of cellular factors in both avian and mammalian cell extracts. Binding of factors to the promoter and enhancer domains was cooperative when these domains were joined in cis.