THE HUMAN FOAMY VIRUS BEL-1 TRANSCRIPTION FACTOR IS A SEQUENCE-SPECIFIC DNA-BINDING PROTEIN

The Bel-1 transcriptional transactivator encoded by human foamy virus (HFV) can efficiently activate gene expression directed by both the HF V long terminal repeat (LTR) and internal (Int) promoter elements. By DNA footprinting and gel retardation analysis, we demonstrate that Bel -1 can specifically bind to discrete sites in both the LTR and Int pro moter elements in vitro. However, transactivation of the HFV LTR by Be l-1 was observed to require not only the promoter-proximal Bel-1 bindi ng site identified in vitro but also additional promoter-distal sequen ces. These data suggest that Bel-1 binding is necessary but not suffic ient for efficient transactivation of Bel-1-responsive promoters in ma mmalian cells and therefore raise the possibility that Bel-1 function may require the action of a cellular DNA binding protein(s). Important ly, these data demonstrate that Bel-1 is unique among retroviral regul atory proteins in being a sequence-specific DNA binding protein.