Human foamy virus Bel 1 transactivator/estrogen receptor fusion proteins allow inducible transactivation of both human foamy virus promoters.

Recombinant plasmids that express human foamy virus (HFV) Bel 1 transactivator and human estrogen receptor (ER) fusion proteins were constructed. The HFV bel 1 gene was inserted up- and downstream of the ER gene. Recombinant Bel 1-ER and ER-Bel 1 fusion proteins were expressed in eukaryotic cells. In the absence of estrogen, the ER moiety of the fusion proteins suppressed Bel 1-mediated transactivation as measured in CAT reporter gene-based transactivation assays. However, transactivation of the HFV LTR and the HFV internal promoter by Bel 1-ER and ER-Bel 1 fusion proteins was recovered in the presence of estrogen. Thus, the transactivation function of the Bel 1 moiety of the chimeric Bel 1-ER fusion proteins can be efficiently, specifically, and intentionally activated and inactivated by simply adding the low-molecular weight effector estrogen.