TRANSCRIPTIONAL TRANSACTIVATORS OF HUMAN AND SIMIAN FOAMY VIRUSES CONTAIN A SMALL, HIGHLY CONSERVED ACTIVATION DOMAIN

The Bel-1 protein of human foamy virus is a potent transcriptional tra ns activator of its homologous long terminal repeat promoter element. Here, we demonstrate that Bel-1 can also efficiently activate gene exp ression when targeted to a heterologous promoter by fusion to the DNA- binding motif of the yeast GAL4 protein. Analysis of a series of delet ion mutants of Bel-1 generated in this hybrid protein context suggests the presence of a single transcription activation domain that is full y contained within a discrete, approximately 30-amino-acid segment loc ated proximal to the Bel-1 carboxy terminus. Although this short motif can be shown to function effectively in eukaryotic cells of mammalian , avian, and fungal origin, it does not bear any evident sequence homo logy to the known classes of eukaryotic activation domain. However, th is Bel-1 activation domain was found to be fully conserved, in terms o f both biological activity and location, in the distantly related Taf trans activator of simian foamy virus type 1.