Transactivation of the two promoters of SFV-3 by different mechanisms

Simian foamy virus type 3 (SFV-3), a member of the spumavirus genus of retroviruses, has a complex genome organization and encodes two open reading frames (ORFs), in addition to the structural genes gag, pol, and env. ORF-1 encodes a viral transcriptional transactivator designated Taf (transactivator of foamy viruses) which augments transcription from the viral long terminal repeat (LTR). It was recently shown that human foamy virus, as well as the simian viruses SFV-1 and SFV-3, contains a second internal transcriptional promoter in the transmembrane domain of the env gene; this promoter also is transactivated by Taf. Here we report the characterization of the internal promoter of SFV-3. The transcriptional start site of this promoter has been mapped in two different SFV-3-infected cell lines to nt position 9761 in the proviral genome of SFV-3. All cis-regulatory elements required for transactivation by Taf are located between -202 and -32 (+1 representing the transcription initiation site in the internal promoter). Analysis of hybrid promoter constructs and deletion mutants in transient expression assays revealed that this region contains two elements which are independently responsive to Taf. In addition, we employed an in vivo DNA competition assay to determine whether the transactivation mechanisms of both SFV-3 promoters are similar or different. The differences observed utilizing this competition assay suggest that Taf transactivates the internal promoter and the LTR through different cellular transcription factors.