Identification of the simian foamy virus transcriptional transactivator gene (taf).

Simian foamy virus type 1 (SFV-1), a member of spumavirus subfamily of retroviruses, encodes a transcriptional transactivator that functions to strongly augment gene expression directed by the viral long terminal repeat (LTR). The objective of this study was to identify the viral gene responsible for transactivation. Nucleotide sequences between the env gene and the LTR of SFV-1 were determined. The predicted amino acid sequence revealed two large open reading frames (ORFs), designated ORF-1 (311 amino acids) and ORF-2 (422 amino acids). In the corresponding region of the human foamy virus, three ORFs (bel-1, bel-2, and bel-3) have been identified (R. M. Flugel, A. Rethwilm, B. Maurer, and G. Darai, EMBO J. 6:2077-2084, 1987). Pairwise comparisons of the ORF-1 and ORF-2 with bel-1 and bel-2 show small clusters of homology; less than 39% overall homology of conserved amino acids is observed. A counterpart for human foamy virus bel-3 is not present in the SFV-1 sequence. Three species of viral RNA have been identified in cells infected with SFV-1; an 11.5-kb RNA representing full-length transcripts, a 6.5-kb RNA representing the env message, and a 2.8-kb RNA from the ORF region. Analysis of a cDNA clone encoding the ORF region of SFV-1 reveals that the 2.8-kb message is generated by complex splicing events involving the 3' end of the env gene. In transient expression assays in cell lines representing several species. ORF-1 was shown to be necessary and sufficient for transactivating viral gene expression directed by the SFV-1 LTR. The target for transactivation is located in the U3 domain of the LTR, upstream from position - 125 (+ 1 represents the transcription initiation site). We propose that OFF-1 of SFV-1 be designated the transcriptional transactivator of foamy virus (taf).