cis-acting element located in the bovine foamy virus internal promoter possesses the properties of a transcriptional enhancer

Bovine foamy virus encodes a transcriptional transactivitor, Ths or Borf-1, which governs the level of viral transcripts initiated by both the promoter in the long terminal repeat (LTR) and the internal promoter (IP) located in the env gene through their cis-acting targets. We have identified and characterized a 72 bp Tas (Borf-1) responsive element located in BFV3026 internal promoter (TREIP) by deletion mutant and transient expression assay. This cis-acting target element in the internal promoter has the properties of a transcriptional enhancer which functions independently of its orientation, position and also in heterologous promoters (BFV LTR and bovine immunodeficiency virus, BIV LTR). Alignments reveal that there are positional similarity and sequence homology among BFV TREIP, SFV-1 TREIP proximal element and SFV-3 (TREII)-I-IP, which suggests that this kind of cis-acting elements possesses some common functional character.