A downstream regulatory element activates the bovine leukemia virus promoter

Recent studies demonstrated, that the R/U5 region of the Bovine Leukemia Virus (BLV) long terminal repeat (LTR) up-regulates the virus promoter. It is also known that this effect is independent from the BLV trans activator protein, p34tax, encoded by the virus genome. Deletions were constructed in the R/U5 region to localize the sequences responsible for this effect. The activity of the different constructs was determined in a transient expression system. Our results show that a 64 bp long sequence (called DAS), present at the 3 end of the R region, is involved in the activation. The in vivo results indicate that DAS could be divided into two independent but overlapping elements (DAS1,2). Sequence comparison allows the identification of three conservative boxes in these elements. Our results suggest, that these boxes are functional only together. The gel-shift assay with DAS2, in good agreement with the in vivo data, demonstrates that only the full length element forms a low mobility DNA-protein complex.