Transcription factor AP-4 participates in activation of bovine leukemia virus long terminal repeat by p34 Tax.

Three 21bp repeats can be found in the bovine leukemia virus long terminal repeat, which are crucial for the LTR directed gene expression by the trans activator protein Tax. Previous studies demonstrated that the major target of the Tax directed activation are the CRE-like elements in the center of these repeats. In this work we report that another motif of the 21bp repeats is also required for the Tax activation. Gel retardation--with the wild type or mutant 21bp repeats--revealed that cellular factors from HeLa cells were specifically bound to the center (CRE-like element) and the 3' region of the repeats, which contains a CAGCTG consensus AP-4 binding site. In vivo analysis using the synthetic 21bp repeats indicated that beyond the consensus CRE-like motif, the AP-4 site is also essential for Tax activation. To determine the role of AP-4 in BLV Tax trans activation, we used the AP-4 cDNA in antisense transient assays. In the in vivo experiments the antisense AP-4 RNA resulted in strongly decreased Tax activation. On the basis of these results we conclude that AP-4 is a good candidate of cellular factors involved in BLV Tax trans activation.