The bovine leukemia virus transactivator, BLV Tax, augments transcription from three imperfect 21-bp repeats in the viral transcriptional regulatory region. Each BLV 21-bp repeat contains a cAMP response element (CRE) in the center and unique 5' and 3' neighboring sequences which are crucial for the transcriptional activation by BLV Tax. Here we describe the interactions of recombinant BLV Tax with cellular bZip proteins. The recombinant BLV Tax, tagged at the carboxy terminus with a hexahistidine extension, was prepared by solubilization in 6 M guanidine hydrochloride and renaturation on the Ni(2+)-chelating Sepharose gel matrix. The renatured BLV TaxH6 activates the BLV LTR when introduced into HeLa cells by scrape loading. Furthermore, the purified BLV TaxH6 enhances binding of members of the CREB/ATF family of bZip proteins to CRE motifs by interacting with their bZip domains in vitro. Chemical cross-linking indicates that dimerization of bZip proteins such as CREB becomes greatly facilitated in the presence of BLV Tax. These results suggest that BLV Tax interacts directly with CREB/ATF-like factors to activate viral mRNA transcription.