A NEW REGULATORY ELEMENT THAT AUGMENTS THE TAX-DEPENDENT ENHANCER OF HUMAN T-CELL LEUKEMIA-VIRUS TYPE-1 AND CLONING OF CDNAS ENCODING ITS BINDING-PROTEINS

The Tax protein of human T-cell leukemia virus type 1 (HTLV-1) trans a ctivates the 21-bp enhancer of HTLV-1. A sequence of more than two cop ies of the 21-bp enhancer is efficiently activated by Tax, but one cop y is not activated extensively. Another sequence (TRE-2, positions - 1 63 to - 117) adjacent to the 21-bp enhancer in the long terminal repea t of HTLV-1 can enhance a single copy of the 21-bp enhancer activity i n trans activation by Tax. This sequence contains motifs related to th e Ets- and NF-KB-binding sequences, but mutations at these sites indic ated that neither is responsive to cooperation with the 21-bp enhancer . A deletion mutation of TRE-2 identified 25 bases at positions - 158 to - 134 (TRE-2S) as an essential sequence, and TRE-2S was sufficient to give maximum cooperation with one copy of the 21-bp enhancer in tra ns activation by Tax protein. Using TRE-2S as a probe, we screened a c DNA library of HUT102 cells by the Southwestern (DNA-protein) procedur e and isolated two cDNA clones, THP-1 and -2. These two clones encode TRE-2S-binding proteins, and they differ by only an extra 17 amino aci ds in THP-2. Both THP proteins contain five zinc finger motifs which a re strikingly similar to those of the GLI family, an amplified gene pr oduct in glyoma cells. The binding site of THP-1 and -2 was GAACCACCCA in TRE-2S, which is highly homologous to the GLI-binding site. These results suggest that binding of THP to TRE-2S may be involved in coope ration with one copy of the 21-bp enhancer in responding to Tax trans activation.