S. Pacauccaralertkun et al., IN-VITRO SELECTION OF DNA ELEMENTS HIGHLY RESPONSIVE TO THE HUMAN T-CELL LYMPHOTROPIC VIRUS TYPE-I TRANSCRIPTIONAL ACTIVATOR, TAX, Molecular and cellular biology, 14(1), 1994, pp. 456-462
The human T-cell lymphotropic virus type I (HTLV-I) transactivator, Ta
x, the ubiquitous transcriptional factor cyclic AMP (cAMP) response el
ement-binding protein (CREB protein), and the 21-bp repeats in the HTL
V-I transcriptional enhancer form a ternary nucleoprotein complex (L.
J. Zhao and C. Z. Giam, Proc. Natl. Acad. Sci. USA 89:7070-7074, 1992)
. Using an antibody directed against the COOH-terminal region of Tax a
long with purified Tax and CREB proteins, we selected DNA elements bou
nd specifically by the Tax-CREB complex in vitro. Two distinct but rel
ated groups of sequences containing the cAMP response element (CRE) fl
anked by long runs of G and C residues in the 5' and 3' regions, respe
ctively, were preferentially recognized by Tax-CREB. In contrast, CREB
alone binds only to CRE motifs (GNTGACG[T/C]) without neighboring G-
or C-rich sequences. The Tax-CREB-selected sequences bear a striking r
esemblance to the 5' or 3' two-thirds of the HTLV-I 21-bp repeats and
are highly inducible by Tax. Gel electrophoretic mobility shift assays
, DNA transfection, and DNase I footprinting analyses indicated that t
he G- and C-rich sequences flanking the CRE motif are crucial for Tax-
CREB-DNA ternary complex assembly and Tax transactivation but are not
in direct contact with the Tax-CREB complex. These data show that Tax
recruits CREB to form a multiprotein complex that specifically recogni
zes the viral 21-bp repeats. The expanded DNA binding specificity of T
ax-CREB and the obligatory role the ternary Tax-CREB-DNA complex plays
in transactivation reveal a novel mechanism for regulating the transc
riptional activity of leucine zipper proteins like CREB.