Models for the interaction of DNA-bound GAL4-VP16
and the TBP-TFIIA-TFIIB-promoter (TATA) quaternary complex.
A, schematic representation of the expected location of GAL4-
VP16 dimers on templates carrying a single GAL4 binding site. The
figure shows a transversal view of the DNA with GAL4-VP16 dimers
positioned on various faces of the helix. The color code represents the
activation level obtained with each template: green, forward orientation;
red, reverse orientation. Darker colors corresponds to stronger
activation levels and lighter colors to lower activation levels. B, one
GAL4-VP16 dimer bound to upstream promoter DNA (GML 258) can
contact a TBP-TFIIA-TFIIB complex positioned at the TATA box
through looping of the DNA helix. Although our results do not determine
the interface of the TBP-TFIIA-TFIIB complex that is contacted
by DNA-bound GAL4-VP16, one possibility is a domain of TFIIB (Val-
135, Arg-137, Asn-139 and Asn-140; depicted in red) that previously has
been shown to be essential for transcriptional activation (62). However,
our results indicate that this TFIIB domain alone is not sufficient for
the interaction of DNA-bound GAL4-VP16 with the transcriptional
machinery bound to core promoter. As shown, DNA looping increases
the contact of TFIIA with the DNA helix around positions -45/-48.
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