We quantitate the 'activating potentials' of deletion and point mutati
on variants of a 42 amino acid yeast transcriptional activating region
excised from the yeast activator GAL4 and, using surface plasmon reso
nance, we measure the relative affinities of these molecules for a var
iety of proteins, including plausible target proteins as well as GAL80
, a specific inhibitor of GAL4. We fond a remarkable correlation betwe
en the relative activating potentials of the derivatives and their rel
ative affinities for yeast TBP and for yeast TFIIB; other tested prote
ins interacted significantly more weakly, if at all. These results pro
vide an especially strong argument that TBP and TFIIB are activating r
egion targets. We also show, using one set of yeast activating region
mutants, that activator-target interactions are strongly correlated wi
th the length of the activating region, that the effect of point mutan
ts is highly dependent on the length of the activating region mutated
and that, unlike interactions with TBP and TFIIB, interaction with the
specific inhibitor GAL80 is destroyed by deletion of certain critical
residues in the C-terminal half of the 42 amino acid activating regio
n.