RECENT discoveries of activator proteins that distort DNA but bear no
obvious activation domains have focused attention on the role of DNA s
tructure in transcriptional regulation(1). Here we describe how the tr
anscription factor MerR can mediate repression as well as activation t
hrough stereospecific modulation of DNA structure. The repressor form
of MerR binds between the -10 and -35 promoter elements of the bacteri
al mercury-detoxification genes, P-T, allowing RNA polymerase to form
an inactive complex with P-T and MerR at this stress-inducible promote
r(2,3). Upon mercuric ion binding, Hg-MerR converts this polymerase co
mplex into the transcriptionally active or 'open' form(2-4). We show h
ere that MerR bends DNA towards itself in a manner similar to the bact
erial catabolite-activator protein CAP, namely at two loci demarked by
DNase I sensitivity, and that the activator conformation, Hg-MerR, re
laxes these bends. This activator-induced unbending, when coupled with
the previously described untwisting of the operator(5), remodels the
promoter and makes it a better template for the poised polymerase.