ACTIVATION OF GENE-EXPRESSION BY A NOVEL DNA STRUCTURAL TRANSMISSION MECHANISM THAT REQUIRES SUPERCOILING-INDUCED DNA DUPLEX DESTABILIZATION IN AN UPSTREAM ACTIVATING SEQUENCE

We have previously demonstrated that integration host factor (IHF)-med iated activation of transcription from the ilvP(G) promoter of Escheri chia coli requires a supercoiled DNA template and occurs in the absenc e of specific interactions between IHF and RNA polymerase. In this rep ort, we describe a novel, supercoiling-dependent, DNA structural trans mission mechanism for this activation. We provide theoretical evidence for a supercoiling-induced DNA duplex destabilized (SIDD) structure i n the A + T-rich, ilvP(G) regulatory region between base pair position s +1 and -160, We show that the region of this SIDD sequence immediate ly upstream of an IHF binding site centered at base pair position -92 is, in fact, destabilized by superhelical stress and that this duplex destabilization is inhibited by IHF binding. Thus, in the presence of IHF, the negative superhelical twist normally absorbed by this DNA str ucture in the promoter distal half of the SIDD sequence is transferred to the downstream portion of the SIDD sequence containing the ilvP(G) promoter site. This IHF-mediated translocation of superhelical energy facilitates duplex destabilization in the -10 region of the downstrea m ilvP(G) promoter and activates transcription by increasing the rate of open complex formation.