OPEN COMPLEX-FORMATION BY ESCHERICHIA-COLI RNA-POLYMERASE - THE MECHANISM OF POLYMERASE-INDUCED STRAND SEPARATION OF DOUBLE-HELICAL DNA

Escherichia coli RNA polymerase is able to site-specifically melt 12 b p of promoter DNA at temperatures far below those normally associated with DNA melting. Here we consider several models to explain how RNA p olymerase destabilizes duplex DNA. One popular model proposes that upo n binding to the promoter, RNA polymerase untwists the spacer DNA betw een the -10 and -35 regions, which results in a destabilization of the -10 region at a TA base step where melting initiates. Promoter untwis ting may result, in part, from extensive wrapping of the DNA around RN A polymerase. Formation of the strand-separated open complex appears t o be facilitated by specific protein-DNA interactions which occur pred ominantly on the non-template strand. Recent evidence suggests that th ese include important contacts with sigma factor region 2.3, which we propose binds the displaced single strand of DNA.