SUPERCOIL-INDUCED EXTRUSION OF A REGULATORY DNA HAIRPIN

Bacteriophage N4 virion RNA polymerase (N4 vRNAP) promoters contain in verted repeats, which form a 5- to 7-base-pair stem, 3-base loop hairp in that is required for vRNAP recognition, We show that, contrary to c ertain theoretical predictions, hairpin extrusion can occur at physiol ogical superhelical densities in a Mg2+-dependent manner, Specific seq uences on the template strand are required for hairpin extrusion, Thes e sequences define stable DNA hairpins that are relatively unreactive to single strand-specific probes, In addition, a specific stable hairp in-inducing sequence can regulate transcription in vivo, Thus, a DNA s tructure, in its natural environment, is involved in transcriptional r egulation.