X. Dai et al., SUPERCOIL-INDUCED EXTRUSION OF A REGULATORY DNA HAIRPIN, Proceedings of the National Academy of Sciences of the United Statesof America, 94(6), 1997, pp. 2174-2179
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.