TRANSCRIPTION REGULATION BY INFLEXIBILITY OF PROMOTER DNA IN A LOOPEDCOMPLEX

The gal operon of Escherichia coil is negatively regulated by represso r binding to bipartite operators separated by 11 helical turns of DNA. Synergistic binding of repressor to separate sites on DNA results in looping, with the intervening DNA as a topologically closed domain con taining the two promoters. A closed DNA loop of 11 helical turns, whic h is in-flexible to torsional changes, disables the promoters either b y resisting DNA unwinding needed for open complex formation or by impe ding the processive DNA contacts by an RNA polymerase in flux during t ranscription initiation. interaction between two proteins bound to dif ferent sites on DNA modulating the activity of the intervening segment toward other proteins by allostery may be a common mechanism of regul ation in DNA-multiprotein complexes.