Effect of ethylation of operator-phosphates on Gal repressor binding. DNA contortion by repressor

Gal repressor inhibits transcription by binding to two operators (OE and OI) in the gal operon. By ethylating DNA, we have identified 23 phosphate groups (11 on OE and 12 in OI) in the DNA backbone of gal operators that when ethylated interfere with repressor binding. By inference, either (1) such a phosphate is contacted or closely approached by Gal repressor, or (2) the structure of DNA generated by ethylation of such a phosphate, although not a site of direct contact, is not compatible with repressor binding. Within an operator, these phosphates are arranged with a perfect symmetry aligned with the operator dyad symmetry, indicating that each half-symmetry is contacted by a subunit of repressor dimer. Unlike in many other similar DNA-protein systems, the same phosphates in the gal operator are distributed around a B-form of DNA helix cylinder covering greater than 180 degrees. Models have been proposed to describe the disposition of the Gal repressor-operator complex, which would explain the layout of the participating phosphate groups around the surface of the DNA helix. We have discussed two ways by which Gal repressor can induce structural changes in DNA.