The quaternary structure of Tet repressors bound to the Tn10-encoded tet gene control region determined by neutron solution scattering.

The spatial arrangement of Tet repressor dimer, both free and in complex with an 80 bp DNA fragment spanning the wild-type Tn10-encoded tet transcriptional control sequence containing a tandem repeat of two operators, has been determined by neutron small-angle scattering. The active, free Tet repressor dimer is an elongated and flat molecule with a maximum dimension of 11 +/- 1.5 mm which can be approximated by an ellipsoid with the half-axes 6 nm, 2.5 nm and 1 nm. The overall conformation undergoes no detectable change when the repressor dimer is bound to a DNA fragment containing a single tet operator. The normal distance between the centre of gravity of the protein and the DNA axis is 3.0 +/- 0.1 nm, indicating that the repressor dimer is mainly located on one side of the DNA. When bound to the wild type tet control DNA, the two repressor dimers have a centre-to-centre distance of 11.0 +/- 0.5 nm. Their minimal distance is 5 +/- 2 nm. Protein-protein contacts via loop formation of the DNA by repressor binding is excluded. The repressors are well separated and have no direct contact. A model is proposed where the two repressor dimers are located on opposite sides of the DNA and the DNA is not strongly bent in the complex.