Lac repressor binding to poly (d(A-T)). Conformational changes

The binding of $\text{lac}$ repressor to poly d(A-T)] at low ionic strength has been investigated by circular dichroism, fluorescence and light scattering. Poly d(A-T)] undergoes an important conformational change upon binding to $\text{lac}$ repressor. The maximum number of binding sites corresponds to about one tetrameric repressor per 11 base pairs of polyd(A-T)]. The inducer isopropyl β-D-thiogalactoside (IPTG) does not affect the binding of $\text{lac}$ repressor to polyd(A-T)]. It binds equally well to free and polyd(A-T)] -bound repressor.