Crystal structure of the [lamda] repressor and a model for pairwise cooperative operator binding

Abstract

In the case of LexA, a conformational equilibrium was shown to govern the rate of the self-cleavage reaction14, and structures revealed the two conformations: one in which the cleavage loop folds over the active site (with the Ala-Gly bond appropriately positioned for cleavage) and one in which the scissile bond is about 20 Å from the catalytic centre15. Strikingly, in either case, the third repressor dimer (shown in dark colour at the top of each part of Fig. 4a, b) is positioned in such a way as to preclude its binding to an operator site located adjacent to the pair of operator sites shown at the bottom of each part of Fig. 4a, b. Thus, these models make it clear how the geometry of the CTD interactions precludes contact between a cooperatively bound dimer pair at O^sub R^1 and O^sub R^2 and an adjacently bound third dimer at O^sub R^3.