Functional inferences from crystals of Escherichia coli trp repressor

We have reproducibly grown crystals of L-tryptophan . trp aporepressor and indole-3-propionate . trp aporepressor complexes from Escherichia coli which are suitable for x-ray diffraction analysis. The active repressor, L-tryptophan . aporepressor, crystallizes in both trigonal (P3(1)21 or P3(2)21) and tetragonal (P4(1)22 or P4(3)22) forms which diffract to at least 2.0 and 2.5 A, respectively. The trigonal form has one-half of the functional dimer/asymmetric unit; therefore, the trp repressor molecule has an axis of 2-fold rotational symmetry corresponding to the lattice dyad. The inactive complex, indole-3-propionate . aporepressor, or "pseudorepressor," forms tetragonal crystals that also diffract to at least 2.5 A and are isomorphous to those of the active repressor. Slight differences between their diffraction patterns indicate modest structural differences between active and inactive complexes that are presumably mediated by the alpha-amino group of L-tryptophan and account for operator-specific binding.