The Escherichia coli RecA protein triggers both DNA repair and mutagenesis in a process known as the SOS response. The 81-residue E. coli protein DinI inhibits activity of RecA in vivo. The solution structure of DinI has been determined by multidimensional triple resonance NMR spectroscopy, using restraints derived from two sets of residual dipolar couplings, obtained in bicelle and phage media, supplemented with J couplings and a moderate number of NOE restraints. DinI has an α/β fold comprised of a three-stranded β-sheet and two α-helices. The β-sheet topology is unusual: the central strand is flanked by a parallel and an antiparallel strand and the sheet is remarkably flat. The structure of DinI shows that six negatively charged Glu and Asp residues on DinI's kinked C-terminal α-helix form an extended, negatively charged ridge. We propose that this ridge mimics the electrostatic character of the DNA phospodiester backbone, thereby enabling DinI to compete with single-stranded DNA for RecA binding. Biochemical data confirm that DinI is able to displace ssDNA from RecA.

The relative orientations of adjacent structural elements without many well-defined NOE contacts between them are typically poorly defined in NMR structures. For apo-S100B(ββ) and the structurally homologous protein calcyclin, the solution structures determined by conventional NMR exhibited considerable differences and made it impossible to draw unambiguous conclusions regarding the Ca2+-induced conformational change required for target protein binding. The structure of rat apo-S100B(ββ) was recalculated using a large number of constraints derived from dipolar couplings that were measured in a dilute liquid crystalline phase. The dipolar couplings orient bond vectors relative to a single-axis system, and thereby remove much of the uncertainty in NOE-based structures. The structure of apo-S100B(ββ) indicates a minimal change in the first, pseudo-EF-hand Ca2+ binding site, but a large reorientation of helix 3 in the second, classical EF-hand upon Ca2+ binding.