NMR and CD spectroscopy have been used to characterize, both structurally and dynamically, the 82-amino-acid ParD protein of the post-segregational killing module of the broad-host-range plasmid RP4\RK2. ParD occurs as a dimer in solution and exercises two different control functions ; an autoregulatory function by binding to its own promoter P parDE and a plasmidstabilizing function by inhibiting ParE toxicity in cells that express ParD and ParE. Analysis of the secondary structure based on the chemical-shift indices, sequential nuclear Overhauser enhancements (NOEs) and $J H α -NH scalar coupling constants showed that the N-terminal domain of ParD consists of a short β-ribbon followed by three α-helices, demonstrating that ParD contains a ribbon-helix-helix fold, a DNA-binding motif found in a family of small prokaryotic repressors. "&N longitudinal