The raf repressor from Escherichia coli regulates the expression of the plasmid-borne raf operon by switching between active and inactive conformational states. Ultracentrifugal analysis of the largely purified repressor proves the DNA-free protein to undergo concentration-dependent dissociation-association. High-speed sedimentation equilibria show that the 72 kDa dimer prevails under meniscus depletion conditions. At intracellular concentrations the 144 kDa dimer-of-dimers is the dominating species. It is suggested that the tetrameric structure of the raf repressor is involved in the recognition of the 18-basepair operator DNA.
