Purification and characterization of a protein from Escherichia coli which forms complexes with superhelical and single-stranded DNAs.

From the cells of an Escherichia coli K-12 strain, a 22,000-dalton protein which has an affinity for the superhelical DNA molecule was purified to apparent homogeneity by monitoring the DNA-binding activity using the filter binding assay. In the sedimentation analysis of the DNA-protein complex, the protein has an affinity for the superhelical or single-stranded DNA molecule but neither for the open-circular nor for the linear DNA molecule. The amino acid composition of the protein resembled those of the other prokaryotic histone-like proteins and also to eukaryotic histones H2A and H2B. The protein precipitated upon heating, which is in contrast to the heat-stable feature of the other histone-like proteins. Furthermore, DNA and RNA syntheses in vitro were not affected by the presence of the protein. In view of these characteristics, this protein may play a role in maintaining the bacterial nucleoid structure.