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.