Comparative immunochemical analysis of the antigenic structures of the DNA-binding histone-like HU protein from Escherichia coli and eukaryotic histones.

The general antigenic structures of the DNA-binding HU protein from E. coli, histones H1, H2A, H2B, H3, and H4 from calf thymus, and histone H5 from chick erythrocytes were compared in immunoassays with the aid of monospecific polyclonal antibodies to the HU protein and the individual histones. A partial cross-reaction between the HU protein and antibodies to histones H1 and H5 was demonstrated. The reaction titres obtained in a solid-phase enzyme immunoassay indicated that the cross-reactions were equivalent to 30% of the reaction with homologous antigen for antibodies to H1, and to 20% and 12% (on dilution of HU protein in solutions of low and high ionic strength, respectively) of the reaction with homologous antigen for antibodies to H5. Cross-reactions between antibodies to the HU protein and histones H1, H5, H2A, H2B, H3, and H4 were not detected. We suggest that the resemblance between the general antigenic structures of the DNA-binding HU protein from E. coli and the lysine-rich linker histones H1 and H5 reflects the structure of the binding sites between these proteins and DNA, which presumably have a similar DNA-binding pattern and a similar functional role.