UV DIFFERENTIAL STUDY OF THE HISTONES H2A-H2B-H3-H4 OCTAMER

The individual calf thymus histones H2A, H2B, H3 and H4, the dimer H2A-H2B, the tetramer (H3-H4)2 and the octamer (H2A-H2B-H3-H4)2 were studied by differential UV absorption, i.e., absorption shifts of tyrosyl residues, due to thermal perturbations were observed. The histone octamer was studied in 2 M NaCl pH 7.5 a condition under which it is stable, as demonstrated by Eickbush et Moudrianakis. The interactions, which maintain the 4 histones as an octamer may involve the weak association of 1 (H3-H4)2 tetramer with 2 H2A-H2B dimers and might be due essentially to histidine-lysine or histidine-tyrosine hydrogen bonds. The octamer was studied by UV differential absorption using the tyrosyl residues as a natural probe to follow their interaction with different residues in their neighborhood. The tetramer (H3-H4)2 has all its tyrosyl residues exposed to the solvent whereas the octamer has no tyrosine exposed, suggesting that with this polymer no DNA-tyrosine interactions could take place.