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. observing absorption shifts of tyrosyl residues due to thermal perturbations. The histone octamer was studied in 2 M NaCl pH 7.5 a condition under which it is stable, as demonstrated by Eickbush and Moudrianakis [18]. In addition these authors suggested that the interactions which maintain the four histones as an octamer involve the weak association of one (H3-H4)2 tetramer with two H2A-H2B dimers and might be due essentially to histidine-lysine or histidine-tyrosine hydrogen bonds. We performed the study of the octamer by UV differential absorption using the tyrosyl residues as a natural probe to follow their interaction with different residues in their neighbourhood. The main result obtained shows that 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.