Differences in the mode of iodination of H2a variants in chromatin

Modification of H2a variants with radioactive iodine was used to study under different ionic conditions the accessibility of their tyrosine residues in chromatin, in monosomes and when free in solution. The modification of tyrosine 57 in the hydrophobic part of H2a was found responsible for the appearance of new fractions with a reduced electrophoretic mobility in the presence of Trition X 100, detected only by autoradiography (radioactive "ghosts"). At low ionic strength a very small number of molecules were iodinated in chromatin, the modification affecting only their hydrophobic region. At moderate ionic strength the tyrosine residues near the N-terminal region of the molecule were predominantly modified. In chromatin the accessibility of the tyrosine residues of H2a1 was much greater than that of H2a2, a difference not observed with free histones.