DIFFERENTIAL UNDERACETYLATION OF HISTONES H2A, H3 AND H4 ON THE INACTIVE X-CHROMOSOME IN HUMAN FEMALE CELLS

It has previously been shown- that the acetylated forms of histone H4 are depleted or absent in both constitutive, centric heterochromatin a nd in the facultative heterochromatin of the inactive X chromosome (Xi ) in female cells. By immunostaining of metaphase chromosomes from hum an lymphocytes with antibodies to the acetylated isoforms of histones H2A and H3, we now show that these histones too are underacetylated in both Xi and centric heterochromatin. Xi shows two prominent regions o f residual H3 acetylation, one encompassing the pseudoautosomal region at the end of the short arm and one at about Xq22. Both these regions have been shown previously to be sites of residual H4 acetylation. H2 A acetylation on Xi is higher overall than that of H3 or H4 and is par ticularly high around the pseudoautosomal region, but not at Xq22. The results suggest that the acetylated isoforms of H3 and H4 have at lea st some effects on chromosomal structure and function that are not sha red by acetylated H2A.