Acetylation of histone H4 in complex structural transitions of spermiogenic chromatin

Abstract

In spermiogenic nuclei of the cephalopod mollusc Sepia officinalis histones are replaced by a precursor-protamine molecule, which is later converted into protamine. Simultaneously, spermiogenic chromatin undergoes a complex structural change. Somatic-like chromatin belonging to the earliest spermatid is progressively reorganized into: (a) granules of 20 nm diameter, (b) fibres of 30-35 nm, and (c) fibres of 40-50 nm. In the final phases of spermiogenesis these fibres of 40-50 nm join to form larger structures of condensed chromatin, and lastly, the uniformly packed chromatin in the sperm nucleus. Using specific antibodies for mono- and hyperacetylated forms of histone H4, in this work we show that the first structural remodelling of chromatin (from somatic-like organization into 20 nm granules) is given concomitantly with a massive mono-acetylation of H4 (acetylation in lysine 12), whereas the structural remodelling from 30-35 to 40-50 nm fibres is produced simultaneously with hyperacetylation of H4 and the nuclear removal of histones.