Chromatin reorganization during spermatogenesis in the winter flounder

During spermatogenesis in the winter flounder, the average repeat length of nucleosomal DNA in the testis increases from 195 +/- 2 base pairs in prespermatid nuclei to 222 +/- 3 base pairs in sperm. This increase in repeat length apparently occurs in the linker region since there is no change in the pattern of DNA fragments produced during micrococcal nuclease digestion of the nucleosome core. The timing of the increase coincides with the loss of phosphate from the high molecular weight basic nuclear proteins and histones H2A and H4. When prespermatid nuclei are digested with micrococcal nuclease to the point where 10% of the DNA is acid-soluble, mononucleosomes and higher oligomers are readily released. However, when sperm chromatin is digested to the same extent, these products are no longer soluble and only traces of H1 and small DNA fragments are released. This situation is not changed in sperm chromatin that has been depleted of H1 by extraction with 0.4 M NaCl. However, if nuclease-treated sperm chromatin is lightly digested with trypsin, mono- and oligonucleosomes are released. At this level of proteolysis, the high molecular weight basic nuclear proteins are completely broken down, but the core histones are largely intact. These data are consistent with a model in which the unphosphorylated high molecular weight basic nuclear proteins function in cross-linking nucleosomes together within the sperm nucleus.