Decondensation of mouse sperm chromatin and reassembly into nucleosomes mediated by polyglutamic acid in vitro.

The organization of the mammalian sperm nucleus was probed with staphylococcal nuclease. Although isolated nuclei are resistant to cleavage, following reduction and alkylation, 30% of the sperm DNA could be digested and the remaining DNA had a heterodisperse size distribution. By morphological criteria, a model acidic protein, polyglutamic acid was capable of decondensing purified sperm nuclei that had been reduced and alkylated. The maximal extent of nuclease digestion increased to 85-90%. The subsequent addition of purified, exogenous core histones in 0.1 M NaCl partially reversed this vulnerability to nuclease cleavage such that only 55% of the DNA was digested. Furthermore, analysis of the remaining DNA revealed a nucleosome ladder pattern with unit length repeat of 150 bp. These results strongly suggest that polyglutamic acid can mediate not only decondensation of sperm nuclei but also the assembly of sperm chromatin into nucleosomes.