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We describe features of the chromatin structure of the early histone gene family of Strongylocentrotus purpuratus during development. Before and after the histone genes are transcriptionally active, chromatin structure is quite similar with well-defined spaced nucleosomes and no major 5'-flanking sites hypersensitive to nucleases. During the period when the genes are active, marked changes in chromatin structure occur. Micrococcal nuclease digestion generates monomer nucleosomes and only trace amounts of higher multimers. Regions hypersensitive to an endogenous nuclease and DNAase I appear in the 5'-flanking regions of genes for H2A, H2B and H3. Each region consists of four sites spanning a DNA length of 200-250 base pairs. In each case, one major cutting site is near the TATA box; the bulk of the sensitive region is in the nontranscribed spacer. Other sites, in 3'-flanking regions of the genes, are sensitive to nucleases only when the histone genes are no longer transcribed.

Transient alterations of the chromatin structure of sea urchin early histone genes during embryogenesis