Major changes in the 5' and 3' chromatin structure of sea urchin histone genes accompany their activation and inactivation in development.

The major histone gene repeat (h22) of the sea urchin Psammechinus miliaris is transiently expressed for several hours during early embryonic development. Several major alterations in chromatin structure coincide with changes in the pattern of early histone gene expression and are reversed later. During the early (128-cell) blastula stages when h22 DNA is maximally expressed, promoter regions of all five histone genes are sensitive to both micrococcal nuclease and DNAase I. Hypersensitivity to micrococcal nuclease remains the same throughout the early hours of development, but disappears abruptly at hatching blastula stage when transcription has completely ceased. Some sequences near the 3' end of active histone genes are very resistant to micrococcal nuclease cutting and map just downstream of the inverted DNA repeats essential for generating faithful 3' ends of histone mRNAs. These same histone DNA sequences are readily cut in free DNA or when histone genes are transcriptionally inactive.