Chromatin structure in pre- and postblastula embryos of Drosophila.

Early in embryogenesis of Drosophila melanogaster, DNA synthesis is extremely rapid while RNA synthesis is virtually undetectable. We have examined the chromatin structure of nuclei from preblastula embryos to determine whether these unusual rates of replication and transcription correlate with any alteration in the chromatin. DNase I-hypersensitive sites at the 5' end of genes have been postulated to be necessary but not sufficient for activity of the associated gene and have been shown to be established prior to the onset of transcription. In order to ascertain whether the apparent transcriptional incompetence of the early embryos is the result of the absence of such chromatin structure, we have examined nuclei from cleavage-stage embryos to determine whether the DNase I-hypersensitive sites have been established. A variety of genes, including inducible heat-shock genes, a constitutively expressed ribosomal protein gene, and two developmentally regulated genes, have been examined. In every case the pattern of DNase I-hypersensitive sites in preblastula embryos duplicates that in the later (6-18 hr after oviposition) embryos. In addition, two extra sites are observed in the early embryos, one at the 5' end of the hsp 70 gene and one in a gene at chromosomal locus 67B1. These sites do not correlate with any known function; however, neither can functional significance be ruled out. In a further investigation of the chromatin structure of early embryos, a nucleosomal array was generated. The pattern produced from nuclei of early embryos is extremely similar to that from 6- to 18-hr embryos, but somewhat less distinct. Both nucleosomal arrays and DNase I-hypersensitive sites must therefore be established very rapidly following DNA replication. The chromatin structure of cleavage-stage embryos detected by these tests appears to be essentially the same as that of older embryos, both in general, and at specific loci.