Abstract
The chromatin structure adjacent to the Drosophila glue protein gene Sgs4 changes drastically when the gene is active. In nuclei from embryos or tissue culture cells in which Sgs4 is inactive, there are three DNAase I-hypersensitive sites 3' to the gene, but none near its 5' end. In the nuclei of late third instar salivary glands, Sgs4 is actively transcribed, and a complex of five DNAase I-hypersensitive sites appears 5' to the gene. Two of the sites are near the point of transcription initiation, at -70 and +30. The others are much farther from the gene at -330, -405 and -480 and are affected by small deletions: one deletion reduces expression about 50-fold and removes sequences corresponding to the -330 hypersensitive site; another makes no Sgs4 RNA and removes sequences corresponding to two hypersensitive sites, -405 and -480. Thus the hypersensitive sites, or DNA sequences within 50 bp of them, seem to be required for normal gene expression Distinct changes are seen in the chromatin from salivary glands of these mutant strains. The first strain is simply missing the -330 hypersensitive site, while the second is missing all of the tissue-specific 5' sites, even though sequences corresponding to three of them remain. This suggests that hierarchical interactions among the regions 5' to Sgs4 are required for its full expression. A sequence of 14 bp at the most prominent hypersensitive site (-405) is closely related to sequences 5' to several other eucaryotic genes.
