DNase I hypersensitive sites in Drow ends of regions of transcription (chromatin structure/Drosophila melanogaster/heat shock)

By using a map of the unique region of DNA en- coding the four small heat-shock proteins of Drosophila melano- gaster (hsp 22, hsp 23, hsp 26, and hsp 28), and a simple mapping technique, the positions of the DNase I hypersensitive sites of chromatin in the vicinity of these genes have now been deter- mined. The major chromatin-specific sites occur at the 5' ends of each of the four heat-shock protein genes in embryo nuclei. These genes are not active in the nuclei analyzed but can be quickly in- duced in these cells by the heat-shock stimulus. The chromatin structure indicated by DNase I hypersensitivity may be a neces- sary factor in the general mechanism of gene activation. During the last few years, considerable progress has been made in the analysis of chromatin structure through the use of various nucleases. Recently it has been observed that there are sites in chromatin that are hypersensitive to cleavage by DNase I (1-4). Such sites occur at specific positions along the chromatin fiber at intervals of about 2-10 kilobases (kb) in Drosophila (2). The positions of such sites relative to the restriction map of a unique, cloned region of the genome may be determined by a simple mapping procedure (5). We have studied the region encoding the four small heat-