THE ROLE OF A POSITIONED NUCLEOSOME AT THE DROSOPHILA-MELANOGASTER HSP26 PROMOTER

The regulatory region of Drosophila melanogaster hsp26 includes a posi tioned nucleosome located between the two DNase I hypersensitive (DH) sites that encompass the critical heat shock elements (HSEs). To test the role of this nucleosome in regulated expression, transgenic flies containing hsp26-lacZ fusion genes with alterations in the nucleosome- associated region have been generated, The positioned nucleosome is as sociated with a DNA sequence that does not itself contain any critical regulatory elements for heat shock-inducible expression. The nucleoso me-associated sequence can be deleted, reversed, duplicated or replace d by a random sequence with no significant effect on DH site formation and gene expression, Analyses of hsp26 and hsp70 transgenes with spac ing changes within the promoter region indicate that the location of t he (CT)(n) .(GA)(n) elements dictates the location of DH site formatio n. Wrapping the DNA between the regulatory elements around a nucleosom e is as effective for gene expression as placing the regulatory elemen ts close to each other. A loss of inducible gene expression was observ ed when the nucleosome-associated DNA was replaced with sequences whic h appear to misdirect nucleosome placement. The results indicate consi derable flexibility in the spacing between DH regulatory sites.