Deletion of an Insulator Element by the Mutation facet-strawberry in Drosophila melanogaster

Corresponding author: Julio Vazquez, Department of Biochemistry and Biophysics, School of Medicine, Box 0448, University of California, San Francisco, CA 94143-0448., vazquez{at}msg.ucsf.edu (E-mail)

Communicating editor: S. HENIKOFF

Eukaryotic chromosomes are thought to be subdivided into a series of structurally and functionally independent units. Critical to this hypothesis is the identification of insulator or boundary elements that delimit chromosomal domains. The properties of a Notch mutation, facet-strawberry (fa^swb), suggest that this small deletion disrupts such a boundary element. fa^swb is located in the interband separating polytene band 3C7, which contains Notch, from the distal band 3C6. The fa^swb mutation alters the structural organization of the chromosome by deleting the interband and fusing 3C7 with 3C6. Genetic studies also suggest that fa^swb compromises the functional autonomy of Notch by allowing the locus to become sensitive to chromosomal position effects emanating from distal sequences. In the studies reported here, we show that a DNA fragment spanning the fa^swb region can insulate reporter transgenes against chromosomal position effects and can block enhancer-promoter interactions. Moreover, we find that insulating activity is dependent on sequences deleted in fa^swb. These results provide evidence that the element defined by the fa^swb mutation corresponds to an insulator.

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