Molecular limits on the size of a genetic locus in Drosophila melanogaster.

Abstract
The outer limits on the size of the DNA region required for expression of a D. melanogaster gene were determined. This region, termed the unit of expression, includes not only the structural gene but also any cis-acting sequences that modulate its activity. The locus studied, Sgs-4, codes for one of the glue proteins secreted by larval D. melanogaster salivary glands. Cytological deficiencies were identified that eliminate sequences on one side or the other of Sgs-4 without affecting its expression. The ends of these deficiencies were localized accurately with respect to restriction endonuclease sites in and near the locus. These endpoints limit the Sgs-4 structural gene and essential flanking sequences to a 16 to 19 kilobase region of the X chromosome. There is no DNA sequence rearrangement in the Sgs-4 region during development of either the polytene larval salivary glands or adult flies.