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

This report places outer limits on the size of the DNA region required for expression of a Drosophila gene. 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 we have chosen, Sgs-4, codes for one of the glue proteins secreted by larval Drosophila salivary glands. Cytological deficiencies have been identified that eliminate sequences on one side or the other of Sgs-4 without affecting its expression. The ends of these deficiencies have been 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. The results also show that there is no DNA sequence rearrangement in the Sgs-4 region during development of either the polytene larval salivary glands or adult flies.