Interactions between the regulatory regions of two Adh alleles.

A region (NS1) that acts like an enhancer is located approximately 300 bp upstream of the larval cap site in the Adh gene of D. melanogaster. When this sequence is deleted (delta NS1), the gene fails to express ADH protein. Gene expression can be restored by placing a second Adh gene with an intact enhancer elsewhere on the same plasmid. In these circumstances, both genes are expressed equally regardless of their orientation on the plasmid. In this report we further characterize the interactions that occur when a single enhancer activates expression from a proximal and distant promoter. We have made the following observations: (1) While the two genes are expressed equivalently, their expression relative to a plasmid carrying two intact genes is reduced by a factor of 2 to 6 depending on the orientation of the two genes. (2) The single enhancer drives expression of both genes on any given plasmid molecule. (3) The enhancer does not interact with the Adh gene from which the NS7 region (which spans the larval TATA box) is removed. (4) Expression of the delta NS1 gene can be restored by an intact gene when both are inserted together into the Drosophila genome via P element-mediated transformation. (5) Increasing the separation between the two genes on a plasmid by up to 15 kbp does not prevent the restoration of expression of the delta NS1 gene. We propose a model that explains how a single enhancer can stimulate equal expression from two genes.