Enhancer action in trans is permitted throughout the Drosophila genome

Abstract

Interactions between paired homologous genes can lead to changes in gene expression. Such trans-regulatory effects exemplify transvection and are displayed by many genes in Drosophila, in which homologous chromosomes are paired somatically. Transvection involving the yellow cuticle pigmentation gene can occur by at least two mechanisms, one involving the trans-action of enhancers on a paired promoter and a second involving pairing-mediated bypass of a chromatin insulator. A system was developed to evaluate whether the action of the yellow enhancers in trans could be reconstituted outside of the natural near telomeric location of the yellow gene. To this end, transgenic flies were generated that carried a yellow gene modified by the inclusion of strategically placed recognition sites for the Cre and FLP recombinases. Independent action of the recombinases produced a pair of derivative alleles, one enhancerless and the other promoterless, at each transgene location. Transvection between the derivatives was assessed by the degree of interallelic complementation. Complementation was observed at all eight sites tested. These studies demonstrate that yellow transvection can occur at multiple genomic locations and indicate that the Drosophila genome generally is permissive to enhancer action in trans.

Figure 1

Strategy for the generation of structurally distinct yellow alleles from a parental transposon. The P[yellowCF] transposon carried a yellow gene with target sites for the Cre (open arrowheads) and FLP recombinases (black arrowheads). The P element ends are shown as boxes labeled 5′ and 3′. Cre recombinase catalyzed excision of the 5′ wing and body enhancers (ovals), producing the P[enh⁻] derivative. FLP recombinase catalyzed excision of the promoter region (arrow) and intronic (Br, bristle; T, tarsal claw) enhancers, producing the P[pro⁻] derivative. Males that carried either the P[enh⁻] or P[pro⁻] allele were crossed to females that carried either the P[enh⁻] or P[pro⁻] allele, and the wing (W) and body (B) cuticle pigmentation of the resulting female progeny was examined to determine if interallelic complementation had occurred.

Figure 2

Southern analysis of P[yellow CF] and derivative lines. The restriction maps of the parental P[yellowCF] (A), P[enh⁻] (B), and P[pro⁻] (C) transposons are shown, with the expected sizes of the EcoRI (R) fragments noted above the map. Other enzyme sites shown are HindIII (H) and BamHI (B). W, wing; B, body; Br, bristle; T, tarsal claw. Genomic DNA from two transgenic lines (100F and 84A) was isolated and digested with EcoRI (D). Southern analysis was completed by using a 7.7-kb yellow fragment (marked probe in A). The two lines differ in the size of the flanking band (*), with 100F showing a 12-kb band and 84A showing a 7.5-kb band. The sizes of other bands are shown at the left of each blot.

Figure 3

Body and wing phenotypes of P[yellowCF] flies and derivatives. The top two rows show representative flies from two parental P[yellowCF] transgenic lines (100F and 84A), the corresponding P[enh⁻]/+ and P[pro⁻]/+ flies and complementing P[enh⁻]/P[pro⁻] flies. The bottom row shows representative flies that have wild-type (Canton S/y⁻) or mutant (y²/y² and y^1#8/y^1#8) wing and body pigmentation and a representative fly that displays interallelic complementation (y²/y^1#8).