In the absence of cut gene activity in Drosophila, external sensory organs are transformed into chordotonal organs. Here we show that the cut locus encodes a large protein containing a homoeodomain and is expressed in nuclei of cells in external sensory organs but not in cells within chordotonal organs.
Data are presented that indicate that the z locus of Drosophila melanogaster represses w locus activity, but the repression is effective only on paired or physically adjacent w loci. Various mutant alleles of z and w were combined in a series of different doses to determine the effect of dosage and physical position in the nucleus on gene expression. In z/z individuals, paired white alleles fail to be expressed, while unpaired alleles are expressed normally. The results are discussed in terms of a model postulating that the z gene product represses the transcription of wr by complexing with an RNA produced by part of the white locus itself. In order to be effective in repression, there must be two w+ genes producing the RNA in a limited volume in the nucleus. Such a model necessarily imposes a specific architecture on the chromatin of interphase nuclei.Examples of regulatory interactions between loci in eukaryotes are rare; communication between two alleles at a single locus is even less well documented. We present evidence that a particular locus of Drosophila melanogaster functions as a repressor of another locus. Repression is generally apparent, however, only if two alleles of the repressed locus are physically adjacent or paired.Gans (1) discovered the mutation zeste (z; 1-1.0) that produces yellow eye color in females but is wild type in males. She showed that the expression of the zeste phenotype in the females is due to the presence of two wild-type alleles of the white locus (w; 1-1.5). Certain duplications of w+ allow zeste phenotype expression in males. On the other hand, females heterozygous for a white locus deletion (z w + /z w -) are wild type in eye color. Some mutant alleles of w act in the same fashion as a deletion of the locus (1). Such white locus alleles were determined by Green (2) to map in the proximal part of the white locus (here designated as wPrX). Those alleles of white located in the distal portion of the locus (wdst) allow expression of zeste just as w + does. In fact, all the mutations that appear to upset regulatory function of the white locus map in WPrx. Pattern mutants that produce mosaics of pigmented and nonpigmented facets are located in WPrX, and all mutants in the region fail to show dosage compensation, which is characteristic of w+. A tandem duplication of only the wPorx portion of the locus behaves with respect to z as two doses of w+ (3). On the basis of these and some other distinguishing properties, Judd (4) has suggested that WPrX is involved in the regulation of the white locus while wdst contains the structural sequence.Historically the zeste-white interaction has been stated in terms of white locus deletions and mutants in wPrx acting as dominant suppressors of zeste. We believe this is not an accurate description of the system, and we will here describe the interaction from the point of view of repression of w+ by z. The reduction in pigmentation in z w + /z w + females results from the continued repression of w+ even when the gene should be active. Th...
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