Title: Trans interactions at the yellow gene in Drosophila melanogaster
Description: Trans interactions, homologous and nonhomologous, have been described at diverse loci in many different organisms, highlighting both their importance and ubiquity. In Drosophila, where somatic pairing occurs, they have been studied as a form of gene regulation at a number of loci, where they can lead to both gene activation and gene silencing. In this work, I have used the yellow gene, which is non-essential and relatively easily manipulated at its natural location, as a tool to investigate the mechanisms by which trans interactions may be controlled. Previous studies at yellow have demonstrated that the cis promoter is essential in either preventing or allowing yellow enhancers to act in trans. In particular, mutation of core elements in the yellow promoter release enhancers and allow trans -activation, while the substitution of intact heterologous promoters for the yellow promoter do not. Chapter 2 describes experiments that further dissect the role of the promoter in trans interactions at yellow. First, I show that mutation of core elements in the heat-shock protein 70 ( hsp70 ), even-skipped ( eve ) and white ( w ) promoters, in the context of the yellow gene, release yellow enhancers to act in trans, with a correlation between transcriptional strength and maintenance of cis -preference. Second, I demonstrate that promoters other than yellow can receive input from yellow enhancers in trans. Third, I describe how insulator bypass, a form of pairing dependant gene regulation found at yellow, can occur when the eve and white promoters are substituted for the yellow promoter and when the gypsy insulator is substituted for the gypsy retrotransposon.

Chapter 3 describes the study of the mechanism by which trans interactions of gypsy insulators at yellow can increase insulator activity. That is, I observe that gypsy insulators located on homologous chromosomes effectively enhance repression of the hsp70 promoter, which is able to partially overcome a single gypsy insulator. The interaction of these insulators, located in trans, is dependent upon the suppressor of Hairy wing [Su(Hw)] protein and can be partially disrupted by rearrangements that alter chromosomal pairing. Chapter 4 describes the development of recombinase-mediated cassette exchange (RMCE) in Drosophila, a transgenic technology that allows the insertion of any genetic construct into the exact same genomic location. In addition, this event can occur without the requirement for a phenotypic marker. As tran s interactions, particularly the enhaner-promoter interactions described in Chapter 2, may require the precise placement of constructs in direct opposition on homologous chromosomes, the development of this technology was necessary in order to continue and expand the study of trans interactions.