Heterochromatic Loci and Position Effects in Drosophila

University Of Washington, Seattle WA

Investigators

Abstract

The chromosomes of most higher organisms show two cytologically distinct domains known as euchromatin and heterochromatin. These domains differ in biases in DNA sequence composition and chromosomal protein distributions along the length of the chromosome. They also differ in gene expression, as first revealed by studies of chromosome rearrangements in Drosophila. Rearrangements that create novel junctions between euchromatin and heterochromatin result in the mosaic inactivation of genes near the breakpoint, a phenomenon known as Position Effect Variegation (PEV). The most popular models of PEV propose that it results from the invasion of one chromatin state into the other across the rearrangement breakpoint. These models view heterochromatin as a highly condensed chromatin state that represses gene expression. This project is aimed at critically evaluating existing models of heterochromatin to account for its activating influence on transcription of heterochromatic sequences. The studies will focus on a block of heterochromatin located on the left arm of Chromosome 2 (2Lh) of Drosophila melanogaster. The transcription of specific 2Lh sequences will be examined in individual flies with structurally normal chromosomes and in mutant strains carrying chromosome rearrangements that induce PEV on heterochromatic genes. Studies of transcription will involve comparing promoters of the 2Lh genes to determine if they differ in structure from euchromatic gene promoters. The effects on 2Lh transcription of reducing the nuclear concentrations of three proteins, HP1, ORC, and HOAP, which modify and bind heterochromatin, will be assayed genetically and molecularly. The chromatin immunoprecipitation technique will be used to determine if these proteins associate directly, as a complex or individually, with 2Lh sequences in vivo. These studies should identify cis- and trans-regulators of heterochromatin. The findings should help reveal the molecular determinants of heterochromatin and the role of its binding proteins in activating transcription. In her position as Director of Biology Programs at the University of Washington, the PI helps guide the education of all undergraduate biology students there. Each year in her own laboratory she typically mentors several undergraduates carrying out research projects.