We are interested in the role that chromatin structure plays in gene regulation, both effects from packaging large domains and local effects of the nucleosome array. Working with Drosophila melanogaster, we have used a transposable P element containing a copy of the white gene, a visible marker for gene silencing, and a copy of hsp26 , a well-characterized inducible gene, to examine the effect of insertion into different chromosomal domains. While these genes are fully active in euchromatic domains, silencing (similar to Position Effect Variegation) is observed on insertion into pericentric heterochromatin, telomeres, and sites within the small fourth chromosome. Both changes in the local nucleosome array, and the spatial organization of the nucleus, appear critical in determining gene silencing. While many P element insertion sites on the fourth chromosome induce silencing, others allow full expression; these sites are interspersed, indicating closely interspersed heterochromatic and euchromatic domains. We are characterizing these domains and their boundaries. Mapping experiments indicate that heterochromatin formation is targeted by the presence of a repetitious element, 1360 , and perhaps by other similar elements. Genetic analysis has shown that heterochromatic silencing is dependent on the RNAi machinery. Work is ongoing to determine the mechanism of heterochromatin targeting, and to analyze the role of critical heterochromatin-associated proteins, including HP1 and HP2. |
