The Elgin Lab is 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. The fourth chromosome, which appears entirely heterochromatic by many criteria, but has ~80 genes, is the focus of many of our studies, which aim to determine how gene activity is achieved in this heterochromatic environment; both comparative genomics and DNA manipulation are being used to identify critical regulatory elements.
Mapping experiments indicate that heterochromatin formation on the fourth chromosome can be targeted by the presence of a repetitious element, 1360, and perhaps by other similar elements. Genetic analysis has shown that this heterochromatic silencing is initially dependent on the RNAi machinery, required during early embryogenesis, but can be maintained by the HP1a/H3K9me system. Recent experiments have demonstrated that heterochromatin-dependent silencing can be induced by tandem arrays of repetitious sequences inserted into euchromatin but close to heterochromatin; either a GAA310 repeat or a lacO256 repeat, both essentially “foreign” DNA, is sufficient. While the GAA310-induced heterochromatin appears typical in its sensitivity to known suppressors of variegation, that induced by lacO256 has a number of unique properties. Both cases are under current investigation.