Characterization of Sequences Responsible for trans-Inactivation of the Drosophila brown Gene

When a gene is moved to a chromosomal position where it encounters structural or biochemical features different from those at its native location, variegated expression can result. This mosaicism, known as position-effect variegation (PEV), has been observed for many euchromatic genes juxtaposed to pericentric heterochromatin and for a few heterochromatic genes moved to distal euchromatin (for review, see Henikoff 1990; Reuter and Spierer 1992). In PEV of a euchromatic gene, the heterochromatic state “spreads” into the gene and inactivates it in some cells of a tissue, but not in other cells. This leads to a variegated pattern of gene expression. It is not known what specific property of heterochromatin inhibits euchromatic gene expression or how this property spreads from the junction into neighboring euchromatin. In some way, the condensed state that is characteristic of heterochromatin causes neighboring euchromatin to be similarly condensed and inaccessible to the transcriptional machinery (Spofford 1976)....