Characterization of sequences associated with position-effect variegation at pericentric sites in Drosophila heterochromatin

In a variety of organisms, euchromatic genes brought into juxtaposition wit h pericentric heterochromatin show position-effect variegation (PEV), a sil encing of gene expression in a subset of the cells in which the gene is nor mally expressed. Previously, a P-element mobilization screen identified tra nsgenic Drosophila stocks showing PEV of an hsp70-white(+) reporter gene; t ransgenes in many of these stocks map to the chromocenter of polytene chrom osome. A screen at an elevated temperature identified two stocks that under standard culture temperatures show complete repression of the hsp70-white transgene. The transgenes in both cases map to the chromocenter of polyten e chromosomes. Different types of middle repetitive elements are adjacent t o seven pericentric transgenes; unique sequences are adjacent to two of the perimetric transgenes. All of the transgenes show suppression of PEV in re sponse to a mutation in the gene encoding heterochromatin protein 1 (HP1). This suppression correlates with a more accessible chromatin structure. The results indicate that a pericentric transgene showing PEV can be associate d with different types of DNA sequences, while maintaining a common associa tion with the chromosomal protein HP1.