In vivo chromatin accessibility correlates with gene silencing in Drosophila

Gene silencing by heterochromatin is a well-known phenomenon that, in Droso phila, is called position effect variegation (PEV). The long-held hypothesi s that this gene silencing is associated with an altered chromatin structur e received direct support only recently. Another gene-silencing phenomenon in Drosophila, although similar in its phenotype of variegation, has been s hown to be associated with euchromatic sequences and is dependent on develo pmental regulators of the Polycomb group (Pc-G) of gene products. One model proposes that the Pc-G products may cause a local heterochromatinization t hat maintains a repressed state of transcription of their target genes. Her e, we test these models by measuring the accessibility of white or miniwhit e sequences, in different contexts, to the Escherichia coli dam DNA methylt ransferase in vivo. We present evidence that PEV and Pc-G-mediated repressi on mechanisms, although based on different protein factors, may indeed invo lve similar higher-order chromatin structure.