Polycomb group repression reduces DNA accessibility.

The Polycomb group proteins are responsible for long-term repression of a number of genes in Drosophila melanogaster, including the homeotic genes of the bithorax complex. The Polycomb protein is thought to alter the chromatin structure of its target genes, but there has been little direct evidence for this model. In this study, the chromatin structure of the bithorax complex was probed with three separate assays for DNA accessibility: (i) activation of polymerase II (Pol II) transcription by Gal4, (ii) transcription by the bacteriophage T7 RNA polymerase (T7RNAP), and (iii) FLP-mediated site-specific recombination. All three processes are restricted or blocked in Polycomb-repressed segments. In contrast, control test sites outside of the bithorax complex permitted Gal4, T7RNAP, and FLP activities throughout the embryo. Several P insertions in the bithorax complex were tested, providing evidence that the Polycomb-induced effect is widespread over target genes. This accessibility effect is similar to that seen for SIR silencing in Saccharomyces cerevisiae. In contrast to SIR silencing, however, episomes excised from Polycomb-repressed chromosomal sites do not show an altered superhelix density.