Recruitment of components of Polycomb Group chromatin complexes in Drosophila.

Polycomb Group complexes assemble at polycomb response elements (PREs) in vivo and silence genes in the surrounding chromatin. To study the recruitment of silencing complexes, we have targeted various Polycomb Group (PcG) proteins by fusing them to the LexA DNA binding domain. When LexA-PC, -PSC, -PH or -SU(Z)2 are targeted to a reporter gene, they recruit functional PcG-silencing complexes that recapitulate the silencing behavior of a PRE: silencing is sensitive to the state of activity of the target chromatin. When the target is transcriptionally active, silencing is not established but when the target is not active at syncytial blastoderm, it becomes silenced. The repressed state persists through embryonic development but cannot be maintained in larval imaginal discs even when the LexA-PcG fusion is constitutively expressed, suggesting a discontinuity in the mechanism of repression. These proteins also interact with other PC-containing complexes in embryonic nuclear extracts. In contrast LexA-PHO is neither able to silence nor to interact with PC-containing complexes. Analysis of pho mutant embryos and of PRE constructs whose PHO-binding sites are mutated suggests that, while PHO is important for silencing in imaginal discs, it is not necessary for embryonic PcG silencing.