Figure 3. A shuttling model for the assembly of PcG target genes into nuclear bodies.

A generalized PcG target gene is presumed to be associated with one or more of each Enhancer, PRE, and Insulator elements. In this speculative model the insulator drives the association of such PcG target genes into PcG bodies when they are repressed. When the gene switches to the transcriptionally active state, it generally forms a domain that binds N-ter Trithorax and ASH1 proteins [33]. The insulator now targets the active gene to a Trithorax transcription factory. The switch in insulator preference for transcription factory vs PcG body localization might depend on modifications such as, for example, acetylation vs sumoylation.