Abstract
Polycomb group (PcG) proteins are conserved chromatin factors that maintain silencing of key developmental genes outside of their expression domains. Recent genome-wide analyses showed a Polycomb (PC) distribution with binding to discrete PcG response elements (PREs). Within the cell nucleus, PcG proteins localize in structures called PC bodies that contain PcG-silenced genes, and it has been recently shown that PREs form local and long-range spatial networks. Here, we studied the nuclear distribution of two PcG proteins, PC and Polyhomeotic (PH). Thanks to a combination of immunostaining, immuno-FISH, and live imaging of GFP fusion proteins, we could analyze the formation and the mobility of PC bodies during fly embryogenesis as well as compare their behavior to that of the condensed fraction of euchromatin. Immuno-FISH experiments show that PC bodies mainly correspond to 3D structural counterparts of the linear genomic domains identified in genome-wide studies. During early embryogenesis, PC and PH progressively accumulate within PC bodies, which form nuclear structures localized on distinct euchromatin domains containing histone H3 tri-methylated on K27. Time-lapse analysis indicates that two types of motion influence the displacement of PC bodies and chromatin domains containing H2Av-GFP. First, chromatin domains and PC bodies coordinately undergo long-range motions that may correspond to the movement of whole chromosome territories. Second, each PC body and chromatin domain has its own fast and highly constrained motion. In this motion regime, PC bodies move within volumes slightly larger than those of condensed chromatin domains. Moreover, both types of domains move within volumes much smaller than chromosome territories, strongly restricting their possibility of interaction with other nuclear structures. The fast motion of PC bodies and chromatin domains observed during early embryogenesis strongly decreases in late developmental stages, indicating a possible contribution of chromatin dynamics in the maintenance of stable gene silencing.
Highlights
The biological mechanisms allowing one genome to translate into the many epigenomes that characterize different cell types involve binding of regulatory factors to chromatin and specific post-translational histone modifications [1]
Polycomb group proteins are important developmental regulators controlling the expression of hundreds of genes
We investigated the dynamics of Polycomb bodies during Drosophila embryonic development, demonstrating that two Polycomb proteins, Polycomb and Polyhomeotic, gradually assemble onto bodies enriched in histone H3 trimethylated on lysine 27, a hallmark of Polycomb silencing
Summary
The biological mechanisms allowing one genome to translate into the many epigenomes that characterize different cell types involve binding of regulatory factors to chromatin and specific post-translational histone modifications [1]. The second level of this organization involves the clustering of individual PREs into large Polycomb domains marked with histone H3K27me and, to a lower extent, by the PC protein Within these genomic regions, PREs may interact to form three dimensional (3D) structural domains inside the cell nucleus, as has been recently demonstrated by chromosome conformation capture for individual PREs of the BX-C locus [9]. Large Polycomb domains may mediate long-range contacts to form a Polycomb network inside the cell nucleus, since PREs have been shown to contact loci at long-distance on the same chromosome or other chromosomes [10,11,12,13,14] These contacts are functionally regulated because the frequency of interaction depends on PcG proteins [12,14] as well as on proteins involved in RNAi and in chromatin insulator function [14,15]
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