Biology Reference
In-Depth Information
activated
Hox
clusters display a more decompacted state (
Chambeyron &
Bickmore, 2004; Chambeyron et al., 2005;Morey et al., 2007
). In cells lacking
RING1B, the derepressed
Hox
clusters also appear less compacted, despite
being coated with H3K27me3 marks (
Eskeland et al., 2010
), suggesting that
chromatin compaction is specificallymaintained by the PRC1 complex, rather
than by PRC2 and the associated H3K27me3 mark. In distal E10.5 limb bud
cells, where the
HoxD
cluster is partially active and decompacted, RING1B
occupancy is indeed strongly decreased (
Williamson et al., 2012
). In such cells,
however, this decrease in RING1B occupancy is paralleled by a decrease in
H3K27me3 occupancy as well (
Montavon et al., 2011
).
6. 3D CHROMATIN ORGANIZATION AND COLLINEARITY
IN DROSOPHILA
The development of dedicated techniques like the Chromosome
Conformation Capture (3C) approach, its derivatives 4C, 5C, and Hi-C,
and the Dam-ID technique, has allowed to assess the 3D organization of
genomic loci at high resolution (
Simonis, Kooren, & de Laat, 2007; van
Steensel & Dekker, 2010
). In
Drosophila
, Dam-ID, 3C, and 4C have been
used to investigate the chromatin architecture of BX-C in embryos, larvae,
and cell lines (
Fig. 4.6
;
Bantignies et al., 2011; Cleard, Moshkin, Karch, &
Maeda, 2006; Lanzuolo et al., 2007
). The 3D organization of the inactive
BX-C was assessed by using the
Fab-7
PRE as a viewpoint in S2 cells.
Fab-7
interacts with other well-known PREs located in the cluster, as well
as with the promoters and transcription termination sites of BX-C
Hox
genes. Interactions were also scored with interspersed sequences within
the cluster, though at considerably lower levels. Likewise, the
abd-A
gene
contacted various PREs and genes from the cluster, both in inactive S2 cells
and in a mixed population of embryonic cells (
Lanzuolo et al., 2007
). A sim-
ilar interaction between the
Fab-7
PRE and the inactive
Abd-B
promoter
was previously identified in the head of adult flies, using a smaller Dam-
ID screen (
Cleard et al., 2006
).
Of note, the interactions within BX-C are significantly higher than with
the chromatin located outside, as shown by 3C and 4C (
Fig. 4.6
B;
Bantignies
et al., 2011; Lanzuolo et al., 2007
). Consequently, when inactive, BX-C
adopts a compartmentalized conformation, which is nucleated around the
PREs, the promoters and the transcription termination sites (
Fig. 4.6
C).
The extent of this 3D compartment largely matches the domain decorated
with H3K27me3 marks. Accordingly, the H3K27me3 marked domains is
