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inactive situation in S2 cells (
Fig. 4.6
C), though the interactions between the
Fab-7
PRE and other PREs (and the inactive genes) were essentially
maintained. Likewise, in the same context, the
bxd
PRE kept most of its
interactions, yet with a decreased frequency with both
Fab-7
and
Fab-8
PREs
and an almost complete loss of interactions with the
Abd-B
promoter
(
Lanzuolo et al., 2007
). A loss of interactions had been previously observed
between the active
Abd-B
gene and
Fab-7
in the abdomen of adult flies,
when compared to the “inactive” brain tissue (
Cleard et al., 2006
). The
active
Abd-B
promoter thus loops out of the H3K27me3 labeled chromatin
domain, whereas it locates inside when inactive (
Fig. 4.6
C). Whether or not
such looping out from the inactive BX-C and ANT-C 3D compartments is
a common feature during collinear expression in
Drosophila
remains to be
determined.
4C studies with
Drosophila
larvae have identified significant interaction
frequencies, though at low level, between Polycomb bound genomic
regions, including BX-
C
and ANT-C (
Bantignies et al., 2011; Tolhuis
et al., 2011
). Fluorescent microscopy in embryos confirmed that about 20
percent of those chromosomes carrying both
Abd-B
and
Antp
inactive alleles
visually overlapped. This proximity decreased below ten percent when one
of the two alleles was active. The deletion of
Fab-7
, located in the BX-C,
only moderately decreased the interactions between the two gene clusters,
but seemed to derepress
Antp
(
Bantignies et al., 2011
). In the
Drosophila
nucleus, the genomic regions marked by Polycomb and H3K27me3 are pre-
sent in foci called “Polycomb bodies” (reviewed in
Bantignies & Cavalli,
2011; Pirrotta & Li, 2012
). This apparent clustering of repressed genes is
mostly stochastic, though the size of the H3K27me3 domain, their proximity
and their location on the same chromosome arm impact upon the interaction
frequency (
Bantignies et al., 2011; Sexton et al., 2012; Tolhuis et al., 2011
).
Also, such long-range clustering might be further organized by insulators,
genomic elements that provide some spatial restriction to PREs (
Li et al.,
2011
). Clustering at Polycomb bodies may provide another level where
the repressive machinery can be concentrated, and hence, it may further
secure and/or maintain repression.
To summarize, in the nuclear space, inactive
Hox
genes from the same
complex cluster together. They may also contact other Polycomb targets,
though at a lower frequency and on a more stochastic basis. At the time
of their activation,
Hox
genes loop out from these inactive 3D structures.
Because
Drosophila Hox
gene are regulated individually, by distinct combi-
nations of
factors
inherited from the maternally driven segmentation
