Biology Reference
In-Depth Information
9. CLUSTERING, COATING, COMPACTION,
COMPARTMENTALIZATION, AND CONTACTS:
THE FIVE C's OF COLLINEARITY?
Recent studies have looked at chromatin dynamics at
Hox
clusters
during collinear expression and drastic changes were observed in both
Drosophila
and mammals, concerning the profiles of histone posttranslational
modifications (and associated proteins) and the higher order chromatin orga-
nization. While based on similar grounds, the collinear mechanisms come as
different flavors in various animal classes, depending on which early devel-
opmental strategies are used. These findings nevertheless start to shape a
framework wherein the multiple declinations of collinearity can be under-
stood and which incorporates the parameters clustering, coating, compac-
tion, compartmentalization, and contacts. We argue that these five
parameters are important, if not sufficient, to account for the different out-
comes of collinear programs in bilateria.
9.1. Clustering
Collinearity is the translation of a genomic topology into coordinated tran-
scription programs. In this context, clustering of
Hox
genes may help to
secure and enhance the necessary repression of
Hox
genes, at developmental
times and in embryonic territories where these genes need to be transcrip-
tionally inactive. Also, an uninterrupted
Hox
gene cluster is necessary for
temporal collinearity to be fully implemented.
9.2. Coating
The coating, either of active
Hox
genes by H3K4me3 or of inactive genes by
H3K27me3, labels the progression of collinear programs. Members of the
Trithorax and Polycomb complexes deposit these marks over the
Hox
clus-
ters in both
Drosophila
and mammals, yet their recruitment and distribution
differ considerably. Selectivity appears to be achieved by two nonexclusive
mechanisms. The first involves the binding of transcriptional regulators to
local enhancers, whereas the second has a polar component and may provide
both the entry point and the dynamics for temporal collinearity. In the
course of vertebrate evolution, the co-optation of collinear programs
involved distant regulatory elements, which could override the built-in
polarity of this coating.
