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site-specific action of these factors. The so-called TAGteam clusters have
been shown to be a key determinant in the timing of transcription initiation
in
Drosophila melanogaster
and
Aedes aegypti
embryos (
Biedler, Hu, Tae, &Tu,
2012; ten Bosch, Benavides, &Cline, 2006
; reviewed in
strup et al., 2012
).
Whether DNA sequence determinants of embryonic transcription also exist
in zebrafish and whether factors such as
b
-catenin may contribute to spatially
and temporally prepattern gene expression during pre-MBT stages are ques-
tions of high interest (
Chan, Chao, Wang, Yu, & Yuh, 2009; Chan,
Longabaugh, et al., 2009
).
6. DEVELOPMENTAL EPIGENETIC FATE MAP THROUGH
THE MBT: A PATH TO MULTIVALENCY?
Despite the potential of pre-MBT H3K4me3-marked genes to be
activated after ZGA, a number of these genes have a distinct transcriptional
fate shortly after ZGA. An epigenetic fate map of pre-ZGA H3K4me3-
marked genes drawn by examining histone modifications associated with
these genes during development through the MBT (
Lindeman et al.,
2011
) shows that a significant proportion of these genes, not expressed upon
ZGA, becomes enriched in H3K27me3, H3K9me3, or apparently in both
marks, while retaining H3K4me3 (
Fig. 3.4
C).
In zebrafish embryos, H3K4me3/H3K27me3 co-occupancy prevails over
H3K4me3/H3K9me3 at the time of ZGA (
Andersen,
strup, et al., 2012
).
Yet, while most H3K4me3-marked genes are devoid of H3K9me3 or
H3K27me3, the latter marks are rarely seen without H3K4me3. Over
apparently coenriched regions, H3K4me3 and H3K27me3 may overlap,
whereas H3K4me3 andH3K9me3 are mutually exclusive. Intriguingly, how-
ever, H3K4me3 andH3K9me3 occasionally coincide, onlywhenH3K27me3
also marks the overlapping area. Furthermore, on metagenes H3K27me3
correlates with local alteration in H3K4me3 density, and H3K9me3 is linked
to alterations in the profiles of bothH3K4me3 andH3K27me3. Thus, enrich-
ment in trimethylatedH3K9 or H3K27 seems to be linked to local remodeling
of chromatin manifested by changes in H3K4me3 density (
Andersen,
strup,
et al., 2012
). This raises the possibility of actual proximity of these marks,
supporting a view of bi- or trivalent chromatin domains in the zebrafish
embryo, at least at this stage of development.
The coincidence of permissive H3K4me3 with repressive H3K27me3
without or with H3K9me3 on a subset of developmentally regulated
