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loci. Over 100 posttranslational histone modifications have been reported to
date, on the tail and core regions of all four core histones (H2A, H2B, H3,
and H4) and of the linker histone H1 (
Ruthenburg, Li, Patel, & Allis, 2007
).
Methylation marks recently examined in the context of embryo develop-
ment include H3K4 trimethylation (H3K4me3). H3K4me3 marks the tran-
scription start site (TSS) of active and of some inactive genes, and is
perceived as a transcriptionally permissive modification. In opposition,
H3K9me3 marks inactive promoters and, in contrast to H3K4me3, tends
to occupy broader domains over promoter regions. Nevertheless,
H3K9me3 can also mark active gene bodies, and a role in the inhibition
of spurious transcription from cryptic intragenic start sites has been ascribed
to this intragenic enrichment (
Carrozza et al., 2005
). Another histone mod-
ification associated with gene repression, H3K27me3, also occupies pro-
moters where, as H3K4me3, it overlaps with the TSS; its occupancy,
however, may often extend upstream of the TSS to cover broader promoter
and even enhancer regions. Whether H3K9me3 and H3K27me3 can
together form a corepressive module is currently a formal possibility, and
evidence suggests that H3K9me3/K27me3 co-occupancy may occur in
zebrafish embryos (
Andersen,
strup, et al., 2012; Lindeman et al., 2011
)
and in human somatic progenitor cells (
Sørensen et al., 2010
; see below).
Trimethylation of H3K36 is found on the bodies of active genes, often in
parallel with RNA polymerase II (RNAPII), and it is as such considered
as a mark of transcriptional elongation (
Kolasinska-Zwierz et al., 2009
).
Additional histone marks have been identified on enhancer regions.
H3K4 monomethylation (H3K4me1) and H3K27 acetylation (H3K27ac)
have been useful in identifying distal regulatory regions (
Rada-Iglesias
et al., 2011
). In embryonic stem cells (ESCs), H3K27ac occupancy on
enhancers correlates with transcriptional activity of the nearest gene,
whereas enhancer-bound H3K27me3 is associated with enhancer inactivity
(
Creyghton et al., 2010; Rada-Iglesias et al., 2011
). Interestingly, a recent
tissue-specific analysis of histone modifications and RNAPII occupancy
on enhancers in
Drosophila
embryos has extended our understanding of
the spatio-temporal setting of histone marks as they relate to gene expression
during development (
Bonn et al., 2012
). These findings have recently been
reviewed elsewhere (
strup et al., 2012
) and are not discussed here. Rather,
we focus on promoter-associated changes in chromatin states, defined by
combinations of histone modifications, during zebrafish development
before, during, and after ZGA.
