Biology Reference
In-Depth Information
catalyzes both H3K9 and H3K27 methylation and is required for the forma-
tion of constitutive heterochromatin (
Liu et al., 2007
). Another example of
PRC2-dependent heterochromatin formation comes from the green alga
Chlamydomonas reinhardtii
, where the H3K27 methylation activity is needed
for the silencing of repetitive sequences (
Shaver et al., 2010
).
During evolution and as a result ofmultiple genome duplication events, the
two pathways diverged and were attributed with specific functions in mul-
ticellular organisms. Below, we will describe themost important developmen-
tal roles of these two pathways and how they contribute to gene regulation.
2. THE H3K9/HP1 PATHWAY AND ITS ROLE IN
DEVELOPMENT
The seminal discovery of enzymes mediating H3K9 methylation on
pericentric heterochromatin (PCH;
Rea et al., 2000
) and thereby generating
a binding site for HP1 (
Lachner, O'Carroll, Rea, Mechtler, & Jenuwein,
2001
) emphasizes the role of this pathway in regulation of heterochromatin
structure (
Cheutin et al., 2003
). The discovery of the Suv39h enzymes was
soon followed by the identification of other H3K9-methylating enzymes
(H3K9 KMT) (see
Table 8.1
) catalyzing different degrees of methylation
and regulating repression of different classes of sequences (
Krauss, 2008
).
The concept of division of labor applies also to HP1, as most eukaryotes
express different isoforms of HP1 that differ in their nuclear localization
(
Zeng, Ball, & Yokomori, 2010
) and serve partially redundant as well as
isoform-specific functions (
Rosnoblet, Vandamme, V
¨
lkel, & Angrand,
2011
). For example, these proteins have been implicated in maintenance
of heterochromatin, chromosome segregation, transcriptional silencing/
activation (
Kwon & Workman, 2011; Schotta, Ebert, & Reuter, 2003
),
DNA replication (
Hayashi, Takahashi, Nakagawa, Nakayama, &
Masukata, 2009; Schwaiger, Kohler, Oakeley, Stadler, & Sch¨beler,
2010
), and the DNA damage response (
Dinant & Luijsterburg, 2009
).
2.1. Developmental role through regulation of gene
expression
2.1.1 Su(var)
With their pleiotropic roles in diverse biological pathways, H3K9-
modifying enzymes and HP1 proteins are of particular interest for embry-
onic development. Functional studies in several organisms revealed that with
the exception of the
Suv39h1
and
Suv39h2
(
Suv39h1/2
) enzymes, removal
