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2000; Bender 2004). A critical turning point also occurred in 1996 when
the group of David Allis reported the puri
cation and identi
cation of
the
first histone acetyl transferase protein (HAT) (Brownell et al. 1996).
When this HAT was partially sequenced and compared with the cloned
homologous gene in yeast, it was realized that HAT is a transcriptional
regulator. This became the
first example of the control of transcriptional
activation by a mechanism other than the recruitment and control of Pol
II RNA polymerase to
cis
-elements of DNA by DNA binding proteins
(Hochheimer and Tijian 2010). These
findings proved to be the tip of the
iceberg, indicating the existence of additional layers of templating the
DNA that would challenge the Central Dogma. More important clues
emerged when chromatin was found to be involved in the regulation of
TF master controls, such as the HOX genes (Gellon and McGinnis 1998).
Genes that are required to establish the stability or memory of the HOX
and other transcriptional cascades through mitosis by chromatin modi-
ed. Especially important are those
encoding the chromatin modifying TRITHORAX and POLYCOMB
group proteins in
Drosophila
that regulate HOX gene suppression (Lewis
1978; Mahmoudi and Verrijzer 2001; Grossniklaus and Paro 2007;
Kingston and Tamkun 2007). The universal nature of this control
process became apparent when many homologous proteins were found
to exist over wide taxonomic distances such as CLF (Goodrich et al.
1997) in plants, MES in
Caenorhabditis elegans
, and RING1 in mammals
(Grossniklaus et al. 1998; Kiyosue et al. 1999; Satijn and Otte 1999;
Kinoshita et al. 1999; Fong et al. 2002; Alvarez-Venegas et al. 2006;
Kouzarides and Berger 2007). It has also become increasingly estab-
lished that the role of chromatin in cell memory is mediated by many
other proteins involved in the epigenetic machinery (Lewis 1978;
Lafos and Schubert 2009). These master controllers also mediate home-
otic structural identity in plants, such as during
fications were eventually identi
floral development
(Grossniklaus et al. 1998; Alvarez-Venegas et al. 2006). Now it is recog-
nized that chromatin structure plays an immensely important role in
exquisitely coordinated activation and repression of genes (Goodrich
et al. 1997; Grewal and Jia 2007).
D. The Rebirth of Epigenetics
In the 1940s, Conrad Waddington, a developmental geneticist, began
using the term epigenetics, literally meaning
(as if to
say, on top of genetics, we have something else going on here) to describe
these somatic cell
“
on top of genetics
”
inherited patterns
that controlled ontogeny
