MINIREVIEW
Nuclear receptor-dependenttranscriptionwith chromatin
Is itallabout enzymes?
W Lee Kraus
1,2
and Jiemin Wong
3
1
Department of Molecular Biology and Genetics, Cornell University, Ithaca, NY, USA;
2
Department of Pharmacology, Weill Medical
College of Cornell University, New York, USA;
3
Department of Molecular and Cellular Biology, Baylor College of Medicine,
One Baylor Plaza Houston, TX, USA
Nuclear receptors (NRs) are ligand-regulated, DNA-bind-
ing transcription factors that function in the chromatin
environment of the nucleus to alter the expression of subsets
of hormone-responsive genes. Itis clear that chromatin,
rather than being a passive player, has a profound effect on
both transcriptional repression and activation mediated by
NRs. NRs act in conjunction with at least three general
classes of cofactors to regulate transcription in the context of
chromatin: (a) chromatin remodelers; (b) corepressors; and
(c) coactivators, many of which have distinct enzymatic
activities that remodel nucleosomes or covalently modify
histones (e.g. acetylases, deacetylases, methyltransferases,
and kinases). In this paper, we will present a brief overview of
these enzymes, their activities, and how they assist NRs in
the repression or activation of transcription in the context of
chromatin.
Keywords: chromatin; chromatin remodeling; coactivators;
corepressors; histone acetyltransferase; histone deacetylase;
histone kinase; histone methyltransferase; nuclear receptor;
transcription.
INTRODUCTION
Nuclear receptors (NRs) comprise a large superfamily of
DNA-binding transcriptional regulatory proteins that con-
trol the expression of distinct subsets of genes in the
chromatin environment of the nucleus [1–3]. In many cases,
the activities of the receptors are modulated by the binding
of hormonal ligands (e.g. steroids, retinoids, thyroid
hormone, and vitamin D
3
), which function as key regulators
in numerous physiological processes (e.g. growth, develop-
ment, metabolism, homeostasis, and reproduction) [1,2].
Most nuclear receptors share a conserved structural and
functional organization, including a highly conserved
DNA-binding domain, a C-terminal ligand-binding
domain, and two transcriptional activation functions (an
N-terminal AF-1 and a C-terminal AF-2) (Fig. 1) [1,2]. The
two most widely studied classes of NRs can be categorized
based on their dimerization and DNA binding properties:
(a) class I contains the steroid hormone receptors, which
function primarily as homodimers, and (b) class II contains
the vitamin and thyroid hormone receptors, which function
primarily as heterodimers with RXR [2]. The structural
organization of the receptors makes them ideally suited for
the transduction of hormonal signals into gene-regulatory
transcriptional responses.
During the regulation of hormone-responsive genes, NRs
must gain access to their cognate receptor binding sites
(hormone response elements, or HREs) in promoter DNA
that is assembled into chromatin, the physiological template
for transcription [3,4]. The packaging of genomic DNA into
nucleosomes (protein–DNA structures which are the repeat-
ing units of chromatin) restricts the access of the transcrip-
tional machinery to the promoters of hormone-regulated
genes, thereby reducing the transcription of those genes [3–5].
Although chromatin was at one time largely overlooked or
considered a passive player in NR-dependent transcription,
it is now clear that it plays a critical role. The importance
of chromatin in achieving a proper ligand-regulated,
NR-dependent transcriptional response (i.e. on/off switch-
ing with/without hormone) has been demonstrated experi-
mentally using both in vitro and cell-based assays [6,7].
NRs make use of chromatin to apply an exquisite level
of transcriptional control to the genes that they regulate
(i.e. repress or activate), but they do not carry out this alone.
NRs act in conjunction with at least three general classes of
cofactors to regulate transcription in the context of chro-
matin, namely: (a) chromatin remodelers; (b) corepressors;
and (c) coactivators, many of which have distinct enzymatic
activities [4,8,9]. For example, chromatin remodeling
Correspondence to W. L. Kraus, Department of Molecular Biology
and Genetics, Cornell University, 465 Biotechnology Building, Ithaca,
NY 14853, USA.
Fax: + 1 607 255 6249, Tel.: + 1 607 255 6087,
E-mail: wlk5@cornell.edu,
Abbreviations: CARM, coactivator associated arginine methyltrans-
ferase; CBP, CREB-binding protein; HAT, histone acetyltransferase;
HDAC, histone deacetylase; HMT, histone methyltransferase; NCoR,
nuclear receptor corepressor; NR, nuclear receptor; PCAF, p300/
CBP-associated factor; PRMT, protein arginine methyltransferase;
SMRT, silencing mediator for retinoid and thyroid hormone recep-
tors; SRC, steroid receptor coactivator.
Note: a homepage for W. L. Kraus can be found at
http://www.mbg.cornell.edu/kraus/kraus.html. The Cornell University
Department of MBG homepage can be found at
http://www.mbg.cornell.edu.
Dedication: This Minireview Series is dedicated to Dr Alan Wolffe,
deceased 26 May 2001.
(Received 8 October 2001, accepted 7 December 2001)
Eur. J. Biochem. 269, 2275–2283 (2002) Ó FEBS 2002 doi:10.1046/j.1432-1033.2002.02889.x
complexes contain ATPase subunits [10]. Likewise,
corepressor and coactivator complexes contain histone
modifying enzymes (e.g. acetylases, deacetylases, methyl-
transferases, and kinases) that covalently modify specific
lysine, arginine, or serine residues in the N-terminal tails of
the core histones [4,11–14]. One consequence of these post-
translational modifications may be to specify a Ôhistone
codeÕ that directs the binding of various regulatory factors,
via specific chromatin-binding domains (e.g. bromodomains
and chromodomains), to the histone tails [13]. The various
enzymatic activities listed above are recruited to hormone-
responsive promoters via direct or indirect interactions with
NRs and subsequently modify their chromatin substrates to
regulate transcription by RNA polymerase II (RNA pol II)
(Fig. 2) [3,4,8].
In this paper, we will present a brief overview of these
enzymes, their activities, and how they assist NRs in the
repression or activation of transcription in the context of
chromatin. Although the methods used to generate this
information will not be presented in detail, many of them
have recently been reviewed elsewhere [15].
CHROMATIN MODIFYING ENZYMES
AND THE REPRESSION
OF TRANSCRIPTION BY NRs
Histone deacetylases (HDACs)
Many class II NRs, including thyroid hormone receptor
and retinoic acid receptor, have the capacity to actively
repress the transcription of their target genes in the absence
of their cognate ligands [3,4,9,16]. Early competition
experiments indicated that NR-dependent transcriptional
repression requires cellular accessory proteins, termed
corepressors [17,18]. Two closely related corepressors,
SMRT and NCoR, were subsequently identified in yeast
two-hybrid screens based on their ability to interact with
unliganded NRs [19,20]. The requirement of corepressors
for repression by unliganded NRs is clearly illustrated by a
lack of NR-dependent repression in isolated NCoR
–/–
mouse embryo fibroblasts [21]. Although transcriptional
repression can be mediated via direct targeting of the basal
transcription machinery by unliganded NRs and/or core-
pressors [22,23], strong evidence indicates that chromatin
structure plays a pivotal role in repression by unliganded
NRs [6,24,25]. The importance of chromatin in NR-
dependent repression was demonstrated in studies using
Xenopus oocytes showing that repression of the TRbA
promoter by unliganded TR/RXR heterodimers requires
the proper assembly of the promoter into chromatin [6].
Following the identification of SMRT and NCoR as
corepressors for NRs, a number of studies were published
implicating histone deacetylase (HDAC) enzymes in
SMRT/NCoR-dependent transcriptional repression by
unliganded NRs (reviewed in [4,9,11,12,16]). The identifi-
cation of HDACs as components of NR corepressor
complexes, in conjunction with a well documented correla-
tion between hypoacetylated core histones and transcrip-
tionally inactive chromatin [13,26], has led to a major effort
to clarify the roles of HDACs in repression by unliganded
NRs. Currently, three major classes of HDACs have been
identified. Class I includes HDACs 1, 2, 3, and 8, which are
related to the yeast transcriptional regulator Rpd3p, and
class II includes HDACs 4, 5, 6, and 7, which are related to
yeast Hda1p [27]. The third class of HDACs are the
NAD
+
-dependent Sir2 family proteins [28]. While itis clear
that HDAC activity is essential for repression by unliganded
nuclear receptors [24,25], questions remain regarding which
HDACs are involved and how they are recruited by
unliganded NRs.
Class I HDACs, specifically HDACs 1 and 2, were the
first to be implicated in NR-dependent transcriptional
repression, as they were found to interact with mammalian
Sin3 proteins (Sin3A and Sin3B), which in turn were found
to interact with both SMRT and NCoR [29,30]. The
observed interactions between Sin3 and the corepressors led
to a model where unliganded NRs could repress tran-
scription, at least in part, through the recruitment of
Sin3-HDAC1/2 complexes by SMRT or NCoR [29,30].
However, the role of Sin3 proteins in repression mediated by
SMRT and NCoR is not as straightforward as the model
suggests. Some reports indicate that purified native SMRT–
NCoR complexes contain Sin3 [31,32], while others indicate
that the purified repressor complexes contain none [31,33–
35]. Whether this discrepancy reflects differences in the
purification protocols used, the existence of distinct or
heterogeneous SMRT–NCoR complexes, or variations in
the strength of association of Sin3 complexes with different
corepressor complexes under different cellular conditions is
presently unclear. Further studies will be required to sort
out these possibilities.
Class II HDACs, including HDACs 4, 5, 6, and 7, have
been shown to interact directly with SMRT and NCoR,
thus representing a second possible mechanism by which
HDAC activities can be recruited to unliganded NRs
[36,37]. However, the extent to which class II HDACs
contribute to repression by unliganded NRs is an open
question, as native SMRT and NCoR complexes purified
from human cells or Xenopus oocytes have so far only been
found to contain class I HDACs (primarily HDAC3, but
also HDACs 1 and 2) [31–33,35]. Thus, the relative
importance of class I and class II HDACs in NR-dependent
transcriptional repression remains undetermined. Interest-
ingly, the class I HDAC3 has also recently been shown to
interact directly with SMRT and NCoR, leading to a
stimulation of HDAC activity [38]. Such interactions that
alter enzyme activity could provide an additional level of
regulatory control.
AthirdmechanismbywhichclassIorIIHDAC
complexes might contribute to NR-dependent transcrip-
tional repression is through a nontargeting mechanism. In
this scenario, interactions with histone-binding proteins
such as RbAp46/48, which are found in HDAC com-
plexes [11,12], could direct the HDACs to the chromatin
template rather than HDAC-corepressor–NR interactions.
Whether this occurs in vivo has not been determined.
Fig. 1. NRs share a conserved structural and functional organization,
including a highly conserved DNA-binding domain (DBD), a C-terminal
ligand-binding domain (LBD), and two transcriptional activation func-
tions (AF-1 and AF-2).
2276 W. L. Kraus and J. Wong (Eur. J. Biochem. 269) Ó FEBS 2002
Regardless of how the HDACs are brought to the
chromatin template, itis clear that they play an important
role in transcriptional repression by unliganded NRs in
the context of chromatin. Although HDACs have also
been shown to deacetylate nonhistone substrates, inclu-
ding acetylated p53 and NF-jB [39–41], a role for factor
deacetylase activity in NR-dependent transcription has not
been demonstrated.
HDAC complexes containing chromatin remodelers
At least two types of HDAC complexes containing ATP-
dependent chromatin remodeling activities have been iden-
tified. The first includes variations of the Mi-2/NURD
complex [42–48], which in its most complete form contains
HDACs 1 and 2, the histone-binding proteins RbAp46/48,
the Snf 2-related ATPase Mi-2, and methyl-DNA binding
proteins such as MBD2 and MeCP1 [49]. The second is the
NCoR-1 complex, which contains NCoR, HDAC3, and
several subunits of the SWI/SNF complex, including the
Snf2-related ATPase Brg1 [32]. Of the two types of
complexes, only the Mi-2/NURD complex has been shown
to repress transcription in biochemical assays, which it does
through remodeling and histone deacetylation of nucleo-
somes assembled from methylated DNA [49]. The role of
HDAC/chromatin remodeler complexes in NR-dependent
transcriptional repression has not been investigated exten-
sively. However, one study has shown that microinjection of
neutralizing antibodies against Mi-2 (also known as CHD4)
partially relieves TR-dependent transcriptional repression in
Xenopus oocytes [46]. These results suggest a role for
HDAC–chromatin remodeler complexes in NR-dependent
repression. However, further studies in this area are clearly
needed. Interestingly, the expression of genes encoding a
number of NRs (including estrogen receptor and retinoic
acid receptor b) has been shown to be inhibited through CpG
methylation of the NR gene promoter DNA in some cancer
cell lines [50]. Thus, methylation-targeted Mi-2/NURD
complexes might play a role in regulating NR activity by
altering NR expression in some pathological states.
Histone methyltransferases (HMTs)
In addition to acetylation, core histones, especially H3 and
H4, are also targets for methylation. A number of histone
methyltransferases (HMTs) have been identified, including:
(a) the H3 lysine 9 (H3-K9)-specific HMTs Suv39H1 and
G9a, which are involved in transcriptional repression or
silencing [51,52]; (b) the H3 lysine 4 (H3-K4)-specific HMT
Set 9 (also known as Set7), which is involved in transcrip-
tional activation [53,54]; and (c) members of the protein
arginine methyltransferase (PRMT) family, such as PRMT1
and CARM1, which are also involved in transcriptional
activation [55–58]. While no specific methyltransferase has
been reported to participate in transcriptional repression by
unliganded NRs, itis worth mentioning these enzymes
because they may indirectly reduce the transcriptional
activity of NRs. For example, Suv39H1 is a heterochroma-
tin-associated, SET domain-containing protein with intrin-
sic H3 lysine 9-specific HMT activity [51]. The methylation
of H3 lysine 9 in nucleosomes generates a binding motif for
the chromodomain of the heterochromatin-associated pro-
tein HP1, which can promote the formation of higher order
chromatin structures that are repressive to transcription
[59,60]. Previous studies have shown that the incorporation
of linker histones into chromatin, which also promotes the
formation of higher order chromatin structures, reduces
NR-dependent transcription [61–63]. Thus, itis possible
that a similar effect will be observed with HP1. Whether
unliganded NRs use lysine-specific HMTs to actively
repress transcription, however, remains to be determined.
CHROMATIN MODIFYING ENZYMES
AND THE ACTIVATION
OF TRANSCRIPTION BY NRs
ATP-dependent chromatin remodelers
As mentioned above, the packaging of genomic DNA into
nucleosomes restricts the receptor-dependent assembly of
transcription complexes at the promoters of hormone-
regulated genes. Unlike many DNA-binding transcriptional
regulators, NRs bind stably and with reasonably high
affinity to DNA even when their cognate HREs are
assembled into chromatin [3]. Thus, the relevant issue seems
to be how the receptors promote the formation of an open
chromatin architecture at the promoter. One way is through
the ligand-dependent recruitment of chromatin remodeling
complexes, which are multipolypeptide enzymes categorized
by the type of ATPase subunit that they contain, including
yeast Snf2-like (e.g. SWI/SNF) or Drosophila ISWI-like
(e.g. RSF, CHRAC, ACF) [10]. Human SWI/SNF (hSWI/
SNF) represents a family of related complexes usually
containing eight or nine subunits, with either hBrg1 or
hBrm as the ySnf2-related ATPase subunit; however, the
exact composition of the complexes can vary from one cell
type to the next [10]. Chromatin remodeling complexes use
the energy stored in ATP to mobilize or structurally alter
nucleosomes, allowing greater access of the transcriptional
machinery to promoter DNA, thus facilitating transcrip-
tional activation [3,4,8,10,64].
The involvement of SWI–SNF complexes in NR-
dependent transcription was originally suggested by studies
in yeast and mammalian cells showing a stimulatory effect
Fig. 2. Multiple proteins withchromatin remodeling or histone modify-
ing activities facilitate transcriptional regulation (repression and activa-
tion) by NRs. See the text for abbreviations and details.
Ó FEBS 2002 Nuclear receptors and chromatin (Eur. J. Biochem. 269) 2277
of SWI–SNF components on NR-dependent activity [65–
68]. Since then, additional cell-based approaches have
supported these results, including experiments showing a
requirement for hBrg1-receptor interactions in estrogen
receptor and glucocorticoid receptor gene regulatory activ-
ity [69,70] and chromatin immunoprecipitation (ChIP)
experiments showing the recruitment of hBrg1 to an
estrogen-regulated promoter upon hormonal stimulation
[70]. Recently, a direct demonstration of the requirement for
the hSWI/SNF complex in receptor-dependent transcrip-
tion was made using the purified complex and an in vitro
chromatin assembly and transcription system with retinoic
acid receptor/RXR heterodimers [71]. Additional in vitro
transcription experiments have shown that recombinant
ISWI can support progesterone receptor-dependent trans-
cription withchromatin templates [72]. These in vitro
studies, in conjunction with previous cell-based studies,
make the important point that although ATP-dependent
chromatin remodeling is required for NR-dependent trans-
cription, itis not sufficient [71,73,74]. Chromatin remode-
ling may set the stage for subsequent actions by coactivators
with histone modifying activities, such as histone acetyl-
transferases [8,71,74,75].
Histone acetyltransferases (HATs)
Numerous studies in yeast and higher eukaryotic organisms
have demonstrated a link between the acetylation of specific
lysine residues in the N-terminal tails of core histones (e.g.
histone H3 lysine 14) and the activation of transcription
[26,76]. An intriguing connection between NRs and chro-
matin was made when some nuclear receptor coactivators,
including p300 and CBP (two closely related factors
commonly referred to collectively as p300/CBP), as well as
PCAF (p300/CBP-associated factor), were found to possess
intrinsic nucleosomal HAT activity [77–79]. Although initial
studies suggested that both p300/CBP and PCAF bind
directly to NRs [80,81], more recent results indicate that the
interaction of p300/CBP with many NRs is mainly indirect,
and mediated by the SRC (steroid receptor coactivator)
family of bridging factors [82–85]. Interestingly, some
reports [86,87], but not others [84,88], have suggested that
members of the SRC family also possess a weak intrinsic
HAT activity. Recent biochemical experiments indicate that
a critical function of ligand-activated, DNA-bound NRs is
to serve as nucleation sites for the recruitment of HAT
enzymes to promoters in chromatin [83], a conclusion
consistent with ChIP assays showing ligand-dependent
recruitment of HATs to hormone-regulated promoters
in vivo [89,90].
The link between histone acetylation and transcriptional
activation is, by now, well-established, yet the exact
mechanism of how histone acetylation leads to enhanced
activation is not clear. Although histone acetylation was
originally thought to facilitate chromatin remodeling by
ÔlooseningÕ the association of the histone octamer with DNA
through the neutralization of positive charges in the histone
tails, more recent results suggest that histone acetylation
may require prior chromatin remodeling or may occur at a
post-remodeling step [8,71,74,75]. The results of one study
suggest that post-remodeling histone acetylation by p300
may direct the transfer of histone H2A–H2B dimers from
nucleosomes to a histone chaperone [75]. Such an effect may
help to establish and maintain an open chromatin confi-
guration conducive to transcription. The differences
observed in the order of action of chromatin remodelers
and HATs in different experimental systems have not been
adequately explained, but may represent promoter-specific
types of regulation [8]. Recent results suggest another role
for histone acetylation, namely to create binding sites on the
amino-terminal tails of core histones for acetylated lysine
binding domains, such as the bromodomain (reviewed in
[91]), similar to the way that methylated H3-K9 serves as a
binding site for chromodomain-containing proteins (des-
cribed above). A mechanism like this may allow for the
recruitment of bromodomain-containing factors (e.g. the
HAT TAF
II
250) to promoters that have nucleosomal
histones with specific patterns of acetylation [91]. Another
question related to histone acetylation is why a number of
different HATs are required by NRs to activate the
transcription of genes in chromatin. The fact that p300/
CBP and PCAF have different histones [77–79,92] and
nonhistones (see, for example [93–95]), substrate specificities
may provide the answer, as each can acetylate a distinct set
of targets, possibly directing a distinct set of outcomes.
Although HAT activity is critical for NR-dependent
transcription, itis important to note that coactivators
such as PCAF (which is found in a large multipolypep-
tide complex with a number of other transcription-related
factors [96]) and p300/CBP contribute other activities to
the transcription process. For example, p300/CBP inter-
acts with RNA pol II complexes [97] and possesses a
glutamine-rich C-terminal region similar to the gluta-
mine-rich activation domains found in some transcrip-
tional activators, suggesting that p300/CBP may also
function as ÔclassicalÕ coactivator by interacting with
RNA pol II [98]. Furthermore, both PCAF and p300/
CBP can acetylate nonhistone, transcription-related fac-
tors,whichinmanycaseshasbeenshowntoalterthe
activity of those factors (reviewed in [99]). For example,
the acetylation of SRC3 (also known as ACTR, an SRC
family member) by p300 was shown to cause a
disruption of receptor–coactivator complexes, leading to
a decrease in receptor-mediated gene activation [90].
Estrogen receptor alpha has been shown to be a target
for p300-mediated acetylation, which may alter the
transcriptional activity of the receptor [100]. Thus some
HATs, such as p300/CBP and PCAF, serve as multi-
functional coactivators for NR-dependent transcription,
contributing multiple activities to the process.
HMTs
Although some HMTs, such as Suv39H1 described above,
are involved in gene silencing, other HMTs are involved in
gene activation. In a recent collection of experiments, two
PRMT (protein arginine methyltransferase) family mem-
bers, CARM1 (coactivator-associated arginine methyl-
transferase) and PRMT1, were shown to interact with
SRC2 (also known as GRIP1, an SRC family member) and
enhance the activity of a variety of nuclear receptors in
mammalian cell-based reporter gene assays [55,56], as well
as Xenopus microinjection experiments [57]. The stimulation
of receptor activity by CARM1 and PRMT1 was shown to
be dependent on the presence of the SRC protein and
require the intrinsic methyltransferase activities of CARM1
2278 W. L. Kraus and J. Wong (Eur. J. Biochem. 269) Ó FEBS 2002
and PRMT1 [55–57]. Both CARM1 and PRMT1 can
methylate histones in vitro [56,57], and recent studies suggest
that both do so in vivo as well [57,58,101]. Interestingly,
CARM1 and PRMT1 exhibit different HMT specificities;
CARM1 primarily methylates arginines 17 and 26 of
histone H3 (H3-R17 and R26) [102], whereas PRMT1
methylates arginine 3 of histone H4 (H4-R3) [57]. The fact
that these two methyltransferases have distinct, rather than
overlapping, HMT specificities may underlie the synergism
that has been observed between them during the stimulation
of NR activity [55]. Like the HATs mentioned above,
CARM1 and PRMT have also been shown to methylate
nonhistone substrates (e.g. p300/CBP by CARM1; STAT1
by PRMT1), thereby regulating the transcriptional activity
of the target proteins [103,104]. However, the role of factor
methylation in NR-dependent transcription has not yet
been explored in detail.
Interestingly, cooperative functional interactions between
HMTs and HATs have been observed during the stimula-
tion of NR activity (e.g. synergism between CARM1 and
p300 with estrogen receptor) [105]. The interplay between
HMTs and HATs can be observed at the enzymatic level, as
well. For example, methylation of H4-R3 by PRMT1 was
shown to increase acetylation of H4-K8 and K12 by p300
[57]. In contrast, preacetylation of H4 reduced subsequent
H4-R3 methylation by PRMT1 [57]. Although not demon-
strated in the context of NR-dependent transcription,
H3-K4 methylation by Set9 (also known as Set7) has been
shown to antagonize H3-K9 methylation by Suv39H1, and
vice versa [53,54]. Furthermore, pre-methylation by Set9 and
Suv39H1 of H3 in a core histone mixture had different
effects on the subsequent acetylation of H3 and H4 by p300,
with Set9 stimulating and Suv39H1 inhibiting the p300-
mediated acetylation [54]. These results illustrate the com-
plex interactions that can occur between different factors
with histone modifying activities, generating interplay at the
level of the nucleosome that results in a Ôcombinatorial
histone codeÕ [91]. Furthermore, they suggest that dissecting
the individual contributions of particular HMTs and HATs
in the context of a large, NR-dependent transcriptional
regulatory complex may be difficult, as each effect may be
subtle and may influence the activity of other factors in the
complex. This will be an important area of continued
investigation.
Linker and core histone kinases
Linker histones, such as histone H1, bind to the linker
DNA flanking the nucleosome core. In doing so, they
facilitate the compaction of chromatin into higher order
chromatin structures, which can lead to the repression of
transcription with a variety of DNA-binding activators,
including NRs [61,106,107]. Interestingly, the repression of
NR activity by histone H1 can be relieved by the ligand-
dependent phosphorylation of H1 by cdk2 (and possibly
other kinases, as well), which leads to the removal of H1
from the promoter region [62,108,109]. Thus, post-trans-
lational modification of linker histones must be considered
when evaluating the transcriptional activity of NRs with
chromatin. Interestingly, recent studies suggest that
another post-translational modification of H1, namely
ubiquitination, may also play a role in regulating the
repressive effects of H1 [110]. However, this has not yet
been shown to play a role in modulating NR transcrip-
tional activity.
In addition to the phosphorylation of linker histones, the
phosphorylation of the core histone H3, specifically at serine
10, has also been shown to enhance the transcription of
genes in chromatin (reviewed in [13]). The phosphorylation
of H3-S10 appears to be tied to intracellular protein kinase
signaling pathways [111,112] and can enhance the subse-
quent acetylation of nearby lysine residues [111,113,114].
Thus, as was observed for methylation and acetylation of
H4 [57], phosphorylation and acetylation of H3 are
functionally linked. Although H3 phosphorylation has not
yet been demonstrated to play a role in NR-dependent
transcription with chromatin, it seems likely to play a role.
In this regard, itis interesting to note that some of the same
signaling pathways that enhance H3-S10 phosphorylation
(e.g. MAP kinase) [111,112] have been shown to enhance
NR transcriptional activity [115,116].
CONCLUSIONS
In closing, we should ask, ÔIs NR-dependent transcription
with chromatin really allabout enzymes?Õ Given the fact
that NRs rely on coactivators that target the basal
transcriptional machinery (e.g. the mediator complexes)
[117] in addition to enzymes involved in chromatin
remodeling and histone modification, the answer is no.
Nonetheless, itis clear that the enzymes described herein
play critical roles in the process of NR-stimulated tran-
scription by RNA pol II, which is itself a complex and
fascinating enzyme. As noted above, gaps remain in our
understanding of which specific enzymes regulate the
transcriptional activity of NRs with chromatin, how they
do it, and how they are regulated. These questions will be
important avenues of investigation in the future.
ACKNOWLEDGEMENTS
The research of W. L. K. is supported by grants from the NIH, the
Burroughs Wellcome Fund, and the Susan G. Komen Breast Cancer
Foundation. The research of J. W. is supported by grants from the
NIH and US Army Medical Research.
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Ó FEBS 2002 Nuclear receptors and chromatin (Eur. J. Biochem. 269) 2283
. MINIREVIEW Nuclear receptor-dependent transcription with chromatin Is it all about enzymes? W Lee Kraus 1,2 and Jiemin Wong 3 1 Department of Molecular Biology and Genetics, Cornell University, Ithaca,. are functionally linked. Although H3 phosphorylation has not yet been demonstrated to play a role in NR-dependent transcription with chromatin, it seems likely to play a role. In this regard, it is interesting. transcription with chromatin really all about enzymes? Given the fact that NRs rely on coactivators that target the basal transcriptional machinery (e.g. the mediator complexes) [117] in addition to enzymes