From the Department of Transcriptional and Post-transcriptional Control of Gene Regulation, Institut de Génétique et de Biologie Moléculaire et Cellulaire, CNRS/INSERM/ULP, BP 163, F-67404 Illkirch Cedex, Communauté Urbaine de Strasbourg, France
Received for publication, January 18, 2001, and in revised form, January 18, 2001
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ABSTRACT |
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Preinitiation complex assembly is nucleated by
the binding of TFIID to the promoters of protein coding genes
transcribed by RNA polymerase II. TFIID is comprised of the
TATA-binding protein (TBP) and TBP-associated factors
(TAFIIs). We investigated the transcription
properties of TBP and TFIID on chromatin templates. On naked templates
both TBP and purified TFIID are able to initiate basal transcription.
However, on chromatin templates only TBP mediates transcription
initiation in a heat-treated extract, whereas TFIID does not. Moreover,
TBP-mediated chromatin transcription is blocked in a nontreated
extract. These observations suggest that a chromatin-targeted repressor
is present in crude extracts and that chromatin per se is
not refractory to transcription mediated by TBP. As TBP can function
through TAFII-independent and
TAFII-dependent pathways, the repression of
TBP-mediated basal transcription may be an additional level to the
control of Pol II transcription initiation on chromatin.
Transcription initiation of protein-encoding genes requires the
assembly of a productive preinitiation complex
(PIC)1 comprising RNA
polymerase II (Pol II) and the general transcription factors
(1). Binding of TFIID is followed by the subsequent ordered
interactions of the other general transcription factors (TFIIA, -IIB,
-IIE, -IIF, -IIH) and Pol II to yield a productive PIC (2). TFIID is a
protein complex composed of TBP and a number of TBP associated factors
(TAFIIs) (3). The discovery that TBP is not only a
component of the TFIID complex but is also present in transcription
complexes functioning in Pol I and III transcription, placed TBP as a
central player in transcription (4).
In vitro recombinant TBP can substitute for TFIID in reconstituted
basal transcription systems but does not support transcription from
TATA-less promoters (5). Moreover, in Drosophila and human cell-free systems TFIID but not TBP could support
activator-dependent transcription. These findings suggested
that TAFIIs are essential both for transcription initiation
and for the response to transcriptional activators in vitro
(3). Evidence showing that TAFIIs may function as
coactivators in vivo has come from either mammalian cell
transfection experiments (6) or from genetic studies in
Drosophila (7). However, in contrast to the above cell-free
systems, activation of in vitro transcription in a
TBP-dependent manner was reported in a
TAFII-depleted nuclear extract (8).
Two active forms of TBP have been recently reported in yeast, a
TAF-dependent and a TAF-independent form, that can mediate Pol II transcription in vivo (9, 10). Novel complexes
consisting of TAFIIs and a plethora of other proteins have
also been identified from yeast and human cells, i.e. TFTC,
PCAF/GCN5, STAGA, and SAGA (11). These complexes do not contain TBP or
TBP paralogues, although they may participate in promoter recognition,
transcription activation, and chromatin modifications. In addition, a
TBP-TFIIA-containing complex (TAC) has recently been described in
embryonic carcinoma cells that does not contain TAFIIs
(12).
Despite their ability to mediate basal transcription on naked
templates, the minimal set of Pol II transcription factors, including
TFIID, are not sufficient for Pol II transcription on DNA templates
packaged into chromatin (13, 14). Although the function of TBP and
TFIID during basal transcription from naked DNA templates has been
widely studied, their precise role during transcription on chromatin
templates remains elusive. Here, we analyzed the transcriptional
properties of TBP and TFIID on chromatin.
Chromatin Assembly--
Drosophila embryo extracts
were prepared, and chromatin was assembled essentially as described
(15). Typically, chromatin templates were assembled with 3 mg of
Drosophila S-190 extract, 1 µg of calf thymus core
histones, 1 µg of supercoiled plasmid (see below), and an ATP
regenerating system (30 mM creatine phosphate, 3 mM ATP, 4.2 mM MgCl2 and 6 µg of
creatine kinase) in a 100- µl volume. After a 5-h incubation period,
chromatin was either analyzed by micrococcal nuclease digestion or
processed for transcriptional analysis.
Transcription in Vitro--
HeLa nuclear extract (NE)
preparation and Gal4-VP16 purification have been described (16). The
heat inactivation of the NE (usually 300 µl) was carried out at
45 °C for 15 min as described in (17). Transcription reactions were
performed using 25 ng of 17M5/pAL7 (AdMLP+1) and 25 ng of pG1 (Glob+1)
templates (16). Purified recombinant TBP (16) or immunopurified TFIID
(18) were incubated with the templates for 10 min at 4 °C. Where
indicated Gal-VP16 was preincubated with the templates for 10 min at
4 °C. Transcription reactions (25 µl) were performed with either
normal or heat-inactivated NEs and incubated for 30 min at 25 °C.
Transcription was initiated by the addition of NTPs (0.5 mM) and MgCl2 (5 mM) and carried
out for 45 min at 25 °C. Specific transcripts were analyzed by S1
mapping (16).
Immunoprecipitation and Immunoblotting--
Immunoprecipitation
was performed as described in (18). Immunoblotting was performed using
anti-hTBP, anti-hTAFII30, anti-hTAFII100, and
anti-hTAFII135 mAbs by standard methods (19).
To analyze the role of TBP and TFIID during transcription
initiation from naked or chromatin-assembled templates, first we normalized the amounts of TBP to be used in the reactions, either as
recombinant TBP or immunopurified TFIID complex, by immunoblotting using a mAb raised against the recombinant human TBP (16) (Fig. 1B). Then using equivalent
units of TBP we compared the basal transcription activities of TBP and
TFIID on naked templates from two different promoters (Fig. 1,
A and C). As a source of remaining general
transcription factors and other accessory factors we used a
heat-treated HeLa cell NE in which the transcriptional activity of TBP
and TFIID could be monitored (17). Transcription in the heated NE was
completely dependent upon the addition of TBP or TFIID (Fig.
1C). Equivalent units of TBP and TFIID resulted in comparable levels of basal transcription initiation from the adenovirus major late promoter (AdMLP) (Fig. 1C). However,
TFIID-dependent basal transcription was more efficient from
the rabbit
INTRODUCTION
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ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES
EXPERIMENTAL PROCEDURES
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES
RESULTS
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES
-globin promoter (Glob) than with TBP (in Fig.
1C, compare lanes 3-5 to 6-8 and in
Fig. 2E, lane 8 to
9). Note, that transcription from the two templates have
been carried out in the same reaction. Thus, our results show that on
certain promoters TAFIIs can have a positive effect on
transcription initiation from naked templates.
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Fig. 1.
Promoter-specific requirements for TBP and
TFIID on naked transcription. A, outline of the
transcription assay. NE(45 °C), inactivation of HeLa
nuclear extract at 45 °C for 15 min. B, normalization of
recombinant TBP and immunopurified TFIID by immunoblotting using mAbs
raised against TBP (3G3) and hTAFII100 (1TA). The sizes of
the protein markers in kDa are indicated. C, TBP- and
TFIID-dependent transcription from AdMLP
(AdMLP+1) and the rabbit -globin (Glob+1)
promoters. Transcription reactions were assembled on naked DNA
templates as depicted in panel A. When used, TBP and TFIID
were prebound to the template for 10 min at 4 °C before nuclear
extracts (NE) were added. Lanes 3 and 6, 4 and 7, and 5 and 8 were
normalized to contain the same amounts of TBP.
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Fig. 2.
TBP-mediated basal transcription is not
blocked on chromatin templates. A, outline of chromatin
assembly and transcription assays. B, chromatin templates
were digested with micrococcal nuclease and the DNA fragments were
analyzed by agarose gel electrophoresis and ethidium bromide staining.
M, 123-base pair DNA ladder. C, TBP- and
TFIID-dependent chromatin transcription. AdMLP
(AdMLP+1) and -globin (Glob+1)
promoter-containing templates were assembled in chromatin, equivalent
units of TBP or TFIID were prebound to these templates, and then normal
(NE, lane 1) or heat-inactivated nuclear extracts
(NE45 °C, lanes 2-10) were added as indicated.
D, TBP- and TFIID-dependent activated chromatin
transcription. The AdMLP template contains 5 GAL4 binding sites
upstream of the adenovirus major late minimal promoter. GAL4-VP16 was
preincubated with the AdMLP and
-globin promoter-containing
chromatin templates for 10 min before TBP or TFIID were added. The
samples shown in panels C and D were analyzed on
the same gel and thus exposed for the same time. E, TBP- and
TFIID-dependent transcription in normal and
heat-inactivated nuclear extracts on naked and chromatin templates.
F, TBP-dependent chromatin transcription is
blocked by sarcosyl at the level of reinitiation. Preinitiation
assembly reactions were assembled as indicated in panel A.
In lanes 5-8 sarcosyl was added 1.5 min after the
addition of NTPs and MgCl2.
Next, the AdMLP and the Glob templates were assembled into chromatin
with an average nucleosome repeat length of 176 base pairs (±3 base
pairs; data not shown), and the efficiency of chromatin assembly was
monitored by micrococcal nuclease digestion (Fig. 2B). To
analyze the transcriptional properties of TBP and TFIID on chromatin
templates, the chromatin-assembled AdMLP and the Glob templates had
been preincubated with TBP or TFIID for 10 min prior to the addition of
the heat-treated NE (Fig. 2A). Surprisingly, recombinant TBP
alone can mediate transcription initiation in a
dose-dependent manner from the chromatin-assembled AdMLP
and, to a lesser extent, from the -globin promoter (Fig.
2C). In contrast, the same amount of TFIID that mediated
efficient transcription initiation from naked DNA templates exhibited
only a very minor enhancement of transcription from the
chromatin-assembled AdMLP and globin promoters (lanes
7-10). However, on the chromatin-assembled AdMLP template that
contains five GAL4 binding sites, the chimeric activator, Gal4-VP16,
strongly stimulated TFIID-mediated transcription (Fig. 2D,
lanes 5-8). These results together suggest that
TAFIIs can impair initiation of basal, but not activated,
transcription when nucleated by TFIID on chromatin templates.
To test whether efficient TBP-dependent transcription on chromatin was solely due to the heat treatment, we used either normal or heat-treated NEs (Fig. 2E). DNA templates were assembled with chromatin or not and preincubated with TBP or TFIID for 10 min, and then the nontreated or the heat-treated NEs were added (Fig. 2E). Using the chromatin-assembled templates neither TBP or TFIID was able to recover or enhance transcription in the normal nuclear extract (Fig. 2E, lanes 4-6). In contrast, in the heat-inactivated NE TBP mediated transcription from the chromatin templates (Fig. 2E). Note that TBP-mediated transcription was much more efficient from the chromatin-assembled AdMLP template than from the Glob template (Fig. 2C, lanes 3-6 and Fig. 2E, lane 11), suggesting that the observed effect may be dependent on the TATA box of the promoter used. However, TFIID had only a weak effect on chromatin transcription when added to the heat-treated NE (Fig. 2E). These results suggest that on certain promoters chromatin is permissive to transcription initiation mediated by TBP but not by TFIID and that the heat treatment may destabilize a chromatin-targeted repressor in the nuclear extract (see also "Discussion").
The action of TBP-mediated transcription on chromatin templates was further analyzed at the level of transcription initiation. To eliminate ongoing initiation in the transcription reaction, we performed single round transcription experiments on the chromatin-assembled AdMLP template. Transcription was initiated for 90 s by addition of NTPs, then sarcosyl was added to a final concentration of 0.08% to prevent de novo PIC formation as well as initiation at preformed PICs (20). Using nontreated NE no differences were observed on chromatin with or without sarcosyl in the absence of TBP and TFIID (Fig. 2F). Moreover, in the heat-treated extract the low TFIID-dependent transcription levels were not influenced by sarcosyl (Fig. 2F). However, a strong decrease was observed in the TBP-mediated transcription initiation on the chromatin template when using heat-treated NE (Fig. 2F). Thus, on chromatin templates TBP and TFIID have little or no effect on single round basal transcription, but TBP seems to be involved in the maintenance of high levels of activator-independent reinitiation.
Next we examined the effect of heat treatment of the NE on endogenous
TFIID. Nuclear extracts were either kept on ice or heat inactivated at
45 °C before immunoprecipitation was performed with an anti-TBP mAb,
and TBP-associated proteins were analyzed by Western blot (Fig.
3). In normal NE, several
TAFIIs were coimmunoprecipitated together with TBP (Fig.
3). In heat-treated nuclear extract TBP dissociated from TFIID because
none of the tested TAFIIs (i.e. TAFII250, TAFII135, TAFII100,
TAFII55, and TAFII30) were coimmunoprecipitated with TBP (Fig. 3 and data not shown). Thus, we hypothesize that after
the dissociation from the TFIID complex, TBP loses its transcriptional activity because it undergoes a conformational change and/or a posttranslational modification. Moreover, it seems that all the other
activities that have been reported to be able to mediate Pol II
transcriptional initiation (TFTC, TAC, and other TBP-like factors (21)
are also heat inactivated in this extract.
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DISCUSSION |
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Our results demonstrate that chromatin per se is not refractory to activator-independent PIC formation and that subsequent transcription initiation on every promoter, such as TBP-dependent basal transcription, is almost as efficient on chromatin-assembled AdMLP template as it is on naked DNA in the heat-treated nuclear extract.
The recent discovery of TAFII-independent and TAFII-dependent forms of TBP (9, 10) and the TAC complex free of TAFIIs (12), give TBP function a new turn. Thus it is important to identify nuclear activities that regulate TBP-dependent (TAFII-independent) basal transcription. Binding of TBP to the TATA sequence seemed to be inhibited by nucleosomes, and this inhibition could be overcome by ATP-dependent alterations in nucleosomal DNA structure mediated by hSWI/SNF (22). Here we show that TBP-mediated basal transcription on chromatin is blocked in a "normal" NE and that heat treatment can relieve this inhibition on certain promoters. Thus, it seems that a chromatin-targeted inhibitory factor exists in NE that blocks transcription initiation by the TAFII-independent forms of TBP only on chromatin. However, activators may relieve the repression by the inhibitory factor as TBP-dependent transcription can be stimulated in a normal NE (data not shown). FACT, a heat labile elongation factor that can relieve chromatin-mediated blocks and that blocks TBP-mediated transcription on naked DNA in the absence of TFIIE and TFIIH (23, 24), may be a candidate for this inhibitory factor. Whether FACT and/or other yet unknown factors are required for this chromatin-targeted inhibitory activity remain to be determined. Alternatively, it is also conceivable that heat treatment causes dissociation of a nuclear multisubunit transcription complex, which subsequently releases a nonheat labile factor that facilitates the observed TBP-dependent basal transcription on chromatin.
When TBP was co-incubated with heat-treated NE under chromatin transcription conditions, we were unable to co-purify TAFIIs together with TBP (data not shown). This observation together with the clear difference between the TBP- and TFIID-mediated transcription on chromatin and with the immunoprecipitation results suggest that TFIID does not reform under our assay conditions. Thus, the observed differences between TBP and TFIID are due to TAFII-independent versus TAFII-dependent transcription initiation on chromatin. In contrast to the TBP-mediated transcription initiation, the inability of TFIID to efficiently initiate basal transcription on chromatin may indicate that TAFIIs are able to exert negative effects on TBP-dependent basal transcription on chromatin or that TFIID is simply sterically excluded from chromatin. On naked templates TAFIIs have been reported to have negative effects on basal transcription (25, 26). Surprisingly, under our assay conditions and on naked templates we have not observed negative effects by TAFIIs on basal transcription as TFIID was as active as free TBP. However, on chromatin templates TAFIIs seem to inhibit basal but not activated transcription as activators strongly stimulated TFIID-mediated transcription (Fig. 2D). Thus, our data suggest that on chromatin TAFIIs may impair only TFIID-dependent basal transcription initiation.
Emerging evidence suggest that reprogramming of chromatin and of TBP-
and TBP-like factor-mediated transcription initiation during distinct
developmental stages of embryogenesis, where fundamentally different
chromatin states exist, are precisely regulated and coupled events (27,
28). Our study thus identifies a potential contributor(s) to this
regulation and provides mechanistic explanations toward the
understanding of chromatin-regulated transcription initiation events
in vivo.
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ACKNOWLEDGEMENTS |
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We thank P. Chambon for his support, H. Ichinose for reagents, and W. Mohan for critical reading of the manuscript.
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FOOTNOTES |
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* This work was supported by funds from INSERM, CNRS, Hôpital Universitaire de Strasbourg, Association pour la Recherche sur le Cancer, Fondation pour la Recherche Médicale, Ligue Nationale contre le Cancer, and Human Frontier Science Program (Grant RG 196/98).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Supported by a Human Capital and Mobility fellowship from
the European Community and by a grant from the Fondation pour la Recherche Medicale. Present address: Div. of Developmental Genetics, National Inst. for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA United Kingdom.
§ Present address: Target Identification, Dept. of Internal Medicine, Sanofi-Synthelabo, 92500 Rueil-Malmaison, France.
¶ To whom correspondence should be addressed. Tel.: 33-3-88-65-34-44; Fax: 33-3-88-65-32-01; E-mail: laszlo@igbmc.u-strasbg.fr.
Published, JBC Papers in Press, January 22, 2001, DOI 10.1074/jbc.M100481200
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ABBREVIATIONS |
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The abbreviations used are:
PIC, preinitiation
complex;
Pol, RNA polymerase;
TBP, TATA-binding protein;
TAFIIs, TBP associated factors;
TAC, TBP-TFIIA-containing
complex;
NE, nuclear extract;
mAb, monoclonal antibody, AdMLP,
adenovirus major late promoter;
Glob, rabbit -globin promoter.
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