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(Received for publication, February
7, 1996) From the
Sp3 is a member of the Sp family of transcription factors, and
it binds to the GC box with an affinity and specificity comparable with
that of Sp1. Previous studies have shown that Sp3 repressed
Sp1-mediated transcriptional activation, suggesting that Sp3 is an
inhibitory member of the Sp family. The experiments described here
demonstrate that Sp3 contains a portable repression domain that can
function independently from the zinc finger DNA-binding domain. We
found that the amino-terminal region of Sp3 tethered to a promoter DNA
by connecting to a heterologous DNA-binding protein domain represses
transcriptional activation by different positive regulators. Moreover,
we determined that Sp3 targeted to a promoter-proximal RNA sequence
acts as a transcriptional repressor. Taken together, our results
suggest that Sp3 functions as a repressor by protein-protein
interaction with components of the general transcription complex.
Originally identified as a cellular transcription factor
required for SV40 gene expression, Sp1 stimulates transcription by
binding to a GC box present in a wide variety of cellular and viral
promoters(1) . Recently, three Sp1-related genes (Sp2, Sp3, and
Sp4) have been cloned based on their homology with the Sp1 DNA-binding
domain(2, 3) . The DNA-binding domains of the Sp3 and
Sp4 proteins are highly conserved, and they recognize GC boxes with
specificity and affinity closely similar to that of Sp1. Contrary to
these factors, Sp2 seems to have different DNA binding
specificities(3) . It has been shown that both Sp1 and Sp3
proteins are ubiquitously expressed at a high level in many mammalian
cell lines, whereas Sp4 expression appears to be restricted to certain
cell types of the brain(2, 4, 5) . Functional
analysis of Sp3 and Sp4 in direct comparison with Sp1, using
transfection experiments into mammalian cell lines and Drosophila SL2 cells lacking endogenous Sp factors, demonstrated that Sp4,
like Sp1, is a transcriptional activator of Sp1-responsive promoters,
whereas Sp3 represses Sp1-mediated
transcription(4, 5, 6, 7) . These
results suggest that Sp3 is an inhibitory member of the Sp family. The intriguing finding that Sp3 represses Sp1-mediated transcription
prompted us to embark on an analysis of the transcriptional properties
of Sp3. We performed in vivo transfections, in which the
non-finger region of Sp3 repressor and a defined activating domain were
both targeted to a promoter. We found that Sp3 contains a portable
repressor activity, which can function independently from the zinc
finger DNA-binding domain. In addition we demonstrated that the Sp3
amino terminus region is a transcriptional repressor of several
activators, and the repression is not influenced by the arrangement of
basal promoter elements. Finally, we determined that the Sp3 repressor
is functional when targeted to a promoter-proximal RNA sequence. Our
results indicate that Sp3 acts as a transcriptional silencing of RNA
polymerase II promoters either when tethered to DNA by fusion to a
DNA-binding protein domain or when targeted to a promoter-proximal RNA
sequence.
Figure 1:
A, schematic
representation of the T7G5-TATA and T7G5-I reporter plasmids containing
the E1b TATA or the AdMLP Inr sequences as core promoter element,
respectively. B, reporter plasmids T7G5-TATA (2 µg, open bars) and T7G5-I (2 µg, solid bars) were
cotransfected into HeLa cells with the GAL4 derivatives (1 µg) as
indicated. The transcriptional activity of each GAL4 derivative
relative to the sample without activator is diagrammed at the right. The values are representative of four independent
duplicated experiments; vertical lines indicate the standard
deviations.
To determine the Sp3-mediated repression of the
activation function of each GAL4 derivative, the reporter plasmids
described in Fig. 1A were co-transfected into HeLa
cells with the indicated GAL4 fusion proteins in the presence of an
increasing amount of the Tet-Sp3 expression vector. Co-expression of
Tet-Sp3 protein was found to repress in a dose-dependent manner GAL4
chimeric protein-mediated activation of both the TATA and
Inr-containing promoters (Fig. 2). The specificity of
Sp3-mediated repression was demonstrated by the results reported in Fig. 2B, showing that neither the Tet-Sp3 fusion
protein affected the activation function of a reporter lacking the tetO
sequences nor did the pCMV-Sp3DBD, encoding the fingerless portion of
Sp3 lacking the DNA-binding domain(6) , influence significantly
promoter activity.
Figure 2:
Sp3 represses transcription in the
presence of defined activators. On the top a schematic
representation of the chimeric protein Tet-Sp3 is shown. A,
reporter plasmids T7G5-TATA (2 µg, open bars) and T7G5-I
(2 µg, solid bars) were cotransfected into HeLa cells
together with 1 µg of the indicated GAL4-activator plasmid in the
presence of increasing amounts (1, 5, and 10 µg) of Tet-Sp3
expression vector in a total of 20 µg adjusted with the parental
plasmid. CAT activities relative to the samples without repressor
(taken as 100%) are shown. The values are representative of four
independent duplicated experiments; vertical lines indicate
the standard deviations. B, repression specifically depends
upon DNA binding. T7G5-TATA (open bars) and G5E1b (striped
bars) reporter plasmids (2 µg each) were transactivated with 1
µg of GAL4-VP16 expression plasmid. Coexpression of CMV-Sp3DBD (10
µg) or Tet-Sp3 (10 µg), respectively, did not affect
transactivation.
Taken together the data from transfection
experiments demonstrate that Sp3 suppresses transcription when allowed
to bind next to the activator, and the extent of repression appears to
be independent of the type of activator domain and of the presence of a
specific core promoter element.
To
clarify this point we chose to attempt regulation of HIV-1 promoter
activity by designing a chimeric Tat negative trans-dominant mutant
fused to the non-finger region of Sp3. We used the previously described
pSVTat22/37, in which cysteines 22 and 37 have been substituted with
glycine and serine, respectively(8) . It has been shown that
these point mutations in the cysteine-rich region abolished the
transacting effect of the protein(17) . The non-finger region
of Sp3 (aa 1-527) was fused to the COOH terminus of Tat22/37
resulting in the pSVTat22/37-Sp3 expression vector. As reporter, the
G5-83HIV-CAT plasmid, which contains five GAL4 DNA-binding sites
located at position -83 of HIV-1 LTR(6) , was used. Using
a similar reporter it has been shown (18) that the GAL4-VP16
fusion protein can potently activate expression from an HIV-1 LTR
bearing multiple GAL4 DNA-binding sites. The G5-83HIV-CAT plasmid
was transfected into HeLa cells together with the GAL4-VP16 effector
plasmid in the presence of increasing amounts of pSVTat22/37 or
pSVTat22/37-Sp3 effectors (see Fig. 3A). Accordingly
with reported results the GAL4-VP16-mediated trans-activation of
G5-83HIV-CAT reporter was very high(18) , and the
presence of Tat22/37 did not change the promoter activity. Conversely,
a dose-dependent repression of GAL4-VP16-mediated activation was
observed in the presence of the Tat22/37-Sp3 fusion protein (Fig. 3A).
Figure 3:
Sp3 represses activated and basal HIV LTR
promoter activity when tethered to TAR. A, GAL4-VP16-mediated
activation of the HIV promoter is repressed by Sp3 targeted to TAR.
HeLa cells were transfected with G5-83HIV reporter (1 µg) in
the presence of GAL4-VP16 effector plasmid (1 µg) together with
increasing amounts of pSVTat22/37 or pSVTat22/37-Sp3 as indicated in a
total of 20 µg adjusted with the parental pSV expression vector.
The bars show the relative CAT activities, with CAT activity
obtained in the presence of GAL4-VP16 alone taken as 100%. B,
Sp3 represses basal HIV LTR promoter activity. HeLa cells were
transfected with the reporter G-83HIV (5 µg) alone (empty
bars) or in the presence of increasing amounts (5 and 10 µg)
of pSVTat22/37 (dashed bars) or increasing amounts (1, 5, and
10 µg) of pSVTat22/37-Sp3 (black bars) effectors as
indicated. C, similar transfections were performed using as
reporter the G5-83HIV
To further substantiate the notion that
Sp3 targeted to nascent RNA is able to repress HIV-1 LTR promoter
activity we analyzed the Sp3-mediated repression of the LTR basal
activity. Dose-dependent repression of the HIV LTR basal activity was
mediated by Tat22/37-Sp3 fusion protein, whereas no significant effect
was observed in the presence of the Tat22/37 mutant (Fig. 3B). Moreover, Tat22/37-Sp3-mediated repression
was dependent on the presence of TAR element (Fig. 3C),
since the activity of the reporter plasmid G5-83HIV- Several models can be envisaged to explain how Sp3
might repress transcription. For example, Sp3 may function by
inactivating or squelching a protein that normally activates polymerase
II expression (14, 15, 19) . This possibility
seems very unlikely because Sp3-mediated repression is strictly
dependent on binding in cis to the promoter or binding to a
promoter-proximal RNA target. A number of repressors appear to use
quenching as their mechanism. For example, the Drosophila Kruppel protein displays the ability to quench some activators but
not others(20) . However, the quenching mechanism does not
account for the ability of Sp3 to repress different types of
activators, and the repression is not influenced by the presence of a
specific core promoter element. Therefore, it appears that Sp3-mediated
repression may act directly on the general transcription machinery. The observation that Sp3 repressor function does not necessarily
require an interaction with a DNA target sequence appears inconsistent
with a repression model involving the formation on the DNA template of
multiprotein complexes composed of Sp3 and other factors. Instead, it
appears more likely that the RNA-bound Sp3 may interact directly with a
component of the general transcriptional machinery and prevent an
isomerization or disassembly step. Alternatively, Sp3 could load a
putative ``corepressor(s)'' into the general transcription
complex, which may then modify the initiation complex so that the rate
of transcription is repressed. However, it remains to be demonstrated
whether the mechanism of Sp3-mediated repression is the same when it is
targeted to nascent RNA as it is when it is bound to promoter DNA.
Volume 271,
Number 15,
Issue of April 12, 1996 pp. 8533-8536
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS AND DISCUSSION
FOOTNOTES
ACKNOWLEDGEMENTS
REFERENCES
Reporter Plasmids
The HIV-1 (
)LTR-based reporters G5-83HIV and
G5-83HIV
TAR have been described (6, 8) .
The T7G5-TATA was constructed by cloning the 7X tetO operator
sequences, obtained by digestion of the plasmid pHUC13-3 (9) with XhoI and SmaI, upstream of the GAL4
DNA-binding domain of G5E1b (10) digested with XhoI-HindIII (filled). The plasmid T7G5-I was
constructed by substitution of the TATA sequences of the T7G5-TATA with
double-stranded oligonucleotide (upper strand,
5`-CTAGAGCGTTCGTCCTCACTCTCTTCCGCGCGGTTAC-3`) containing the AdMLP Inr
sequence flanked by the XbaI and KpnI sites and
cloned into XbaI-KpnI sites of T7G5-TATA. The plasmid
constructions were analyzed by DNA sequencing.Effector Plasmids
The GAL4-VP16, GAL4-Sp1,
GAL4-Sp3, GAL4-E1A, GAL4-p65rel(A), GAL4-E2F, CMV-Sp3DBD, and
pSVTat22/37 have been previously
described(6, 8, 10, 11, 12) .
To construct the Tet-Sp3 effector plasmid the TetR DNA-binding coding
region (aa 1-206) was polymerase chain reaction-amplified from
pUHD15-1 (9) with 5`-HindIII and
3`-EcoRI adapter primers (5`-CCCGCAAGCTTGCCGCGATTTCA-3`,
5`-CGGGAATTCGGACCCACTTTC-3`). The PCR product was subcloned into the
plasmid pSG424 digested with HindIII and EcoRI to
substitute the Gal4 coding region, resulting in pTetR. The Tet-Sp3
clone was obtained by inserting the Sp3 non-finger region (aa
1-527) derived from the digestion of the pBS A31416 plasmid (2) with ClaI-Acc65.I (both filled) in pTetR EcoRI (filled). The Tat22/37-Sp3 clone was constructed by
inserting the aforementioned Sp3 non-finger region (aa 1-527) in
pSVTat22/37 EcoRI (filled). All plasmids were analyzed by DNA
sequencing to confirm correct construction. Full details of each
construction are available upon request.Transfection and CAT Assay
HeLa cells were grown
in Dulbecco's modified Eagle's medium supplemented with 10%
fetal calf serum. Transfection was performed by calcium phosphate
precipitation using subconfluent cell cultures with different amounts
of reporter and effector plasmids. For normalization of transfection
efficiencies a 
-Gal expression plasmid was included in the
cotransfections (pSV--Gal expression plasmid, Promega). CAT assays
were performed with a different amount of extract to ensure linear
conversion of the chloramphenicol with each extract, and results are
presented as the means ± S.D. of a least four duplicated
independent transfection experiments. The CAT activity was quantified
using the Molecular Dynamics PhosphorImager System
.
Transcriptional Activation by Different Positive
Regulators Is Suppressed by Sp3
It has been suggested that Sp3
is an inhibitory member of the Sp family(4) . Transfection
experiments into mammalian cell lines and Drosophila SL2 cells
lacking endogenous Sp factors have demonstrated that Sp3 failed to
activate Sp1-responsive promoter constructs and enforced expression of
Sp3-repressed Sp1-mediated transcriptional
activation(4, 6, 7) . We now wished to
determine whether Sp3 may interfere with the activating function of
different defined activators. To address this point, we developed an in vivo transcription assay in which various well
characterized transcriptional activation domains were fused to the
yeast GAL4 DNA-binding domain and their activity assayed on a reporter
plasmid. To evaluate the Sp3-mediated repression the Sp3 non-finger
region (aa 1-527) was connected in frame to the COOH terminus of
the prokaryotic TetR encoded by Tn10 from Escherichia
coli(9) . Thus, the Tet-Sp3 chimeric protein was able to
bind to the tet operator (tetO) sequences. Two different reporter
plasmids were constructed and used as templates: the T7G5-TATA and the
T7G5-Inr. The two reporters are isogenic except for the presence of the
core promoter element. The T7G5-TATA reporter contains the CAT gene
under the control of the E1b TATA box with five GAL4 DNA-binding and
seven tetO sequences, whereas the T7G5-Inr bears as core promoter
element the AdMLP Inr element. Relevant features of the effectors and
reporter plasmids are outlined in Fig. 1. First the effect of
the various activator domains present in the GAL4 chimeric proteins was
determined. Accordingly with previous results (13) we found
that both TATA and Inr promoters responded to the GAL4-based
activators, with the exception of the E1a-activating domain, which as
previously reported is strictly dependent upon the presence of the TATA
box(10) . Therefore, the GAL4-E1a fusion protein is unable to
trans-activate the T7G5-Inr reporter whereas the T7G5-TATA was fully
responsive.
Sp3 Repressor Is Functional When Targeted to a
Promoter-proximal RNA Sequence
We have previously shown that the
non-finger region of Sp3 tethered to the HIV-1 promoter by a GAL4
DNA-binding domain repressed transcription in a distance-independent
manner(6) . Repression at a distance supports a mechanism
requiring protein-protein interactions between the distantly bound Sp3
repressor domain and proximal factors looping out intervening
DNA(14, 15) . Alternatively, the Sp3 domain tethered
to DNA may alter (bending) promoter topology, such as described for the
YY1 transcription factor(16) . We reasoned that if repression
is due to a local topological alteration of DNA the Sp3 repressor must
be tethered to the template via fusion to a DNA-binding domain.
TAR in which the TAR element was
destroyed by deleting TAR sequences downstream of position +25.
All CAT assays were determined as described in the text, after
normalization for the internal control -galactosidase activity.
The values are representative of four independent duplicated
experiments; vertical lines indicate the standard
deviations.
TAR in
which the TAR sequences have been deleted was not affected by the
presence of the Tat22/37-Sp3 fusion protein. These data clearly
indicated that the Sp3 repressor domain can efficiently repress
transcription from nascent RNA target and demonstrated that
Sp3-mediated repression does not require a stable interaction with the
promoter DNA.
)
We thank Drs. R. Dalla Favera, M. Gossen, H. Bujard,
G. Pengue, and P. Baeuerle for the gift of plasmids and Rosaria
Terracciano for technical assistance.
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.
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