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Molecular and Cellular Biology, April 2001, p. 2736-2742, Vol. 21, No. 8
0270-7306/01/$04.00+0 DOI: 10.1128/MCB.21.8.2736-2742.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Yeast NC2 Associates with the RNA Polymerase II Preinitiation
Complex and Selectively Affects Transcription In Vivo
Joseph V.
Geisberg,1
Frank C.
Holstege,2
Richard A.
Young,2 and
Kevin
Struhl1,*
Department of Biological Chemistry and
Molecular Pharmacology, Harvard Medical School, Boston,
Massachusetts 02115,1 and Whitehead
Institute for Biomedical Research and Department of Biology,
Massachusetts Institute of Technology, Cambridge, Massachusetts
021422
Received 4 December 2000/Returned for modification 10 January
2001/Accepted 22 January 2001
 |
ABSTRACT |
NC2 (Dr1-Drap1 or Bur6-Ydr1) has been characterized in vitro as a
general negative regulator of RNA polymerase II (Pol II) transcription
that interacts with TATA-binding protein (TBP) and inhibits its
function. Here, we show that NC2 associates with promoters in vivo in a
manner that correlates with transcriptional activity and with occupancy
by basal transcription factors. NC2 rapidly associates with promoters
in response to transcriptional activation, and it remains associated
under conditions in which transcription is blocked after assembly of
the Pol II preinitiation complex. NC2 positively and negatively affects
approximately 17% of Saccharomyces cerevisiae
genes in a pattern that resembles the response to general environmental
stress. Relative to TBP, NC2 occupancy is high at promoters where NC2
is positively required for normal levels of transcription. Thus, NC2 is
associated with the Pol II preinitiation complex, and it can play a
direct and positive role at certain promoters in vivo.
 |
INTRODUCTION |
TATA-binding protein (TBP) nucleates
the assembly of the RNA polymerase II (Pol II) transcription machinery
by specifically recognizing TATA promoter elements and directly
interacting with general transcription factors TFIIA and TFIIB
(12, 34, 36, 40). TBP is a component of distinct
multiprotein complexes that affect Pol II transcription in vitro and in
vivo (26, 31). One such TBP complex, TFIID, contains
approximately 14 associated factors (TAFs) that contact initiator or
downstream promoter elements and that may serve as targets for
transcriptional activator proteins (2, 3, 35, 39, 43). It
is generally believed that TFIID is the predominant form of TBP that
mediates Pol II transcription, although an alternative form(s) of
transcriptionally active TBP exists in Saccharomyces
cerevisiae cells (22, 29). TBP also associates
with Mot1 (1, 7), the multiprotein Not-Ccr4 complex (9, 25, 30), and the NC2 (Dr1-Drap or Bur6-Ydr1)
heterodimer (13, 17, 32, 33), and it has been suggested
that these proteins function as general negative regulators that
inhibit some aspect of TBP function (26).
NC2 (Dr1-Drap) was originally identified in human cells as a
biochemical activity that inhibits basal TBP-dependent transcription in
vitro (17, 32). NC2 is a heterodimer between two histone fold proteins (13, 33), and a homologous complex
(Ydr1-Bur6) is required for growth in yeast cells (10, 19,
37). NC2 directly interacts with TBP and DNA, but the
TBP-NC2-TATA complex is transcriptionally inactive in vitro, because it
is unable to bind TFIIA or TFIIB and hence the remainder of the basal
Pol II machinery (13, 33). In this regard, TBP mutations
that inhibit interaction with NC2 are located near surfaces that
mediate TFIIA or TFIIB binding (4, 20). NC2 also interacts
in vitro with the repression domain of the AREB6 repressor
(16) and with the hyperphosphorylated form of Pol II
(5).
After this paper was initially submitted, it was shown that NC2 can
function in vitro as a positive or negative effector of transcription
in a manner that depends on the structure of the core promoter
(45). Specifically, in certain kinds of cell extracts, NC2
can stimulate transcription in vitro from Drosophila
promoters containing downstream promoter elements (DPEs), whereas it
represses transcription from TATA-containing promoters
(45). DPEs have a conserved DNA sequence motif that is
located a precise distance from the TATA element and mRNA initiation
site, and they interact with the TAF60 component of the TFIID complex
(2, 24). DPEs appear to be as widely utilized as TATA
elements in Drosophila core promoters (24),
although they have not been described for yeast promoters. These recent
experiments do not address whether the positive role of NC2 reflects a
productive association with the preinitiation complex, and in this
regard, NC2 blocks the association of TFIIA and TFIIB with promoters in
vitro (13, 33). In addition, the positive role of NC2 in
these experiments may be due to inhibition of another inhibitory factor
in the cell extracts employed. Finally, the physiological significance
of these biochemical observations remains to be established.
Several lines of genetic evidence have suggested that NC2 functions as
a general negative regulator in yeast cells. First, bur6
mutations were identified by their ability to increase transcription from enhancerless promoters, suggesting that NC2 inhibits basal transcription in vivo (37). Second, overproduction of NC2
is toxic, and this toxicity can be reversed by overproduction of TBP
(19). Third, the essential function(s) of NC2 can be
overcome by a mutation in TFIIA (46) or the Sin4 component
of Pol II holoenzyme (18, 27). Fourth, reduced NC2
function permits cell growth and Pol II transcription in cells with
functionally compromised Srb4 (10, 25), a component of Pol
II holoenzyme that is universally required for Pol II transcription
(15, 41). This functional antagonism between NC2 and Pol
II holoenzyme suggests that NC2 is a global negative regulator,
although this global effect is observed under a nonphysiological
condition where Pol II holoenzyme is functionally compromised. There
are a few examples of genes whose transcription decreases upon loss of
NC2 function, suggesting that NC2 might play a positive role in
transcription in vivo (27, 37). However, there is no
evidence addressing whether these positive effects of NC2 on
transcription are direct or indirect.
To investigate the mechanism of transcriptional regulation by NC2 in
vivo, we directly measure NC2 association with yeast promoters by
chromatin immunoprecipitation using an epitope-tagged derivative of Bur6. In addition, we analyze the transcriptional profile
of a bur6 mutant strain on a genomic scale using
microarrays. Our results indicate that, in contrast to the conventional
view, NC2 associates with the Pol II preinitiation complex in vivo. Further, NC2 appears to act directly to increase transcription of
certain genes. Thus, NC2 is not simply a general negative regulator that blocks preinitiation complex formation, but rather it selectively affects transcription both positively and negatively.
 |
MATERIALS AND METHODS |
Chromatin immunoprecipitations were generally performed in yeast
strain FT4 (42) that either did or did not express an
epitope-tagged version of Bur6 containing three copies of the HA1
epitope at its amino terminus. The levels of untagged and tagged Bur6
were comparable, as determined by Western blotting using a Bur6
antibody. For the experiment in Fig. 4, isogenic KIN28 and
kin28-ts16 strains were used as described previously
(8, 23). Cells were grown at 30°C in Casamino Acid
medium lacking uracil supplemented with 2% glucose to an optical
density at 600 nm of 0.6. Chromatin immunoprecipitations used
monoclonal antibodies to the hemagglutinin (HA) epitope (F7 from Santa
Cruz) or polyclonal antibodies to TBP or TFIIB on identical samples.
Quantitative PCR analyses were performed as described previously
(22, 23), except for the experiment in Fig. 2, which was
performed in real time using an Applied Biosystems 7700 sequence
detector. The NC2/TBP ratios were calculated by dividing background-subtracted NC2 binding by background-subtracted TBP binding.
The average of the occupancy ratios for promoters analyzed in Fig. 1
was arbitrarily defined as 1.0. Individual values represent the
averages from at least three independent experiments and have an error
of approximately ±25%. Therefore, promoters showing values of 2.0 and
greater (see Fig. 6) contain relatively high NC2 levels that are
clearly beyond experimental error. Detailed information on experimental
procedures, genetic reagents, high-density array technology, and
data analysis can be found on the World Wide Web at
http://www.wi.mit.edu/young/expression/nc2.
The bur6 temperature-sensitive strain was generated and
kindly provided by Danny Reinberg.
 |
RESULTS |
NC2 specifically associates with Pol II promoters in a manner that
strongly correlates with transcriptional activity and occupancy by TBP
and TFIIB. In previous work, we and others demonstrated that the level
of transcriptional activity in yeast cells is strongly correlated with
the level of TBP association at promoters (23, 28).
Moreover, the relative associations of TBP, TFIIB, and TFIIA are very
tightly correlated with each other; i.e., the TBP/TFIIA and TBP/TFIIB
occupancy ratios are constant at essentially all promoters
(22). In contrast, association of the TAFs in the TFIID
complex is not strictly correlated with TBP occupancy, and the TAF/TBP
occupancy ratio can vary over a 5- to 10-fold range depending on the
promoter (22, 29). Given the biochemical properties of NC2
and the genetic evidence that NC2 functions as a global repressor, we
expected that NC2 occupancy would be inversely correlated with
transcriptional activity and with TFIIA and TFIIB association.
In the initial experiment, we analyzed NC2 occupancy at several Pol II
promoters, whose transcriptional activities span a wide range (Fig.
1A). In contrast to our expectation, NC2
associates with promoters in a manner that is strongly correlated
with TBP and TFIIB occupancy and hence transcriptional activity.
Specifically, the NC2/TBP and NC2/TFIIB occupancy ratios at these
promoters are essentially indistinguishable, indicating that NC2
behaves similarly to TFIIA and TFIIB but differently from TAFs. The
NC2/TBP ratio is not significantly affected by whether the promoter
contains high or low levels of TAFs (and hence TFIID) (Fig. 1B),
suggesting that promoter occupancy by TAFs and NC2 is not mutually
exclusive. NC2 does not associate with a tRNA promoter, which is
transcribed by Pol III, even though this (and most other) tRNA promoter
contains canonical TATA elements that specifically bind TBP (14,
44). In addition, NC2 does not associate with the rRNA promoter,
which is transcribed by Pol I and shows high TBP occupancy. Finally, mapping experiments on the RPS11B locus indicate that TBP
and NC2 colocalize over the promoter (Fig.
2), indicating that NC2 is not associated
with the elongating Pol II complex. Thus, NC2 specifically associates
with the functional Pol II machinery at promoters.

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FIG. 1.
Association of NC2 with promoters strongly correlates
with association of TBP and TFIIB. (A) Bur6, TBP, and TFIIB occupancy
at selected promoters. (B) Bur6 and TBP occupancy at TAF-dependent and
TAF-independent promoters. Cross-linked chromatin preparations from
HA3-Bur6 and untagged Bur6 were immunoprecipitated with
antibodies to the HA epitope, TBP, or TFIIB. Promoter-specific PCR
products were generated from input chromatin or immunoprecipitated DNA,
and the NC2/TBP or TFIIB/TBP occupancy ratios are indicated.
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FIG. 2.
Bur6 association is localized over the promoter. Bur6
and TBP occupancy at the indicated regions of the RPS11B
locus (drawing to scale). PCR analysis was performed in real time.
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|
NC2 rapidly associates with promoters in response to
transcriptional induction.
Although the above analysis was
performed under steady-state growth conditions, the results suggest
that NC2 is recruited to promoters by transcriptional activator
proteins. We addressed this issue directly by analyzing NC2 occupancy
at heat shock promoters under conditions where transcription was
strongly induced upon a rapid heat shock (Fig.
3). Heat shock causes a rapid increase of
NC2 occupancy at all heat shock promoters tested, and the NC2/TBP occupancy ratios are comparable to those of the non-heat-shock promoters. Thus, in accord with their abilities to activate
transcription, the Hsf1, Msn2, and Msn4 activators cause a rapid
association of NC2 with target promoters.

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FIG. 3.
Bur6 is rapidly recruited to promoters by
transcriptional activators. Bur6 and TBP occupancy at heat
shock-inducible and uninducible promoters is shown. Cells were grown at
24°C and were heat shocked for 15 min at 39°C. SSA4
and HSP82 are activated by heat shock factor (Hsf1),
HSP12 and CTT1 are activated by the Msn2
and Msn4 activators, and HSP104 is activated by both
classes of activator. Heat shock inhibits the RPL9A
promoter but does not affect the other promoters tested.
|
|
NC2 association correlates with formation of a Pol II preinitiation
complex, not transcriptional activity per se.
Phosphorylation of
the C-terminal tail of Pol II by the Kin28 subunit of TFIIH is required
for transcription at a step after formation of the preinitiation
complex such as promoter clearance or elongation. Mutational
inactivation of Kin28 results in rapid inhibition of transcription
(8), but the Pol II machinery remains stably associated
with the promoter in vivo (21, 23). Loss of Kin28 function
does not affect NC2 occupancy (Fig. 4),
indicating that NC2 associates with promoters even under conditions in
which the Pol II machinery is assembled at promoters in an
elongation-incompetent state. Thus, NC2 association with promoters
correlates with formation of a Pol II preinitiation complex, not
transcriptional activity per se.

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FIG. 4.
Bur6 remains bound at promoters under conditions in
which transcription is blocked after assembly of the Pol II machinery.
Bur6 and TBP association following thermal inactivation of Kin28, the
TFIIH subunit that phosphorylates the Pol II C-terminal tail, is shown.
Cells were grown at 24°C and were shifted to 37°C for 75 min to
inactivate Kin28.
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NC2 positively and negatively affects transcription in a manner
that overlaps the response to general environmental stress.
To
address the requirement for NC2 at individual promoters, we compared
the transcriptional profiles of wild-type and bur6 mutant
strains on a genome-wide level using microarrays (Fig. 5). Thermal inactivation of Bur6 resulted
in twofold or greater transcriptional effects on approximately 852 genes, which represent 17% of all yeast genes. Of these, 415 genes
show decreased transcription, whereas 437 genes display increased
transcription. Thus, NC2 can positively or negatively affect
transcription of selected genes.

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FIG. 5.
Relationship between Bur6 function and the response to
environmental stress. Venn diagram indicating the number of genes that
are increased or decreased at least twofold in cells in which Bur6 is
thermally inactivated (top circles) or in cells subjected to a wide
variety of environmental stresses (bottom circles). The number of genes
affected by both conditions is indicated at the intersections.
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|
When yeast cells are subjected to a broad range of environmental stress
conditions, there is a common response involving approximately
500 genes (
6,
11). Depending on the specific gene, these
various environmental stress conditions result in positive or
negative
regulation of transcription. Strikingly, the set of NC2-affected
genes
significantly overlaps the set of genes that are coregulated
in
response to a broad range of environmental stress conditions.
Approximately 40% of the 500 genes that are positively or negatively
affected by environmental stress are affected in the same manner
by
loss of NC2 function. This relationship between NC2 regulation
and
environmental stress is specific and not due to thermal inactivation
per se, because the NC2 pattern of expression has never been observed
in comparable analyses of numerous temperature-sensitive mutants
in
other components of the Pol II machinery (
15). One model
to explain this relationship is that loss of NC2 affects the
transcription
of a gene(s) that results in the generation of a stress
signal.
Alternatively, environmental stress could result in the
transient
inactivation of
NC2.
Increased NC2 association at promoters positively affected by
NC2.
Formally, the transcriptional profile of the bur6
mutant strain indicates that NC2 behaves selectively in vivo as both a
positive and negative factor, although it does not establish whether
NC2 acts directly at the affected promoters. To address this issue, we
examined NC2 occupancy at promoters at which NC2 appears to act
positively or negatively (Fig. 6). For
all five promoters in which NC2 appears to act positively (i.e., gene
expression is reduced upon loss of NC2 function), the NC2/TBP occupancy
ratio is 2.5- to 5-fold higher than observed on NC2-independent
promoters. Four of these NC2-stimulated promoters (the exception being
CDC31) are also stimulated in response to general stress
(6, 11). In contrast, the six promoters whose activity
appears to be negatively regulated by NC2 show NC2/TBP occupancy ratios
comparable to those of NC2-independent promoters. Thus, high NC2 levels
are specifically observed on promoters that require NC2 for normal
levels of expression, indicating that NC2 can perform a direct and
positive role in transcription.

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FIG. 6.
Increased Bur6 occupancy relative to TBP at promoters
positively regulated by Bur6. Analysis of genes that are positively or
negatively regulated by NC2 as defined by the microarray analysis shown
in Fig. 4. (A) Absolute promoter binding (in arbitrary units) by Bur6
(left axis) and TBP (right axis) expressed over background. (B) Plot of
background-subtracted Bur6/TBP occupancy ratios. The average occupancy
ratios are 1.0 for NC-independent promoters, 1.1 for NC-inhibited
promoters, and 3.9 for NC2-stimulated promoters.
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|
 |
DISCUSSION |
NC2 associates with the Pol II preinitiation complex in vivo.
Several lines of evidence indicate that NC2 associates with the Pol II
preinitiation complex in yeast cells. First, the Pol II preinitiation
complex is defined in vivo by the constant TBP/TFIIA/TFIIB occupancy
ratio (22) and its very strong correlation with Pol II
occupancy and transcriptional activity (23, 28). In terms of promoter occupancy, NC2 behaves like a general Pol II transcription factor, indicating that it associates with the preinitiation complex. Second, NC2 association is not simply due to its ability to interact with TBP and form TBP-NC2-TATA complexes, because NC2 associates specifically with active Pol II promoters. High TBP occupancy (Pol I
and Pol III promoters) or canonical TATA elements (most Pol III
promoters and many inactive Pol II promoters) are clearly insufficient
for NC2 association in vivo. Third, the hypothesis that the observed
NC2 occupancy represents an association with TBP (or TFIID) alone or
with a partially assembled preinitiation complex (e.g., lacking TFIIB
or Pol II holoenzyme) is inconsistent with the strong correlation of
NC2 occupancy with general factors. Furthermore, loss of TFIIB or the
Srb4 component of Pol II holoenzyme significantly reduces TBP
occupancy, indicating that partial Pol II preinitiation complexes are
unstable in vivo (23, 28).
NC2 functions directly to increase transcription of certain genes
in vivo.
In virtually all microarray (or more limited) experiments
involving yeast strains with mutations in specific transcriptional regulatory proteins, some genes show increased transcription whereas other genes show decreased transcription. By themselves, however, such
experiments do not allow the determination of whether the observed
positive or negative effects on gene expression are due to the direct
action of the transcriptional regulatory protein at the affected
promoter. The possibility of indirect effects is particularly relevant
for proteins that do not exhibit sequence-specific binding to DNA, such
as general factors, TBP-associated proteins, or components of
chromatin-modifying activities. Hence, it is essential to develop
independent criteria for distinguishing direct from indirect effects on transcription.
Here, we utilize relative promoter association in vivo as such an
independent criterion. Specifically, we show that the NC2/TBP
occupancy
ratios at all five NC2-stimulated promoters tested are
significantly
higher (average, 3.9-fold; range, 2.5- to 5-fold)
than the ratios
observed for NC2-independent or NC2-inhibited
promoters. This
observation provides strong evidence that NC2
performs a direct
transcriptional role at the NC2-stimulated promoters
tested and
presumably at most other NC2-stimulated promoters.
Indeed, it is very
difficult to formulate a plausible hypothesis
in which the positive
effects of NC2 are indirect, given that
analysis of more than 25 genes
reveals a strict relationship between
increased NC2/TBP occupancy
ratios and positive NC2 effects on
transcription. Our results do not
distinguish whether NC2 is directly
or indirectly responsible for the
NC2-dependent repression of
selected
genes.
It is important to note that NC2-TBP occupancy ratios are arbitrarily
defined in absolute terms and hence do not provide any
information
about the stoichiometry of NC2 and TBP molecules on
promoters. Although
we presume that a Pol II preinitiation complex
contains one molecule
each of TBP, TFIIB, and TFIIA, we have no
experimental information on
how many molecules of NC2 associate
with a preinitiation complex in
vivo. However, we suspect that
that the high NC2/TBP occupancy ratios
observed at NC2-stimulated
promoters do not reflect multiple NC2
molecules associated with
an individual preinitiation complex but
rather reflect increased
association of NC2 with preinitiation
complexes assembled at these
promoters. For this reason, we believe
that NC2 associates with,
but is not a stoichiometric component of, the
preinitiation complex
at the vast majority of promoters (i.e., those
with NC2/TBP occupancy
ratios of 1.0).
Molecular implications.
Our conclusion that NC2 associates
with functional Pol II preinitiation complexes and can perform a direct
and positive role in transcription is in apparent conflict with the
ability of NC2 to inhibit TBP-TFIIB-TATA and TBP-TFIIA-TATA complex
formation and basal transcription in vitro. However, these biochemical
experiments were performed with purified proteins at nonphysiological
concentrations in the absence of Pol II holoenzyme. We suspect that NC2
interactions (direct or indirect) with Pol II holoenzyme alleviate or
override the inhibitory effects observed with purified general factors, perhaps by competitive binding or conformational alteration of the
relevant protein surfaces. In support of this idea, NC2 interacts genetically with the Srb4 (10) and Sin4 (18,
27) components of yeast Pol II holoenzyme. NC2 also interacts in
vitro with the hyperphosphorylated form of Pol II (5),
although NC2 remains associated with promoters in the absence of Kin28
function (Fig. 4), conditions that block phosphorylation of the Pol II
C-terminal domain (21). Thus, under physiological
conditions, our results are inconsistent with the model that NC2
globally represses Pol II transcription by inhibiting TBP function and
assembly of the preinitiation complex, although we cannot exclude the
possibility that this model operates at certain promoters.
Though unexpected, the ability of yeast NC2 to selectively and directly
increase transcription in vivo is in broad accord
with the concurrent
and unexpected observation that NC2 is selectively
required for
activity of promoters containing DPEs in vitro (
45).
Furthermore, our observation that NC2 and TAFs (and hence TFIID)
can
cooccupy promoters in vivo is consistent with the requirement
for TFIID
(i.e., not TBP) to mediate NC2-dependent activation
of TATA-less
promoters in vitro (
45). However, TAFs are not
required
for NC2 to associate with promoters in vivo, and it is
possible that a
TBP-NC2 complex represents a non-TFIID form of
transcriptionally active
TBP inferred from previous studies (
22,
29).
Finally, the results from the microarray analysis that
NC2 can have
both positive and negative effects on transcription
in yeast cells are
in broad accord with recent results obtained
with
Drosophila
promoters in vitro (
45).
At present, we do not know whether the selective positive effects of
NC2 in yeast cells directly correspond to DPE-dependent
transcription
in vitro. DPEs have yet to be described in yeast
promoters, and it is
unclear whether this reflects the true absence
of DPEs or complications
due to the atypical structure of yeast
core promoters. In
Drosophila melanogaster and most
eukaryotes,
DPEs are located a precise distance downstream of both the
TATA
and initiator elements (
24). In yeast, the distance
between
TATA and initiator elements is considerably larger and highly
variable and promoter regions are AT rich (
38), thereby
making
it difficult to define a TATA-less promoter and to know where
a
yeast DPE should be located. Nevertheless, it is interesting
that NC2
positively affects
his3 transcription that depends on
a weak
TATA element but not on a canonical TATA element (
27).
Although yeast NC2 associates with promoters in a manner analogous to
general transcription factors, it selectively stimulates
or inhibits
transcription of particular genes. This property is
broadly consistent
with the observations in vitro that NC2 acts
during assembly of the
preinitiation complex and functions positively
or negatively depending
on the structure of the core promoter
(
45). This
functional dichotomy and the relatively high NC2
occupancy at
NC2-stimulated promoters might reflect preferential
NC2 interactions
with certain DNA sequences or NC2-dependent conformational
changes of
TBP and/or TFIID that alter promoter recognition. Alternatively,
as
promoter specificity is affected by multiple forms of transcriptionally
active TBP (
22,
29) and perhaps by multiple forms of Pol
II
holoenzyme, NC2 might stimulate or inhibit transcription depending
on which isoform of the Pol II machinery is present at a particular
promoter.
 |
ACKNOWLEDGMENTS |
We are particularly grateful to Danny Reinberg for providing the
bur6 temperature-sensitive strain and for fruitful
discussions on NC2 function. We also thank Jim Kadonaga for
communicating unpublished information about positive and negative
functions of NC2 in vitro and Laurent Kuras and Mario Mencia for advice and commentary throughout the course of the work.
This work was supported by research grants from the National Institutes
of Health to K.S. (GM 30186 and GM 53720) and R.A.Y. (GM 34365).
 |
FOOTNOTES |
*
Corresponding author. Mailing address: Dept. of
Biological Chemistry and Molecular Pharmacology, Harvard Medical
School, Boston, MA 02115. Phone: (617) 432-2104. Fax: (617) 432-2529. E-mail: kevin{at}hms.harvard.edu.
 |
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Molecular and Cellular Biology, April 2001, p. 2736-2742, Vol. 21, No. 8
0270-7306/01/$04.00+0 DOI: 10.1128/MCB.21.8.2736-2742.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
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