(Received for publication, October 20, 1995; and in revised form, February 16, 1996)
From the
The gene encoding UDP-GlcNAc:dolichol phosphate N-acetylglucosamine-1-phosphate transferase (GPT), the enzyme
that initiates the pathway for the biosynthesis of asparagine-linked
glycoproteins, is ubiquitously expressed in eukaryotic cells. However,
its expression in the mammary gland is developmentally and hormonally
regulated; transcription of the mouse mammary GPT gene is stimulated by
the lactogenic hormones, insulin, glucocorticoid, and prolactin. The
involvement of cis-acting elements in regulating the expression of the
mouse GPT (mGPT) gene was investigated by transient transfections of
various GPT promoter/luciferase (Luc) constructs into primary mouse
mammary epithelial cells. A series of 5`-deletions of the GPT promoter
identified a distal negative regulatory region (base pairs -1057
to -968) and deletion of this region results in enhanced hormonal
induction (7-fold) with no effect on basal promoter activity.
Electrophoretic mobility shift assays (EMSA) performed with nuclear
extracts from different developmental stages of mouse mammary gland
demonstrated that the binding activity of the nuclear proteins to the
distal negative regulatory region was predominant in virgin stage as
compared with pregnant and lactating stages. EMSA performed with
nuclear extracts from virgin explants showed that the binding activity
was markedly decreased after cultivation with the combination of the
three lactogenic hormones. DNase I footprinting analysis identified two
pentamer direct repeat motifs, AGGAA and GAAAC, within the negative
regulatory region. EMSA competition experiments showed that mutations
within the direct repeats failed to compete for binding of the nuclear
proteins to labeled wild type oligonucleotide. Transcription from the
promoter containing the mutated direct repeats was increased greatly,
consistent with the conclusion that these motifs functions in vivo to repress GPT gene expression. These data suggest the importance
of the negative regulatory region in hormonal control of mGPT gene
expression in mammary gland.
Asparagine-linked glycosylation in eukaryotes is initiated by a
stepwise, dolichol-linked assembly of
GlcMan
GlcNAc
-P-P-dolichol and its
transfer en bloc to the nascent polypeptide in the endoplasmic
reticulum, followed by co- and post-translational modifications of the
tetradecasaccharide in the secretory pathway(1) . The enzyme
UDP-GlcNAc:Dol-P GlcNAc-1-P transferase (GPT) (
)catalyzes
the first and the committed step, i.e. the transfer of
GlcNAc-1-P from UDP-GlcNAc to Dol-P to form GlcNAc-P-P-Dol, in the
assembly of the oligosaccharide precursor. It is, therefore, a
potential target for the overall regulation of oligosaccharide assembly
and protein N-glycosylation.
GPT has been purified to apparent homogeneity from bovine mammary gland (2) ; the cDNAs encoding GPT enzyme have been isolated from cDNA libraries prepared from mRNA of tunicamycin-resistant Chinese hamster ovary cell lines (3, 4) and mouse mammary gland(5) . The GPT gene has been cloned from yeast(6) , Leishmania(7) , and mouse(8) . These studies show that GPT is a multispan hydrophobic protein in the endoplasmic reticulum, and its catalytic site has a cytoplasmic orientation(9, 10) . We previously reported that the mouse GPT gene maps to the chromosome 17(5) , is encoded in nine exons spanning 7.5 kilobases (kb), and its promoter is of the housekeeping type(8) . The GPT gene promoter lacks TATA box, is GC-rich, and has multiple transcription initiation sites spread over a large region. These features are consistent with the ubiquitous occurrence of N-linked glycosylation process, and the presence of GPT mRNA in many tissues examined. However, an earlier report showed that the activities of three key enzymes of the dolichol cycle, namely GPT, GDP:Man-Dol-P mannosyltransferase, and UDP-Glc:Dol-P glucosyltransferase, are developmentally regulated in mouse mammary gland(11) .
The mammary gland offers a unique model for
studying the regulation of protein N-glycosylation. It
synthesizes and secretes only a very limited number of well
characterized N-linked glycoproteins, e.g. -lactalbumin and transferrin. The gland is intensely
modulated by mammogenic and lactogenic hormones as it repeatedly cycles
between dormancy, growth, and differentiation upon pregnancy,
lactation, and post-lactational regression throughout the reproductive
life of the female(13) . A recent study from our laboratory
showed that GPT enzyme activity, immunoreactive GPT protein, and GPT
mRNA are developmentally regulated in mouse mammary gland (12) . All three parameters are low in tissues from virgin and
pregnant animals, increase steadily during lactation, reaching a peak
around mid to late lactation, and decline thereafter in glands of
post-lactating animals. Furthermore, GPT synthesis is stimulated at the
level of RNA in mouse mammary explants and primary mammary epithelial
cell cultures under the synergistic influence of insulin, prolactin,
and hydrocortisone(12) . These studies suggest that the
developmental and hormonal regulation of GPT gene expression is
mediated at the transcriptional and/or post-transcriptional levels.
The aim of our studies is to identify cis-acting elements involved in the developmental and hormonal regulation of GPT gene in the mouse mammary gland. In this report we show that the pentamer direct repeat motifs, AGGAA and GAAAC, within the distal negative regulatory region of the mGPT promoter play an important role in developmental and hormonal regulation of GPT gene expression in mouse mammary gland.
Figure 5:
Competition EMSA at sequence -1050
to -1004 of mGPT promoter. A, DNA sequence of GPT
promoter region between -1050 and -1004 was used as WT
probe in EMSA. Sequences in bold areas I and II are
sequences protected in DNase I footprinting assays. Arrows indicate the direct repeat motifs. B, mutations used in
EMSA. Letters indicate introduced mutations, * indicates
deleted nucleotides, and indicates inserted nucleotides. C, 30-fold excess competitor oligonucleotides were incubated
with 2 µg of protein from nuclear extracts of virgin mouse mammary
gland prior to addition of labeled WT oligonucleotide. Competitors used
are indicated above appropriate lanes.
The sequence of the mGPT promoter revealed the presence of several potential binding sites for transcription factor such as Sp1 and AP2, sequences similar to the half-palindromic CTF/NF1 element, and a 13-bp direct repeat motif(8) . Using a promoterless Luc reporter vector, deletion mutations were targeted systematically to eliminate these potential binding sites to determine whether the sites and/or any other cis-acting elements were involved in developmental and hormonal regulation of GPT gene expression. The Luc constructs containing the deleted promoter fragments were transiently transfected in the PMME cells and the basal and induced activities were determined (Fig. 1). Deletion from bp -1489 to -264 had little effect on the basal level of Luc activity. When the 5` deletion reached to bp -175, the basal level of Luc activity dropped to half, indicating that the sequence between bp -264 and -175 is important for the basal promoter activity. Since the major transcription initiation sites were located in this region(8) , the possibility that removal of these sites caused the reduction of promoter activity could not be ruled out.
Figure 1:
Functional analysis of the mouse GPT
promoter in PMME cells. A, a schematic representation of the
GPT promoter with the position of the putative regulatory elements:
, CAAT box;
, AP-2;
, Sp1;
, CTF/NF1;
,
13-bp direct repeat;
, imperfect GC box;
, core sequence
for CAAT box. The multiple transcription start sites are shown by the bent arrows, with the major site (nucleotide -202) being
indicated by the dark arrow. B, structure and Luc
activity of the GPT/Luc deletion constructs. Names of the various GPT
promoter/Luc constructs used and the deletion start and end points
relative to the adenosine (+1) of the translation initiation codon
ATG are listed to the left. BV, pGL2-Basic luciferase
vector. Twenty µg of the indicated GPT/Luc construct were
co-transfected with 5 µg of pSV-
-galactosidase plasmid into
PMME cells. Twenty hours after transfection, cells were cultured for 5
days in the presence of either I (control) or IHP. The Luc activity was
determined and normalized to
-galactosidase activity measured in
the same sample. The value show the percent Luc activity relative to
IHP-treated B7 construct (100%). Thin lines represent the
S.E.; n = 3-6.
Hormonally induced Luc activity was increased dramatically by the
removal of the sequence between bp -1057 and -968 and
construct B7 (7-fold) was one of the most active constructs
analyzed. The hormone inducibility was greatly reduced when the
sequence between bp -264 and -92 was deleted (Fig. 1). Again, the decrease in induction activity could result
from the removal of either positive regulatory element(s) or the major
transcription initiation sites in this region. These results
demonstrate that a distal fragment from -1057 to -968
behaved as a negative regulatory region and is crucial for induction of
promoter activity by the lactogenic hormones.
The region involved in the negative regulation contained a 13-bp direct repeat sequence, TGACTGGGAGGAA, at bp -1052 to -1039 and bp -1033 to -1020. Since some transcription factors act through binding to direct repeat sequences (such as vitamin D, thyroid hormone, and retinoic acid response elements, AGGTCA)(20) , it was speculated that this direct repeat sequence was involved in binding a trans-acting factor that down-regulates GPT expression. To examine the possible presence of trans-acting factors that interact with the negative regulatory region, EMSA were performed using a 178-bp DNA fragment FP1 that encompasses the negative regulatory region and nuclear extracts prepared from mouse mammary glands of different developmental stages. As shown in Fig. 2A, FP1 formed a major shifted band with the nuclear proteins from virgin mouse mammary gland. The complex was abolished by addition of 50-fold molar excess of either unlabeled FP1 (data not shown) or a much shorter oligonucleotide WT (lane 3). In contrast, when a unrelated oligonucleotide FP2 was used as a nonspecific competitor, it failed to abolish the binding (lane 2). In addition, the amount of the complex increased with increasing protein concentration (lanes 4-6), suggesting that it was a result of specific interactions. The binding activity to the FP1 was highest in virgin stage (Fig. 2B, lane 1) and was lowest in lactating stage (lane 3). The binding activities were intermediate at pregnant stage (lane 2) and returned to a higher level in post-lactating stage (lane 4). Results from EMSA experiment involving mixing of nuclear extracts from the virgin and lactating tissues showed the same binding pattern as virgin nuclear extract alone (data not shown), suggesting that lactating extracts did not contain any factor(s) that inhibit the DNA-protein complex formation. Since the adipocytes comprise the most mass in the virgin tissue, whereas the epithelial cells predominate in the lactating tissue(13) , we examined the possibility that alterations in cell population were responsible for the results relating to the developmental changes. Fat cells were removed from mammary tissues of different developmental stages, and the resulting epithelial enriched cell fractions were used to prepare nuclear extracts for EMSA experiments. The results were similar to those obtained from the intact glands (Fig. 2C). Possible variations in the efficiency of nuclear isolation and extraction were examined by EMSA using consensus oligonucleotide sequence for known transcription factor, mammary gland factor (MGF). MGF activity in mice is strongly increased at the end of pregnancy and is maintained at high levels during lactation(38) . As shown in Fig. 2D, MGF activity as determined by binding to DNA was very low in the virgin and post-lactating tissues and reached its highest level in the lactating tissue. These results suggest that developmental changes in the level of DNA binding activity are not due to the difference in the efficiency of nuclear extraction in our experiments.
Figure 2: Developmental profiles of nuclear factors that specifically bind to sequence -1161 to -983 of mGPT promoter. Double-stranded FP1 fragment was end-labeled by the Klenow reaction. A, increasing amounts (2, 4, and 6 µg) of protein from virgin nuclear extracts were used (lanes 4-6); lane 1, no extract (NE); lanes 2 and 3, 50-fold excess of unlabeled oligonucleotide FP2 (nonspecific, bp -776 to -623) and WT (specific, detailed in the legend to Fig. 5), respectively, were used as competitors. B, 2 µg of protein from nuclear extracts of virgin (V), pregnant (P), lactating (L), and post-lactating (PL) mouse mammary glands were used (lanes 1-4). C, 2 µg of protein from nuclear extracts of adipocyte-depleted fraction isolated from virgin (V), pregnant (P), and lactating (L) mammary glands were used (lanes 1-3) in electrophoretic mobility shift assays. D, end-labeled oligonucleotide for MGF (nucleotides -104 to -84, 5` CGTAGACTTCTTGGAATTGAA3`) and 2 µg of protein from nuclear extracts of virgin (V), pregnant (P), lactating (L), and post-lactating (PL) mouse mammary glands were used (lanes 2-4). Lane 1, no extract (NE).
The effect of lactogenic
hormones on the FP1 binding activity in mammary epithelial cells was
examined in mouse mammary explant culture system. Earlier studies have
demonstrated that the combination of insulin (I), hydrocortisone (H),
and prolactin (P) stimulate GPT activity (11) and GPT gene
expression in cultured mammary gland(12) . The results in Fig. 3show that the hormone treatments, IH, IE (E, epidermal
growth factor), and IP did not cause significant changes in the binding
activity over control (T, lane 2). On the other
hand, the combination of all three lactogenic hormones, IHP (lane
6), greatly reduced the binding activity.
Figure 3:
Combination of lactogenic hormones IHP
reduces the binding between the nuclear factors in virgin mammary
explants and sequence -1161 to -983 of mGPT promoter.
End-labeled FP1 was incubated with 2 µg of protein from nuclear
extracts of virgin mouse mammary gland explants cultured in the
presence of indicated combinations of hormones for 72 h. T, uncultured explants. The designations for
hormones are as follows: I, insulin; E, epidermal
growth factor; H, hydrocortisone; P,
prolactin.
To localize the protein binding sites, DNase I footprinting analysis was performed using nuclear extracts from the virgin tissue and FP1 probe. The nuclear extracts protected two regions (Fig. 4). The sequence of region I is 5`-AGGAAGAAACTTGAC-3`, between bp -1029 and -1044, and the sequence of region II is 5`-AGGAAACCCACAAGC-3`, between bp -1010 and -1025 (Fig. 5A). Their positions overlap with the 13-bp direct repeat sequence. Comparison of these two sequences revealed the presence of two direct repeats AGGAA and GAAAC. The AGGAA direct repeat is separated by 14 bp, and the GAAAC direct repeat is separated by 11 bp.
Figure 4: DNase I footprinting analysis of sequence -1161 to -983 of mGPT promoter. Double-stranded FP1 was labeled on the coding strand and incubated either with increasing amounts (5, 10, and 20 µg) of protein from virgin nuclear extracts or without the extracts. After the binding reaction, the mixture was subjected to DNase I digestion and electrophoresed on 8% denaturing polyacrylamide gel.
As mentioned above, direct repeats of the hexamer AGGTCA can serve as response elements for vitamin D, thyroid hormone, or retinoic acid (20, 21, 22) . The specificity of the response appears to reside in the spacing between the hexamers. To test whether the direct repeats found in mouse GPT gene are responsible for the observed DNA-protein interaction, a 46-bp double-stranded oligonucleotide probe WT (bp -1050 to -1004) containing these sites along with oligonucleotides containing the mutated sites in the direct repeats were used in EMSA competition experiments (Fig. 5B). As shown in Fig. 5C, the WT probe formed a complex with the nuclear extracts from the virgin tissue and the formation of this complex was efficiently competed by excess unlabeled WT oligonucleotide (lanes 2 and 3). Oligonucleotides MM1 and MM2, which had the mutations at the both sites of the direct repeats AGGAA and GAAAC, respectively, failed to compete, suggesting that these sites were critical to binding (Fig. 5C, lanes 4 and 5). Since the right half-sites of these two pentamers were overlapped, it was difficult to differentiate their effect on the binding. Therefore, mutations were introduced only at the half-sites. Both oligonucleotides M1 and M1-2, which were mutated at the left and right half-site of the direct repeat AGGAA, respectively, showed some competition (Fig. 5C, lanes 6 and 7), whereas MML mutated at both left half-sites of AGGAA and GAAAC had weaker effect on the binding (lane 8). In contrast, oligonucleotide M2 with mutations at the left half-site of the direct repeat GAAAC competed the binding substantially (lane 9). These results suggested that both direct repeats are involved in DNA-protein binding and AGGAA plays a more important role. To evaluate the effect of spacing between the direct repeats on the binding, oligonucleotides with deletions and insertions between the direct repeats were used in EMSA experiment (Fig. 5B). SM2 and SM4 with two and four base deletions, and SMI3 with three base insertions between the two footprints, competed the binding completely (Lanes 11-13), indicating that spacing is not important. However, MS4, deleted four bases within the footprint I, failed to compete efficiently (lane 10), whereas MS2-5 deletion with two bases within the footprint I and three bases between the two footprints competed the binding completely (lane 14). These data suggest that sequences within the footprints are important for DNA-protein contact.
The results from the EMSA experiments indicated the ability of the GPT promoter sequences between bp -1050 and -1004 to form a specific complex with binding proteins in the nuclear extracts of virgin tissue. In order to assess the functional importance of these DNA-protein interaction, 12 GPT/Luc expression constructs, containing the mutations used in the competition EMSA experiments, were prepared. These constructs were transfected into PMME cells in parallel with the unmutated GPT/Luc construct WT (Fig. 6). The WT construct produced the same pattern of transcriptional activity as observed in previous experiments (compare Fig. 6to Fig. 1, B4). The MM1 and MM2 constructs had dramatically increased induction activity. M1, M2, and MMl constructs also showed much higher induction activity than the WT, indicating that mutations at half-site of AGGAA pentamer alone could partially relieve the repressive effect of the negative regulatory region. M2 construct showed only half of the induction activity compared to the M1, consistent with the competition EMSA results that AGGAA is more important than GAAAC. All spacing mutant constructs had no effect except for MS4, and MS4 caused an increase in induction activity. These results suggest that the DNA-protein complex between the factor in the virgin extracts and the sequence containing the direct repeats suppressed the expression of the GPT gene.
Figure 6:
Transient expression of GPT/Luc mutated
constructs in PMME cells. Mutated oligonucleotides used in competition
EMSA were used as primers to generate mutated GPT/Luc constructs, which
contain sequence -1161 to -5 of the GPT gene. The
transfection experiments were preformed as described in Fig. 1.
The Luc activity was determined and normalized to -galactosidase
activity measured in the same sample. The values are presented as the
percent Luc activity relative to IHP-treated MM1 construct (100%). Thin lines represent S.E.; n =
3.
The present work is part of a study focused on the identification of cis-acting elements involved in the developmental and hormonal regulation of GPT gene in the mouse mammary gland. GPT has a ubiquitous function in eukaryotic cells, and GPT gene has several characteristics of a housekeeping promoter. Such promoters often lack both TATA and CCAAT boxes, are GC-rich, and contain one or several Sp1 binding sites. The housekeeping genes are expressed in all cells, and their expression is not subject to control by environmental factors such as hormones(23) . The mGPT gene is unusual in that its promoter contains characteristics of housekeeping genes but its activity is modulated during the developmental state of the mammary gland and by hormones.
Removal of consensus sequences for several potential transcription factors within the promoter region of mGPT gene indicated that these sequences, e.g. a CCAAT box(-1487), two Sp1 binding sites (-754 and -25), and an AP-2 binding site(-577), were not important for expression of the GPT gene in transient transfections of PMME cell assays. On the other hand, a region from bp -1057 to -968 was identified to be critical for developmental and hormonal regulation of GPT gene in mouse mammary gland. This region contained a 13-bp direct repeat sequence, which was a potential site for binding a trans-acting factor. Deletion of this region resulted in the enhancement of hormonally inducible promoter activity. EMSA experiments using a DNA fragment targeted to this region revealed that a nuclear protein(s), a trans-acting repressor, could specifically bind to this region. The binding activity changed during the development of the mammary gland such that it was high in virgin and post-lactating glands, intermediate in pregnant gland, and low in lactating gland. The binding activity was also hormonally regulated since mammary explants cultured with lactogenic hormones, IHP, showed greatly reduced DNA-protein complex formation. Our data are consistent with the results from previous studies that GPT enzyme activity is developmentally and hormonally regulated in the mammary gland. It is high in the lactating gland(11, 12) , as well as in the mammary explants cultured in the presence of IHP.
DNase I footprinting analysis identified that the presence of two pentamer direct repeats, AGGAA and GAAAC, within the protected sequences. EMSA competition experiments, using a 46-bp oligonucleotide containing this direct repeat motifs and oligonucleotides with mutations at the direct repeats, confirmed that the direct repeat AGGAA is largely responsible for the observed DNA-protein interaction and the direct repeat GAAAC is also involved. A dramatic increase in hormonally inducible transcription was observed after introducing the direct repeat mutations into Luc reporter constructs. This is consistent with the view that failure of the repressor, identified by in vitro EMSA, to bind in vivo resulted in the enhancement of transcription activity. The finding that two protected regions separated by 4-bp were identified by footprint analysis and that only one major band was detected by EMSA suggested that the repressor probably binds to this negative regulatory element as a dimer. The nature of this DNA repressor remains to be studied.
Negative regulatory mechanisms appear to be quite common in
eukaryotic cells and negative elements are often associated with
cell-specific gene expression(32) . These promoters contain
basal positive elements that bind ubiquitous transcription factors,
which function in different cell types. Therefore, negative control
mechanisms are required for their repression in these different cell
types(33) . On the other hand, ubiquitous expression of a
promoter can be achieved by activation of different elements by nuclear
proteins in different cell types(31) . Although GPT gene is
expressed in all cells, its expression in the mammary gland is
regulated in a tissue-specific manner by lactogenic hormones during the
development. Our data suggest that high levels of the repressor in the
virgin tissue is responsible for the repression of promoter activity.
Low levels of the repressor and the presence of possible mammary
gland-specific positive factors in the lactating tissue confer the
promoter full responsiveness to the hormonal induction. Therefore,
developmental and hormonal regulation of GPT gene in mammary gland most
likely involves a combination of both negative and positive elements
that are modulated by changing in hormone milieu. Such promoter
structure is correlated with the regulation of the GPT gene and the
functions of the GPT enzyme, which participates not only in the
constitutive pathway of N-linked glycosylation but also in the
tissue-specific, hormonally regulated pathway to synthesize large
amounts of N-linked glycoproteins such as -lactalbumin in
the lactating mammary gland. The
1,4-galactosyltransferase
(
4-GT) gene is another example of a widely expressed gene whose
expression involves negative regulatory
element(29, 30) . Both GPT and
4-GT have
housekeeping function to participate in biosynthesis of N-linked glycoproteins. It was proposed that differential
initiation from the two promoters of the
4-GT gene, a distal
housekeeping promoter for the 4.1-kb transcript and a proximal mammary
cell-specific promoter for the 3.9-kb transcript, is responsible for
regulation of
4-GT enzyme levels. Binding of repressor(s) to the
negative regulatory element could account for the repression of
transcription from the 3.9-kb start site in all somatic tissues except
the lactating mammary gland. Tissue-specific transcriptional regulation
of genes that encode other enzymes involved in N-linked
glycoprotein biosynthesis, e.g.
1,3-galactosyltransferase(34) ,
2,6-sialyltransferase(35, 36) , and N-acetylglucosaminyltransferase I(37) , also have been
reported.
In GC-rich, Sp1 binding site-containing promoters, the
ubiquitous promoter activity is due to the ubiquitous expression of the
Sp1 protein(24) . Binding of Sp1 has been shown to be critical
for transcription initiation(25, 26) . In the case of
mGPT promoter, removal of the two perfect GC boxes did not affect the
basal and inducible promoter activities, suggesting that these elements
are inactive. However, the involvement of Sp1 could not be ruled out,
because there are three imperfect copies of GC boxes with a 1-bp
mismatch between bp -264 and -92. In addition, the
transfection experiments indicated that this region may contain
positive regulatory elements for both basal and inducible promoter
activity. Furthermore, two copies of CCAAT box core sequence, CAAT, and
two sequences similar to the half-palindromic CTF/NF1 element are also
present in this region. A unique half-site CTF/NF1-binding protein has
been identified in lactating mammary glands (27, 28) and implicated in the regulation of murine
4-GT gene (29, 30) and several milk protein
genes(27, 28) . CCAAT box-binding proteins have been
shown to be the major factors involved in basal promoter activity of
the cytosolic aspartate aminotransferase gene. This gene, like mGPT
gene, is also ubiquitously expressed but regulated by hormones in a
tissue-specific manner(31) . The roles of these elements in
regulation of mGPT gene are currently under investigation.
The
nuclear protein(s) that binds to the negative regulatory element is
present in all tissues examined, although the position of the shifted
band differed a little among different tissues (data not shown).
Relative higher levels were found in thymus, spleen, ovary, and virgin
mammary gland. Among these tissues, thymus, ovary, and virgin mammary
gland are targets of prolactin action. Although the mechanism of
prolactin signal transduction is not completely understood, MGF has
been shown to be a Stat (signal transducer and activator of
transcription) protein and confer the prolactin response in
-casein gene(38, 39, 40) . MGF binding
site was found in the promoters of the genes of the most abundantly
expressed milk proteins, namely caseins(41) . However, MGF
binding site is not present in the promoter of mGPT gene. It is
possible that lactogenic hormones exert their influence partially
through reduction of the binding of the repressor to the negative
regulatory element in the mGPT promoter. Further purification and
characterization of the repressor will be necessary to understand its
mode of action and to determine whether it plays a more general role in
transcriptional regulation of TATA-less housekeeping genes.