From the Department of Biochemistry, School of Medicine and Biomedical Sciences, State University of New York, Buffalo, New York 14214
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ABSTRACT |
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The Mac1 protein in Saccharomyces
cerevisiae is required for the expression CTR1 and
FRE1, which, respectively, encode the copper permease and
metal reductase that participate in copper uptake. Mac1p binds to a
core GCTC sequence present as a repeated unit in the promoters of both
genes. We show here that Mac1p DNA binding required an intact
N-terminal protein domain that includes a likely zinc finger motif.
This binding was enhanced by the presence of a TATTT sequence
immediately 5' to the core GCTC, in contrast to a TTTTT one. This
increased binding was demonstrated clearly in vitro in
electrophoretic mobility shift assays that showed Mac1p·DNA complex
formation to a single TATTTGCTC element but not to a TTTTTGCTC
one. Furthermore, the fraction of Mac1p in a ternary
(Mac1p)2·DNA complex in comparison to a binary
Mac1p·DNA complex increased when the DNA included two TATTTGCTC
elements. A similar increase in ternary complex formation was
demonstrated upon homologous mutation of the FRE1
Mac1p-dependent promoter element. The in vivo
importance of this ternary complex formation at the CTR1
promoter was indicated by the stronger trans-activity of this promoter
mutated to contain two TATTT elements and the attenuated activity of a
mutant promoter containing two TTTTT elements that in vitro
supported only a weak ternary complex signal in the shift assay. The
stronger binding to TATTT appeared due to a more favorable
protein contact with adenine in comparison to thymine at this
position. An in vivo two-hybrid analysis demonstrated a
Mac1p-Mac1p protein-protein interaction. This Mac1p-Mac1p interaction may promote (Mac1p)2·DNA ternary complex formation at
Mac1p-responsive upstream activating sequences.
The Mac1 protein is a 46-kDa polypeptide that is essential for the
copper-regulated expression of high affinity copper uptake activity in
the budding yeast Saccharomyces cerevisiae
(1-7).1 Several features of
the regulatory activity of Mac1p have been described. For example,
in vitro EMSA2 has
demonstrated that Mac1p binds to a sequence in upstream activating regions of two genes whose expression is known to be
Mac1p-dependent, CTR1 and FRE1 (4).
These two genes encode a high affinity copper permease (9) and metal
reductase (10), respectively. The latter activity is required for both
copper and iron uptake in yeast, that is reduction of medium Cu(II) and
Fe(III) to the lower valent species is an essential first step of the
accumulation of both metal ions (2, 11). The sequence element
associated with Mac1p binding in these two loci is shown in Scheme I. Inspection of these two fragments in CTR1 (9) and
FRE1 (10) and in other activating sequences thought to be
associated with Mac1p regulation (3, 7, 12) indicates that the core
binding site is given by GCTC (shown in boldface). For example, Mac1p-
and copper-dependent protection of this region in one of
these other loci, CTR3, which encodes a second copper
permease not expressed in all yeast strains, has been demonstrated by
in vivo DNA footprint analysis (7).
INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
REFERENCES
In addition to these studies on Mac1p DNA binding activity, use of
Mac1p fusions to heterologous DNA binding domains (DBD), e.g. that from Gal4 (5) and from the LexA protein (6), has provided evidence that Mac1p also has an inherent transactivation activity that is copper-dependent. The copper dependence of
DNA binding and transactivation activity is negative, that is both activities are expressed in copper-deficient cells but are suppressed in copper-replete ones. The down-regulation of Mac1p activity occurs
between 1 nM and 1 µM; at [copper] medium
1 µM little expression from the CTR1
promoter is observed (5, 9).
As shown in Scheme I, the core GCTC-binding site in the CTR1 promoter is found within a nearly perfect palindrome; this element in the FRE1 locus is a direct repeat (sequences underlined in Scheme I). CTR3 also contains an inverted repeat which, however, is separated by 44 base pairs. That both GCTC sites are essential for transactivation by Mac1p has been indicated by previous studies that demonstrated loss of such activity in reporter promoter constructs derived from the FRE1 (10), CTR1 (4), and CTR3 (7) loci which contained only a single GCTC-containing element. The presence of this repeated motif suggested to us the possibility of Mac1p binding to both sites simultaneously if not cooperatively.
Thus, the objective of the work described herein was to delineate
in vitro the gross structural features of the Mac1p·DNA complex that had been demonstrated by EMSA (4) and suggested by
in vivo DNA footprint analysis (7) and to correlate these features with promoter activity in vivo. We first tested the
hypothesis that the N-terminal domain of Mac1p, which contains a CCHC
zinc finger motif (1, 13) and which is homologous to the N-terminal regions of two other yeast trans-factors, Ace1p (14-16) and Amt1p (17-20), was essential to the binding of Mac1p to DNA. We next evaluated the possible formation of ternary complexes, that is (Mac1p)2·DNA species, and determined what sequence
characteristics in the DNA promoted such complex formation. We show a
correlation between the in vivo activity of a promoter
sequence and its ability to support formation of a
(Mac1p)2·DNA ternary complex in vitro. Finally, we present data from a two-hybrid analysis that demonstrate that in this assay, at the least, a Mac1p-Mac1p interaction can be
demonstrated. The data indicate that Mac1p can and does bind at both
sites in the CTR1 promoter simultaneously and that formation of the resulting ternary complex correlates to downstream
transcriptional activity.
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EXPERIMENTAL PROCEDURES |
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Strains of S. cerevisiae and Growth Media--
Three yeast
strains were used in this study. For the one- and two-hybrid analyses,
strain SFY526 (obtained from CLONTECH) was used as
host (MATa ura3-52 his3-200 ade2-101 lys2-801 trp1-901 leu2-3, 112 canr gal4-542 gal80-538
URA3::GAL1-lacZ). For the promoter activity studies
using CTR1 promoter-lacZ reporter plasmids,
DEY1457 was used as host (MAT ade6 can1 his3 leu2
trp1 ura3). This strain was obtained from David Eide (21). For
test of the complementing activity of mutant forms of Mac1p (
ZFMac1p
and ZF*Mac1p), DEY1457(mac1
) was used as host (DEY1457
mac1::TRP1) (4). The following media were used for
culture growth: YPD was used for routine growth of wild type strains
(2% yeast extract, 2% peptone, 2% glucose); SC medium was used for
routine selective growth of transformed strains (1.67 g/liter yeast
nitrogen base without amino acids, 2% glucose plus the
appropriate drop-out (otherwise complete)) mixture of amino acids; for
growth of copper-depleted cultures, a completely synthetic,
Chelex-treated medium was used that was prepared as described
previously (2). This medium contained an estimated 0.5 nM
residual copper based on the amount of copper in the individual media
components (as determined by flameless atomic absorption
spectrophotometry) and the affinity of the Chelex resin for copper.
This medium was subsequently supplemented with a trace metal mixture
lacking copper to which copper was added as required for a given
experiment. This medium was also used with glycerol replacing glucose
as carbon source to test for the respiration competence of DEY1457
(mac1
) when performing functional tests of mutant forms
of Mac1p. This is a standard test for mac1
-complementing clones that takes advantage of the iron (and, consequently,
respiratory) deficiency in this background (1).
Plasmid Construction--
All PCR-generated fragments,
mutations, and reading frames in the plasmid constructs described below
were confirmed by dideoxy sequencing using the T7 Sequenase 2.0 kit
from Amersham Life Sciences, Inc. The DNA sequence for the Mac1p ORF
was derived from the plasmid MAC16 that was obtained by inserting a
MAC1 SacI (162) to HindIII(+1997) fragment (1)
into the complementary restriction sites in vector pRS316 (22). The ORF
for
ZFMac1p (lacking residues 1-41) was constructed using the
endogenous NcoI site (+121) in MAC1 which includes the codon for Met-42. The NcoI (filled
in)/SalI fragment from MAC16 was ligated into the
SmaI/SalI sites in pGEM3Zf(+) (Promega)
generating pGEM-
ZF for in vitro synthesis of
ZFMac1p and into the bait pGBT9 (giving pGB-
ZF) and catch pGAD424 (giving pGAD-ZF) vectors for one- and two-hybrid analyses (23). To construct whole Mac1p ORF fusions, the endogenous NcoI (+121) site in
MAC16 was destroyed by PCR mutagenesis, and a new NcoI site
was engineered at the +1 nucleotide (translation start). This modified
NcoI(
1)/PstI(+210) fragment from MAC16 was
exchanged with the NcoI(+121)/PstI(+210) fragment
in pGB-
ZF to generate pGB-MAC, comprising the whole Mac1p ORF. The
recombinant NcoI(
1)/SalI(+1927) fragment from pGB-MAC was then subcloned into the catch vector pGAD424 (giving pGAD-MAC) and pGEM3Zf(+) (giving pGEM-MAC) as above. The double mutant
ZF*Mac1p (C23S/H25N) was engineered by two rounds of PCR mutagenesis
using pGEM-MAC as the template. The
NcoI(
1)/HindIII(+1927) fragment from
pGEM-ZF*MAC was filled-in and subcloned into the SmaI sites
in pGBT9 and pGAD424 to make plasmids pGB-ZF*MAC and pGAD-ZF*MAC, respectively.
ZF-Mac1p and ZF*Mac1p were functionally tested by complementation in
DEY1457
mac1 in the following manner. The respective ORFs
from pGEM-
ZF and pGEM-ZF*MAC were subcloned into vector pADH; the
resulting recombinants were subsequently transformed into the Mac1p
minus strain and the transformants tested for respiration competence as described above. pADH was constructed from pGAD424 by
replacing the HindIII fragment in this vector that contained all sequences from GAL4 by a multiple cloning site
containing an NcoI site. Thus, pADH contained the yeast 2µ
replication origin, the promoter and terminator from ADH1,
and LEU2 for plasmid selection (23).
The wild type CTR1 promoter-lacZ fusion reporter plasmid was obtained from Andrew Dancis (see Ref. 9). The two promoter mutants, C5'AT-lacZ and C3'TA-lacZ, were derived from the wild type by site-directed mutagenesis using the Quick-Change kit from Stratagene. For the former plasmid, the complementary primers used were 5'-GCAAATCATGGGATTTTTGCTCAAGAC and 5'-GTCTTGAGCAAAAATCCCATGATTTGC (A to T transition underlined), whereas for the latter the primers used were 5'-CGGTAAAATGAGCAAATATGGCACGATCC and 5'-GGATCGTGCCATATTTGCTCATTTTACCG (T to A transition underlined).
EMSAs-- The double-stranded oligonucleotides used in the EMSAs are given in Table I. A typical binding reaction contained 2-10 fmol of radiolabeled probe, 0.5 µg of salmon sperm DNA, 12% glycerol, 12 mM Hepes-NaOH (pH 7.9), 60 mM KCl, 5 mM MgCl2, 4 mM Tris-HCl (pH 8.0), 0.6 mM EDTA, 0.6 mM dithiothreitol, and competitor DNA when present (14). This 15-µl binding mixture, in addition, contained 3-5 µl of a wheat germ extract transcription-translation reaction (Promega) that had been programmed with either vector alone (typically pGEM3Zf(+)) or with vector containing either wild type or mutated Mac1p-encoding sequences. Binding reactions were performed by preincubation of all components except labeled probe for 10 min at room temperature with or without competitor DNA; 2-10 fmol of radiolabeled probe was then added, and the mixture was incubated for another 10 min at room temperature. The mixture was chilled on ice and then electrophoretically resolved on a 6.0% polyacrylamide gel at 4 °C. The gel was dried and exposed to a PhosphorImager screen and to Kodak BiomaxTM MR film. The screen was then read using a Bio-Rad model GS-505 PhosphorImager, and the digitized intensity data were then quantitated using Molecular Analyst 1.5. The EMSA figures herein were imaged directly from these digitized intensities. In Figs. 2-6, the relative amount of a given complex with respect to the control is shown using a bar graph; the control was the radioactivity associated with the faster migrating complex formed with the WT CTR1 promoter probe which we have assumed represents a binary Mac1p·DNA complex. Note that in some experiments either CuCl2 (up to 100 µM) or bathocuproine disulfonate (100 µM) were added to determine a potential effect of free [copper] on the EMSA results. None of these treatments had a specific effect as has been reported (4).
To determine if the amount of input Mac1p protein was independent of the MAC1 construct used to program the wheat germ extract, each construct was expressed in the presence of [35S]methionine, the reaction mixture resolved on SDS-polyacrylamide gel electrophoresis, and the dried gel developed by autoradiography. In all cases, a single Mac1p product was detected by appropriate molecular weight. Based on quantitation of the film by image analysis (Bio-Rad Gel Doc system) equivalent amounts of protein were produced from all templates.
-Galactosidase Assays--
Yeast transformants were grown in
the appropriate Chelex-treated/copper-supplemented defined medium for
at least 5 doublings until the cultures reached mid-log phase
(O.D.660 nm = 1.5-2.0). Samples (2 × 107 cells) were assayed in triplicate for
-galactosidase
activity which was expressed in Miller units in the standard fashion
(24). The data in Fig. 7 are from three independent experiments
(samples in triplicate), and statistical significance of differences in mean values was determined with the use of InStat (GraphPad, San Diego, CA).
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RESULTS |
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Mac1p N-terminal Zinc Finger Element Is Required for DNA
Binding--
Two mutant forms of Mac1p were tested to demonstrate that
DNA binding was due at least in part to the N-terminal portion of Mac1p
(1) that was homologous to the zinc finger, DNA-binding domain in both
Ace1p (15, 16) and Amt1p (17). One mutant had an N-terminal 41-amino
acid deletion (ZFMac1p). The other was a C23S/H25N double mutant
designated ZF*Mac1p; Cys-23 and His-25 are two of the five conserved
residues in the zinc finger-like motif found in the three
trans-factors. These two constructs, as well as wild type Mac1p, were
then tested by EMSA as shown in Fig. 1
using the wild type CTR1 Mac1p-specific promoter element as
probe (see Scheme I and Table I). Binding
of wild type Mac1p to this probe was clearly evident (lane
4). In contrast, neither mutant protein yielded a detectable level
of protein-DNA complex in this assay (
ZFMac1p, lane 6;
ZF*Mac1p, lane 8). Furthermore, neither mutant construct was
capable of complementing a deletion in MAC1 when expressed
in a
mac1-containing background (see "Experimental Procedures" and data not shown). These results most reasonably suggest that a major determinant of functional Mac1p binding to DNA
resides in the Ace1p/Amt1p homology element in the N-terminal domain of the protein.
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Characterization of Mac1p·DNA Binary and (Mac1p)2·DNA Ternary Complexes at the CTR1 Promoter-- The repeated nature of the core GCTC element suggested to us the possible formation of a transcriptionally active protein-DNA complex, (Mac1p)2·DNA. We tested this model by first establishing the presence of such a complex by EMSA and then by delineating the sequence features of the DNA that promoted its formation.
In fact, two mass species were apparent in an EMSA in which the wild type CTR1 sequence (WT, Table I) was used as probe (Fig. 2, lane 3; see also Fig. 1, wild type Mac1p, lane 4). The more slowly migrating of these two complexes (shaded bar, quantitation below) was present with an abundance of 50% that of the faster migrating species (solid bar, quantitation below). We infer that the more slowly migrating species is larger in mass and arises from separate protein molecules binding to the two core elements. This inference is derived from the disappearance of this slowly migrating species when either core element was mutated indicating that two elements were required for the formation of this complex (lanes 6 and 9, probes R5' and R3', respectively). In probes R5' and R3', the core sequences in either the 5' or 3' element, respectively, were randomized (Table I). However, the data indicated that these two sites were not equivalent since the fraction of the probe found in what we will refer to as a binary Mac1p complex (the smaller, faster migrating species) was essentially absent if the 5' core sequence was mutated (in R5', Fig. 2, lane 6) in comparison to mutation of the 3' core (as in R3', Fig. 2, lane 9). This result suggested that a binary complex at the 5' element was more stable than one at the 3' sequence.
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The primary structural difference between these two core elements resides in the sequence immediately 5' to both. The upstream element is 5'-TATTTGCTC and the downstream one is 5'-GAGCAAAAA (5'-TTTTTGCTC on the opposite strand). The data above suggested the possibility that the presence of the TTTTT sequence in the latter 3' element dampened Mac1p binding to the core or, alternatively, the A in the former 5' element was preferentially recognized by Mac1p. Furthermore, the EMSA indicated that binding of Mac1p to the apparently weaker 3' element in the wild type promoter, as must occur in a ternary complex, was linked to Mac1p binding to the apparently stronger 5' element. This latter conclusion derives from the fact that a single TTTTTGCTC site cannot support formation of a detectable binary complex (Fig. 2, R5' probe, lane 6), whereas in the context of an intact upstream TATTT-containing element, ternary complex formation does occur which must involve Mac1p binding to the 3' element (WT probe, lane 3).
One test of this conclusion was to convert the 3'-TTTTTGCTC element in R5' (to which Mac1p binds weakly if at all, Fig. 2, lane 6) to 5'-TATTTGCTC (again, reading on the opposite strand). The binding of Mac1p to this mutant, designated R5'-3'-TA, would be expected to increase in comparison to binding to R5' itself. This prediction was tested by direct binding with R5'-3'-TA as probe (data not shown; see however, Fig. 6, lane 3) and also by the more quantitative competition assay. Thus, unlabeled mutant oligonucleotides were used as competitor DNA of the Mac1p binding to the WT CTR1 promoter element as probe (Fig. 3, control, lane 3). Based on these competition data as in Fig. 3 we estimate that Mac1p has a 3-4-fold greater affinity for the R5'-3'-TA oligonucleotide (stronger competitor as shown in Fig. 3, lanes 6-9) than for R5' itself (Fig. 3, lanes 4-6; also, see quantitation).
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A second test of this conclusion was to use as probe mutated wild type double-stranded oligonucleotides in which both elements contained either TATTT or TTTTT, followed by GCTC. The TATTT-containing repeat would be expected to support both stronger binding overall and, importantly, a larger fraction of complex in what we propose is (Mac1p)2·DNA. In contrast, the mutant that had T5 at both sites would be expected to have weaker binding overall and little or no ternary complex formation. Both of these predictions were confirmed by experiment as demonstrated by the EMSA shown in Fig. 4A. Specifically, the symmetrical probe containing repeated TTTTTGCTC elements (Fig. 4A, probe 5'-AT, lane 6) supported 75% ternary complex formation compared with WT probe (WT, lane 3), whereas the symmetrical probe containing TATTTGCTC (probe 5'-TA, lane 9) supported 150% of this complex compared with WT (shaded bars, quantitation). Furthermore, competition experiments in which wild type was used as probe and these two mutant oligonucleotides were used as competitors (see Fig. 4B) yielded a pattern in complete agreement with the direct binding data shown in Fig. 4A. Thus, the T5-containing direct repeat (5'-AT, lanes 3 and 4) was a poorer competitor than the TAT3-containing species (3'-TA, lanes 5 and 6; data in bar graph presented as a percent of values for the WT probe alone, see Fig. 4A, lane 3, for this control). In summary, the data in Figs. 2-4 indicate that Mac1p binds more strongly to the sequence TATTTGCTC than to TTTTTGCTC, and this better binding appears to stabilize what is interpreted to be a ternary (Mac1p)2·DNA complex.
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The FRE1 promoter provides a natural test of this model. The EMSA shown in Fig. 5 demonstrates that little complex formation at the FRE1 promoter element was observed (Fig. 5, FRE1-WT probe, lane 6; compare with binding to the CTR1-WT probe, lane 3). Competition by this FRE1 element of Mac1p binding to the wild type CTR1 44-mer showed that, indeed, Mac1p has a weaker affinity for the former sequence (data not shown). To show that this weaker binding was likely due to the absence of even one TATTT-containing site in the WT FRE1 probe, a mutant FRE1 element was used in which the 3' site contained the requisite T to A transition. Consistent with our model, while the wild type FRE1 element gave only a weak signal in the EMSA (Fig. 5, lane 6), the mutant probe supported the formation of both binary and ternary complexes (Fig. 5, FRE1-3'-TA probe, lane 9). Although the stability of these complexes appeared less than with the wild type CTR1 element (which also has one each of the two types of T-rich sequences), the relative amount of the two complexes at the mutant FRE1 oligonucleotide was similar to what was observed with the CTR1 one (see quantitation). Importantly, the fact that a single TATTT at the FRE1 promoter converted this element from one that supported only a very weak interaction overall to one that actually could support ternary complex formation suggests a model in which formation of the ternary complex could be cooperative, i.e. that it might involve a thermodynamically important Mac1p-Mac1p protein-protein interaction linked to Mac1p binding at a TATTT-containing site.3
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Origin of the Stronger Binding of Mac1p to TATTTGCTC--
One
simple explanation for the difference in Mac1p binding to the TATTTGCTC
sequence is that a specific protein-DNA interaction could be more
favorable with A in comparison to T at this 4 position (relative to
the G in the core sequence). To test this possibility, three probes
were constructed based on the random 5' sequence (R5', Fig. 2) in which one of the thymines in
the wild type 3' element was replaced by adenine. Thus, this series
consisted of oligonucleotides containing 5'-TTTTTGCTC
(R5'), 5'-ATTTTGCTC (R5'-A41T), 5'-TATTTGCTC
(R5'-A40T; this is identical to R5'-3'-TA used
as competitor in Fig. 3), and 5'-TTATTGCTC (R5'-A39T;
sequences given for bottom strand, Scheme I). If the specific placement
of the A were important to Mac1p binding, Mac1p binding to only one of
these probes would be observed, presumably to R5'-A40T as
indicated by the competition data in Fig. 3. As the EMSA results in
Fig. 6 show, Mac1p appeared to make a
more favorable contact with A in comparison to T at this
4 position
in as much as binding to the single, 3'-GCTC in R5' was
observed only with the probe containing 5'-TATTTGCTC on the bottom
strand (Fig. 6, R5'-A40T, lane
3). In terms of binding affinity for Mac1p, R5'-A40T
should be equivalent to R3', since both mutant
oligonucleotides have only one TATTTGCTC site. Comparison of the data
for R5'-40T (Fig. 6, lane 3) and R3'
binding (Fig. 2, lane 9) shows this to be the case.
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(Mac1p)2·DNA Complex Formation in Vitro Correlates to
Stronger Promoter Activity in Vivo--
To test whether the ternary
complex formation indicated by the EMSA data might be functionally
important, the WT CTR1 promoter, in a fusion to the
lacZ gene, was mutated to contain either two TTTTT or two
TATTT elements. The transcriptional activity of the wild type and two
mutant promoters was then quantitated by standard -galactosidase
assay. To demonstrate also the relative dependence of the activity of
these promoter constructs on the copper concentration in the medium,
cultures were grown in a copper-depleted medium (estimated 0.5 nM residual copper, "Experimental Procedures") or in
the presence of added copper. The results of these measurements are
shown in Fig. 7; they demonstrate a
positive correlation between transcriptional activity in
vivo and the fraction of Mac1p in (Mac1p)2·DNA
in vitro. Specifically, while the 5'-AT mutant promoter (Fig. 7, squares) supported only 65% the expression seen
from WT CTR1 (Fig. 7, circles), the 3'-TA mutant
supported 115% of this expression (Fig. 7, triangles).
These differences can be compared with those described for the EMSA
results (Fig. 4A) in which the symmetrical
T5-containing 5'-AT and symmetrical
TAT3-containing 3'-TA probes supported 75 and 150%
(Mac1p)2·DNA formation, respectively (in comparison to
the WT CTR1 probe, Fig. 4A, lane
3).
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The mutant promoters were equivalently down-regulated by copper indicating their essential dependence on Mac1p function. However, the dependence of this down-regulation on [copper]medium was shifted to higher values for the more active promoter element, that is in our model, the greater the fraction of ternary protein-DNA complex. From the data in Fig. 7, one can estimate that the [copper]medium that resulted in the half-maximal change in trans-activity for the WT CTR1 promoter was ~20 nM (Fig. 7, circles); for the 5'-AT (triangles) and 3'-TA (squares) mutant CTR1 promoter sequences it was 10 and 30 nM CuCl2, respectively. This pattern would be consistent with a differential Mac1p-DNA interaction in the three promoter constructs. The fact that a single Mac1p-binding site is incapable of supporting transcription in vivo has been demonstrated previously (4, 7, 12). Thus, the quantitative data shown in Fig. 7 are consistent with a model in which a (more stable) ternary Mac1p-containing DNA complex at the CTR1 promoter is active in the subsequent up-regulation of the expression of this gene. Furthermore, they indicate that the structural difference(s) between TATTT and TTTTT that modulate the stability of this complex in vitro apply in vivo as well. It is important to note that the copper-dependent down-regulation of Mac1p trans-activity that occurs between 1 and 100 nM (as in Fig. 7) is entirely independent of the turnover of Mac1 protein that occurs when cells are exposed to [copper]medium >10 µM or under conditions of incipient copper toxicity (25).
A Mac1p-Mac1p Interaction Can Be Demonstrated by a Two-hybrid
Analysis--
We inferred above that formation of what we suggest is a
ternary, (Mac1p)2·DNA complex might involve a
thermodynamic contribution due to a protein-protein interaction between
two Mac1p molecules. However, the data do not exclude an alternative
model in which two Mac1p molecules bind to the DNA completely
independently of one another. We have been unable to produce and
isolate Mac1p that is active in the DNA binding experiments necessary
to distinguish between these two possibilities. Therefore, we chose to
demonstrate a possible Mac1p-Mac1p interaction independent of DNA
binding by the in vivo two-hybrid genetic approach
(23).1 To do so, fusions of wild type Mac1p, the ZFMac1p
truncation, and the mutated zinc finger Mac1p, ZF*Mac1p, were made to
both the Gal4 DNA-binding (DBD) and transactivation domains. These fusions were tested for in vivo interaction by the standard
two-hybrid assay in cells grown in both the absence and presence of
copper. The results of these several assays are summarized in Table
II. Also included in this table (in
parentheses) are representative values for the Mac1p-Gal4 DBD fusions
alone in a one-hybrid assay. These values describe the inherent
transactivation activity of Mac1p and are needed for comparative
purposes in as much as this Mac1p trans-activity is
copper-dependent (5, 6).1
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The data in Table II supported two basic conclusions. First,
irrespective of the strength of the Mac1p-Mac1p interaction indicated by a given fusion pair, the interaction was essentially independent of
medium copper (compare values in first and third rows, all columns).
This was in contrast to the results from the one-hybrid analysis that
showed that the inherent trans-activity of Mac1p was consistently
7-10-fold greater in copper-deficient cells in comparison to
copper-replete ones. This copper dependence is illustrated best by the
transcriptional activity due to the ZFMac1p fusion (Table II, third
column, compare second and fourth
rows).4 Second, a Mac1p-Mac1p
interaction was increased ~10-fold upon removal of the N-terminal
sequences suggested above to have DNA binding activity as in the
ZF
truncation (Table II, compare first to subsequent columns). This fold
increase paralleled the increase in the trans-activity of this
truncated Mac1p noted directly above (Table II, third
column).1 The effect of N-terminal deletion was geometric,
that is this domain in each Mac1p fusion contributed equally in energy
terms to the masking of the strong Mac1p-Mac1p interaction indicated for the
ZFMac1p fusion pair (compare columns 1, 2, and 4, first and
third rows). Significantly, increase in neither trans-activity nor
Mac1p-Mac1p interaction was observed when a fusion of the C23S/H25N
double mutant was used (ZF*Mac1p, last column), a mutant which lacks
DNA binding activity (Fig. 1) but which still has an N-terminal domain.
Interaction between two molecules of this mutant form was identical to
that between the wild type protein, i.e. it was weak, giving
a value only slightly above the blank.
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DISCUSSION |
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The experiments described here establish six new facts and/or
inferences about the structure-function relationships pertaining to the
Mac1p-DNA interaction. These are as follows: 1) Mac1p DNA binding
activity requires at the least the CCHC Ace1p/Amt1p zinc finger
homology domain; 2) a complex of Mac1p and its DNA target forms in an
EMSA that is most reasonably described as a ternary, (Mac1p)2·DNA species in addition to a Mac1p·DNA binary
one; 3) Mac1p binding to any core GCTC element is modulated by the
immediate 5' sequence, specifically, binding is stronger to TATTTGCTC
than to TTTTTGCTC; 4) Mac1p appears to make a specific and favorable contact with adenine at the 4 sequence position (tAttt) in comparison to a thymine (tTttt); 5) a mutated CTR1 promoter that
conforms to a perfect palindrome including the TATTT sequence supports a larger fraction of Mac1p in the ternary complex in vitro
and a stronger Mac1p-dependent expression from the
CTR1 promoter in vivo; and 6) a Mac1p-Mac1p
protein-protein interaction occurs that appears negatively modulated by
the N-terminal domain of the protein that is essential to DNA binding.
Based on these new facts and on previously reported ones, we propose
the following model of how the interactions of Mac1p with itself and
with its DNA binding site are linked to the activation of gene
expression at Mac1p-dependent genes.
The components of this model are as follows. (a) (Mac1p)2·DNA is, at the least, a transcriptionally active species. Mac1p-dependent transactivation requires two Mac1p DNA-binding sites, as in the FRE1 (10), CTR1 (4), and CTR3 promoters (7). A single core GCTC element cannot support Mac1p- and copper-dependent expression from these promoters. Our data indicate that this requirement is most reasonably explained by a model in which the Mac1p·DNA complex that is competent for transcriptional activation is a ternary one, (Mac1p)2·DNA. (b) Mac1p can be in (minimally) two conformational states. The "resting" state is inactive with respect to any of the interactions necessary for Mac1p-dependent transactivation, i.e. DNA binding, Mac1p-Mac1p interaction, and recruitment of components of the pre-initiation complex. The "active" state can participate in each of these intermolecular interactions. (c) The interconversion between these two states is linked to these three interactions, that is Mac1p has the characteristics of a thermodynamically cooperative system: the intermolecular interactions in the active state (which include all interacting components) compensate for the loss of the intramolecular ones (in Mac1p) which stabilize the resting state. (d) The DNA-binding, zinc finger-containing N-terminal domain is the key element in this cooperativity in that in the resting, unbound (to DNA) Mac1p conformation this domain masks those other domains involved in either the Mac1p-Mac1p interaction or in the recruitment of other transcription factors. Reciprocally, in the resting state these other domains mask the N-terminal domain from the DNA.
That the DNA binding activity of Mac1p requires at the least the N-terminal domain including an intact CCHC motif is not surprising. Previous work with Ace1p (16) and Amt1p (19) suggested that in both of those systems the homologous N-terminal domains made important DNA contacts, as, for example, in the major groove in the case of Ace1p binding to the metal-responsive element in the CUP1 promoter (16). However, in neither case are the core cis elements (GCTG in the case of the Ace1p-binding site) and the flanking regions, which also make protein contacts, repeated as they are in CTR1 and CTR3 (and FRE1, albeit as a direct rather than an inverted repeat). Furthermore, the C-terminal core cysteine-rich regions of Ace1p and Amt1p, which when bound to Cu(I) form what has been described as a "copper fist," also make specific DNA contacts (16, 19). Indeed, it is the copper fist domain in Ace1p that makes the major contacts with the Ace1p-core binding site, TTTCCGCTG (16). Structure-function relationships in Mac1p appear to be quite different. Current data are most consistent with a more strict demarcation of function between the N- and C-terminal domains in the protein; the C-terminal domain expresses all of the transactivation activity (and its copper dependence, much as in Ace1p and Amt1p), whereas the N terminus is required for DNA binding. Note also that both Ace1p and Amt1p are positively regulated by copper, the inverse of the behavior of Mac1p in response to copper level.
The presence of the inverted repeat in the CTR1 (and CTR3) promoters (3, 7, 9), TTTGCTC, and the fact that promoter deletion analyses in the CTR1 (4) and CTR3 promoters (7) demonstrated the need for both repeats suggested to us that Mac1p might bind at both sites irrespective of the precise nature of the ternary complex. Our data are the first that support this possibility. Our EMSA results show clearly that two Mac1p-dependent complexes form and, furthermore, that the formation of both, particularly the more slowly migrating one, appears driven by a stronger Mac1p binding to the 5' element, TATTTGCTC. Importantly, this stronger binding in vitro can be correlated with stronger promoter activity in vivo in support of our model that a (Mac1p)2·DNA complex is at the least more active transcriptionally if not the active complex. However, our data do not prove this latter constraint, leaving open the question of whether both, or only the ternary complex, can support downstream transcription initiation.
The stronger Mac1p binding to the TATTT-containing motif appeared due
to a more favorable interaction with A in comparison to T at this 4
position specifically. Of interest is that of the six Mac1p elements
analyzed (two each in CTR1, CTR3, and
FRE1), only two contain an adenine at this position, one in
CTR1 and one in CTR3. In contrast, the equivalent
4 position in the Ace1p and Amt1p core binding sites at various loci
has an invarient thymine, or T(T/C)XXGCTG (8,
19). This conservation suggests that this specific nucleotide
base-protein contact, which has been confirmed by experiment (16, 19),
makes a more substantive contribution to the overall stability of the
protein·DNA complex in the case of Ace1p and Amt1p than it does in
the case of Mac1p. One explanation for this difference would be that in
the case of Mac1p-DNA ternary complex formation, a Mac1p-Mac1p
interaction provides an additional and significant driving force.
One caveat of our work is that we have not provided direct evidence for the presence of two Mac1p molecules in the more slowly migrating DNA complex. Although we have attributed this difference in mobility to a larger size, i.e. to a (Mac1p)2·DNA complex, another possibility is that the more slowly migrating species is a conformer of the Mac1p·DNA binary complex. However, the appearance of this species in the binding of Mac1p to the FRE1 promoter element upon mutation of only one of the core sequences to include an upstream TATTT is more consistent with a ternary complex model. With this mutant oligonucleotide as probe, the distribution of Mac1p-DNA species was equivalent to what was observed with the wild type CTR1 promoter, that is introducing one strong site led to the appearance of both mobility species. This can most simply be explained by a cooperative binding model in which the "good" site recruits Mac1p and that the resulting DNA-protein interaction stabilizes a second Mac1p binding (to an inherently weaker DNA-binding site) through a Mac1p-Mac1p interaction. However, while a cooperative dimer model is consistent with the EMSA results shown, it clearly remains to be tested more rigorously than it has been here.
The two-hybrid results indicated that Mac1p can, in effect,
self-associate. One aspect of these data was particularly significant, namely the potential protein-protein interaction was strongly suppressed by an intact N-terminal domain. Thus, the ZF construct that lacked Mac1p residues 1-40 supported a 10-40-fold stronger interaction in comparison to the fusions with Mac1p (1-417) as both
bait and catch. We interpret this result to show that the N-terminal
domain (whether wild type or mutated, e.g. as in ZF*Mac1p) sterically blocks self-association of Mac1p by an
intramolecular contact with the region in Mac1p involved in
the intermolecular protein-protein interaction. In our
model, we postulate that this intramolecular interaction is replaced
(energetically compensated for) by the intermolecular one the
N-terminal domain has with the DNA. In the Mac1p·DNA complex,
therefore, the DNA-bound Mac1p (conformationally equivalent to
ZFMac1p) is active for binding to another molecule of Mac1p. The
apparent strength of this latter interaction will be
dependent on whether the second Mac1p is also active for binding (is
bound to DNA, resulting in a ternary complex) or, in the two-hybrid
case, whether the second Mac1p is intact or N-terminal deleted.
We interpret the one-hybrid data to indicate that the N-terminal domain also blocks the recruitment by Mac1p of components of the general transcription machinery (as it does the apparent Mac1p-Mac1p interaction) when Mac1p itself is not bound to DNA via its own N-terminal domain. Thus, the 10-fold increase in the (copper-dependent) expressed trans-activity in the N-terminal deleted construct in comparison to Mac1p-(1-417) was nicely consistent with the suggestion that the DNA-binding N-terminal domain masks elements in the C-terminal region involved in both types of protein-protein interaction.
The role of copper in the regulation of the inherent transcriptional activity of Mac1p was not clarified by the results presented here. Clearly, Mac1p, either by itself or as the TA domain in a DBD fusion, e.g. to Gal4 DBD (above and Ref. 5)1 or to the LexA protein (6), is down-regulated by a copper concentration in the medium that is known to down-regulate the expression of CTR1 (cf. Fig. 7). The [copper]medium that causes a 50% suppression of the transcriptional activity of Mac1p is ~20 nM. Furthermore, in vivo DNA footprinting at the CTR3 promoter (Ref. 18, and at the CTR1 promoter as well)5 has demonstrated a Mac1p-dependent protection at the Mac1p-binding sites that is negatively modulated by copper. On the other hand, neither the Mac1p-Mac1p protein-protein interaction indicated by our two-hybrid data nor the in vitro DNA binding experiments employing a full-length Mac1p (EMSA) exhibited a copper dependence (described under "Experimental Procedures"). Whereas the latter results could be ascribed to the use of, for example, in vitro generated protein, the former result cannot be so easily disregarded. The fact that the same fusion exhibited a copper-dependent trans-activity in a one-hybrid assay but a copper-independent protein-protein interaction in a two-hybrid one most reasonably suggests that one specific protein-protein interaction, self-association, is, in fact, not modulated by the alteration in Mac1p that is caused by copper.
Clearly, the model that we propose here requires additional rigorous
experimental evaluation. For example, one could predict differential
protection of the two GCTC elements in the wild type CTR1
promoter in a DNA footprinting experiment, whether in vivo or in vitro. The one published in vivo footprint,
at the CTR3 promoter, was relatively symmetric (7). However,
additional footprinting experiments using altered Mac1p-binding sites
as used here in the EMSA along with Mac1 protein that either has or
does not have the potential to self-associate could provide new and
important insight into the protein-protein and protein-DNA equilibria
involved in the Mac1p-DNA interaction whether or not they specifically
confirm the present model. These experiments are in progress.
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ACKNOWLEDGEMENTS |
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We are indebted to Dennis Winge for communication of the promoter activity results that have been referenced herein prior to their publication. We thank Richard Hassett for the preparation of the copper-free medium used in this work and Annette Romeo for essential and diverse technical assistance. The plasmids received from David Eide and Andrew Dancis and used in this work are gratefully acknowledged.
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FOOTNOTES |
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* This work was supported by National Institutes of Health Grant GM46787 (to D. J. K.).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.
To whom correspondence should be addressed: Dept. of Biochemistry,
140 Farber Hall, 3435 Main St., Buffalo, NY 14214. Tel.: 716-829-2842;
Fax: 716-829-2661; E-mail: camkos{at}acsu.buffalo.edu.
1 M. Serpe and D. J. Kosman, submitted for publication.
3 A double FRE1 promoter mutant was not constructed (both sites containing TATTT) since in making such a mutation at the 5' site, one would be constructing a perfect TATA element due to the flanking sequence. This would likely be target for the transcription factors present in the in vitro transcription/translation mix used to generate the Mac1p for these experiments giving rise to a confusing Mac1p-independent mobility shift(s).
4
Full-length Mac1p (or the mutant ZF*Mac1p) has
very little trans-activity in a one-hybrid fusion. This activity
resides in the C-terminal half of the molecule and is expressed in the
one-hybrid assay only when N-terminal Mac1p truncations, such as
ZFMac1p, are fused to the Gal4 DBD (Table II).1
5 M. Serpe and D. Kosman, unpublished results presented at the Genetics Society of America Meeting, August 6-11, 1996, Madison, WI.
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ABBREVIATIONS |
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The abbreviations used are: EMSA, electrophoretic mobility shift assay; DBD, DNA binding domain; PCR, polymerase chain reaction; ORF, open reading frame; WT, wild type..
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REFERENCES |
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