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Journal of Bacteriology, September 2000, p. 4970-4978, Vol. 182, No. 17
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Understanding the Growth Phenotype of the Yeast
gcr1 Mutant in Terms of Global Genomic Expression
Patterns
M. Cecilia
López and
Henry V.
Baker*
Department of Molecular Genetics and
Microbiology, University of Florida College of Medicine,
Gainesville, Florida 32610-0266
Received 9 May 2000/Accepted 30 May 2000
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ABSTRACT |
The phenotype of an organism is the manifestation of its expressed
genome. The gcr1 mutant of yeast grows at near wild-type rates on nonfermentable carbon sources but exhibits a severe growth defect when grown in the presence of glucose, even when nonfermentable carbon sources are available. Using DNA microarrays, the genomic expression patterns of wild-type and gcr1 mutant yeast
growing on various media, with and without glucose, were compared. A
total of 53 open reading frames (ORFs) were identified as
GCR1 dependent based on the criterion that their expression
was reduced twofold or greater in mutant versus wild-type cultures
grown in permissive medium consisting of YP supplemented with glycerol
and lactate. The GCR1-dependent genes, so defined, fell
into three classes: (i) glycolytic enzyme genes, (ii) ORFs carried by
Ty elements, and (iii) genes not previously known to be
GCR1 dependent. In wild-type cultures,
GCR1-dependent genes accounted for 27% of the total
hybridization signal, whereas in mutant cultures, they accounted for
6% of the total. Glucose addition to the growth medium resulted in a
reprogramming of gene expression in both wild-type and mutant yeasts.
In both strains, glycolytic enzyme gene expression was induced by the
addition of glucose, although the expression of these genes was still
impaired in the mutant compared to the wild type. By contrast, glucose
resulted in a strong induction of Ty-borne genes in the mutant
background but did not greatly affect their already high expression in
the wild-type background. Both strains responded to glucose by
repressing the expression of genes involved in respiration and the
metabolism of alternative carbon sources. Thus, the severe growth
inhibition observed in gcr1 mutants in the presence of
glucose is the result of normal signal transduction pathways and
glucose repression mechanisms operating without sufficient glycolytic
enzyme gene expression to support growth via glycolysis alone.
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INTRODUCTION |
In Saccharomyces
cerevisiae, glucose is the preferred carbon and energy source. The
enzymes of glycolysis, which are required for the utilization of
glucose, make up a major fraction of the soluble cellular proteins
(22, 25). The genes encoding these enzymes are transcribed
at high levels, and the individual transcripts are some of the most
abundant in yeast (26, 54). Glycolytic enzyme gene
expression is brought about by the concerted action of a number of
transcription factors that bind in the upstream activating sequences
(UAS) of these genes (3, 4, 9, 10, 38, 48, 55). The
centerpiece of glycolytic enzyme gene UAS elements is made up of the
closely positioned binding sites for the proteins Gcr1p and Rap1p
(3, 18, 29, 39, 55).
The role of Gcr1p in glycolytic enzyme gene expression was first
realized when mutations were isolated in the gene encoding it
(14). gcr1 mutants exhibited a severe growth
defect when grown in the presence of glucose and were shown by enzyme
assays to have reduced levels of most glycolytic enzyme activities
(14). Sodium dodecyl sulfate-polyacrylamide gel
electrophoresis with extracts from wild-type and gcr1 mutant
strains indicated that the gcr1 mutation affected the
expression of a limited set of several prominent bands that comigrated
with purified glycolytic enzymes (13). Since the first
gcr1 mutation appeared to affect the expression of a few
genes primarily involved in glycolysis, it was named gcr1 to
signify its effect on glycolytic enzyme gene expression. At the time,
however, it was difficult to state the limits of Gcr1p in global gene expression.
Rap1p is a multifunctional protein that can act in transcription as
either an activator or a repressor depending on the sequence context of
its binding site (5-7, 49). From the outset, it was
recognized that Rap1p is one of a class of general transcription factors whose function is required for expression of many different genes. Rap1p has been implicated in the expression of elongation factors, initiation factors, aminoacyl tRNA synthetases, ribosomal protein genes, tRNA and rRNA genes, nutrient transporters, glycolytic enzyme genes, and expression at HMR and HML (5-7, 49). The role of Rap1p as an activator is best understood for glycolytic enzyme
genes. At the UAS elements of these genes, Rap1p achieves its
activating function by facilitating the binding of Gcr1p at adjacent
binding sites (18). Gcr1p is unable to bind these elements in vivo unless Rap1p is bound to an adjacent site (18, 53). In the absence of functional Gcr1p-binding sites or Gcr1p itself, Rap1p, while able to bind to the UAS elements of glycolytic enzyme genes, is unable to mediate the activation of these genes by itself (18). It has been proposed that Rap1p and Gcr1p function
together to mediate ribosomal protein gene expression (44).
In the case of ribosomal protein genes, which as a class do not have
Gcr1p-binding sites, it has been suggested that Rap1p recruits Gcr1p by
complexing with it (57).
The precise mechanism by which Rap1p facilitates the binding of Gcr1p
has yet to be elucidated, and the true nature of the relationship
between Rap1p and Gcr1p has been the subject of research and debate
(18, 34, 44, 48, 50). We have proposed that both Rap1p and
Gcr1p may have additional DNA binding partners, with which they
interact to mediate their roles in transcription (34). Rap1p
is known to make contact with other proteins, namely, Rif1p, Sir3p, and
Sir4p (23, 33, 37); however, these proteins are not known to
make sequence-specific contact with DNA as is the case for Gcr1p
(2, 28, 29, 34). A search of the yeast genome for sequences
that match the proposed consensus binding sites for both Rap1p and
Gcr1p reveals several hundred sites for each, yet in only a limited
number of cases are the two sites found adjacent to one another, most
notably at glycolytic enzyme gene UAS elements (34). The
large number of potential binding sites for each protein raises the
intriguing possibilities that Rap1p may facilitate the DNA binding of
other proteins in addition to Gcr1p and that the binding of Gcr1p may
be facilitated by proteins other than Rap1p.
The advent of DNA microarray technology (11, 45) affords one
the opportunity to understand mutant phenotypes in terms of both the
primary and secondary effects of the specific genetic lesion (17,
21, 27, 56). To gain a greater understanding of the extent of the
role of Gcr1p in yeast gene expression and to more fully understand the
phenotype associated with gcr1 mutants, we used labeled cDNA
prepared from RNA isolated from wild-type and gcr1 mutant
strains to interrogate high-density DNA microarrays of the yeast
genome. These experiments defined a limited set of genes which together
make up a significant fraction of the yeast transcriptome and provide
an explanation for the glucose inhibition observed with gcr1 mutants.
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MATERIALS AND METHODS |
Yeast strains and growth conditions.
Isogeneic strains
S150-2B (MATa leu2-3,112 his3
trp1-289
ura3-52) and HBY4 (MATa
gcr1
::HIS3 leu2-3,112 his3
trp1-289
ura3-52) used in this study have been described previously (47).
Strains were subcultured and grown at 30°C in medium permissive for
growth of the gcr1 mutant. Cultures were grown in YP medium (42) supplemented with 2% lactate and 2% glycerol (YPGL).
Cultures in exponential-phase growth at an optical density at 600 nm of approximately 1 were rapidly harvested on ice, and cells were collected
by centrifugation at 4°C. In some experiments with strain S150-2B,
YPD medium was used to investigate the effect of steady-state growth in
medium with glucose.
For glucose induction and repression experiments, cultures were grown
in YPGL to an optical density at 600 nm of approximately
1, at which
time glucose was added to 2%. Following glucose addition,
the cultures
were allowed to grow for 4 h before being harvested
as described
above. Glucose induction and repression growth conditions
are denoted
throughout the text as YPGL+G.
RNA isolation and cDNA preparation.
Total RNA was isolated
using an RNeasy kit from Qiagen (Chatsworth, Calif.) as recommended by
the manufacturer.
cDNA was prepared from 1 µg of total RNA using reverse transcriptase
following oligo(dT) priming. The cDNA was uniformly labeled
using
[
33P]dCTP during the course of the reverse transcription
reaction.
DNA microarrays and hybridization conditions.
DNA
microarrays of yeast open reading frames (ORFs) were obtained from
Research Genetics (Huntsville, Ala.). In total, four microarray sets,
identified by the numbers 11, 77, 83, and 99, were used. The
microarrays were prewashed and hybridized as recommended by the
manufacturer. Hybridizations were carried out in a Robbins roller drum
hybridization chamber at 42°C for a minimum of 16 h. Following
hybridization, the filters were washed as specified by the protocol
provided by Research Genetics. As recommended by the manufacturer, each
array was interrogated, stripped, and reinterrogated to a maximum of
five interrogations. Table 1 shows the
interrogation schedule for each of the microarray sets used.
Signal detection and analysis.
Hybridization of the
radiolabeled cDNA to the immobilized probe DNA on the filters was
detected by phosphorimaging using a Molecular Dynamics Storm
PhosphorImager scanning at 50 µm. The data obtained from the
PhosphorImager were imported into the Pathways 2.01 software package
(Research Genetics) for normalization and analysis. For comparison
purposes, arrays were normalized between experiments by using the
"all data points" method in Pathways 2.01. The normalization method
used by Pathways 2.01 in effect adjusts the total hybridization signal
between filters such that they are equal and then compares the ratio
between adjusted signals at each element on the array.
In total, RNA was isolated and cDNA was prepared from 17 cultures. Each
labeled cDNA preparation was used to interrogate one
of four DNA
microarray sets (sets 11, 83, 77, and 99) used. Comparisons
were made
between interrogations of a given DNA microarray set.
These experiments
resulted in five different expression profiles.
Two of these
highlighted comparisons between mutant and wild-type
cultures, i.e.,
gcr1 versus
GCR1, under permissive growth
conditions
of YPGL (
gcr1/GCR1 YPGL) (four data sets) and
after a 4-h exposure
to glucose (
gcr1/GCR1 YPGL+G) (three
data sets). The other three
profiles examined the effect of exposure to
glucose on gene expression:
the effect of a 4-h glucose exposure on
gene expression in mutant
and wild-type cultures was examined in the
YPGL+G/YPGL
gcr1 and
the YPGL+G/YPGL
GCR1
comparisons, respectively (three data sets
each); and for the wild-type
culture, expression profiles were
also obtained for comparisons of
steady-state growth in YPD versus
YPGL (YPD/YPGL
GCR1)
(three data
sets).
Algorithms and databases.
During the course of this work,
extensive use was made of the online database resources at the
Saccharomyces Genome Database (J. M. Cherry, C. Ball,
K. Dolinski, S. Dwight, S. Harris, et al., Saccharomyces
Genome Database, 1999 [http:/genome-www.stanford.edu/cgi-bin/SGD/search]) and Proteome
(16). Functional groups of genes were classified in
accordance with the Martinsried Institute of Protein Sciences classification scheme (36). Gene expression data were
subjected to hierarchical clustering analysis and displayed using the
algorithms developed by Eisen et al. (20). Common sequence
motifs were identified using the algorithm AlignACE, written by Roth et
al. (43), and are represented using the logos format of
Schneider and Stephens (46).
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RESULTS AND DISCUSSION |
GCR1-dependent genes.
To identify genes dependent on Gcr1p for
full expression, we carried out a series of global genomic expression
studies with wild-type and gcr1 mutant strains of yeast. In
consideration of the severe growth defect that gcr1 mutants
exhibit when grown in the presence of glucose and to reduce the
difference in gene expression due to differences in the growth rate, we
grew yeast cultures in YPGL, which is permissive for the
gcr1 mutant (13, 14). In this medium, the
doubling times for the wild-type and mutant are ca. 280 and 300 min,
respectively. Radiolabeled cDNA, prepared from total RNA isolated from
cultures harvested during logarithmic growth, was used to interrogate
high-density DNA arrays of yeast ORFs. The relative degree of
hybridization to each of the 6,144 ORFs in the array was compared
between experiments. From four independent experiments, using a twofold
difference cutoff, we identified 53 ORFs that reproducibly displayed
lower levels of hybridization when interrogated with cDNA prepared from RNA isolated from the gcr1 mutant than when interrogated
with cDNA from the wild type. Figure 1,
lanes 13 to 16, show comparisons of the expression
profiles of the 53 GCR1-dependent ORFs in YPGL-grown cultures of the gcr1 mutant and the wild type. The complete
data set containing more than 104,000 observations and 98,000 comparisons is available on-line at
http://cmg.health.ufl.edu/~bakerlab/genomic.htm.

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FIG. 1.
Hierarchical cluster analysis of gene expression
patterns of GCR1-dependent genes in wild-type and
gcr1 mutant backgrounds in the absence and presence of
glucose. (A) Genes whose expression was reduced by twofold or greater
in gcr1 mutants growing in YPGL compared to wild-type cells
growing in the same medium are represented in the figure by using
TreeView (21). The degree of difference between the
expression patterns of the various genes is indicated by the length of
the branches on the tree. As indicated at the top of the figure, lanes
1 to 9 represent glucose induction-repression experiments and lanes 10 to 16 represent comparisons of gcr1 mutant and wild type. For the glucose
induction-repression experiments, the reference condition was growth in
YPGL. In comparisons of mutant and wild type, the wild type was set as
the reference. Lanes: 1 to 3, YPD/YPGL GCR1 represents gene
expression patterns in the wild-type strain growing in YPD medium
compared to growth in YPGL medium; 4 to 6, YPGL+G/YPGL GCR1
represents gene expression patterns in the wild-type strain 4 h
after the addition of glucose to YPGL (YPGL+G) compared to growth in
YPGL medium without the addition of glucose; 7 to 9, YPGL+G/YPGL
gcr1 represents gene expression patterns in the
gcr1 mutant strain 4 h after the addition of glucose to
YPGL compared to growth of the mutant in YPGL without glucose; 10 to
12, gcr1/GCR1 YPGL+G represents a comparison of mutant and
wild-type gene expression 4 h after the addition of glucose to
each culture growing in YPGL; 13 to 16, gcr1/GCR1 YPGL
represents a comparison of mutant and wild-type cultures growing in
YPGL medium. The microarray set (Array) and the individual
interrogations (Interrogations) from which the ratios were calculated
are indicated. The ratios of transcript levels are depicted visually
according to the color scale on the figure. Red indicates increased
expression relative to the reference, and green indicates decreased
expression relative to the reference. On the right of the figure is
listed the ORF and gene name for each expression profile. The number of
occurrences of the common sequence motif shown in panel B, within 600 nucleotides upstream of (CT-Box Up) or inside (CT-Box In) each ORF is
also noted. (B) Logos representation (46) of the common
sequence motif found among GCR1-dependent genes by using
AlignACE (43). The height of each letter is proportional to
its frequency at that position. The overall height of the stack at each
position represents the informational content of that position in bits
of information ranging from 0 to 2 bits. This sequence motif closely
resembles the consensus binding site for Gcr1p proposed by Huie et al.
(29), also known as the CT box.
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As a group, the ORFs most severely affected by the
gcr1
lesion encode glycolytic enzymes. We note that of the enzymes assayed,
the phosphoglycerate mutase and enolase activities are the most
severely affected in
gcr1 mutants (
1,
13,
14).
These enzymes
are encoded by the ORFs most strongly affected in the
microarray
experiments. Hybridization to YKL152C (
GPM1) was
reduced 13.8-fold
with material from the
gcr1 mutant
compared to that obtained with
material from the wild type. Similar
reductions were observed
for YHR174W (9.6-fold) and YGR254W (7.9-fold),
the
ENO2 and
ENO1 ORFs, respectively. Two ORFs
were identified, YKL153W and YCR013C,
which partially overlap ORFs,
YKL152C and YCR012W, encoding the
glycolytic enzymes phosphoglycerate
mutase and phosphoglycerate
kinase. The respective ORF pairs are
capable of hybridizing to
the same cDNAs and thus served as an internal
control. The ORF
specifying Gcr1p was also identified in the screen and
served
as a control for our ability to detect differences in genes
expressed
at low levels; the
gcr1 mutant strain used for
these experiments
harbors a deletion of
GCR1 itself. We are
therefore unable to
comment on whether
GCR1 is autoregulated
based on this study,
although the occurrence of an upstream CT box
suggests the possibility.
In addition to ORFs encoding glycolytic
enzymes, ORFs carried
by Ty elements made a second major class of genes
identified.
All Ty1 and Ty2 ORFs present on the arrays were identified
as
GCR1 dependent. In Fig.
1, the ORFs encoding Gag and
Gag-Pol are
listed separately, although the encoded proteins are
derived from
the same transcript, with the Gag-Pol protein resulting
from translational
frameshifting. Work from several laboratories
previously established
the
GCR1 dependency of Ty gene
expression (
19,
51). Fourteen
other ORFs, listed in Fig.
1,
were also identified as
GCR1 dependent
using the twofold
cutoff criterion with the microarray
screen.
The effects of mutations in genes encoding transcription factors on the
transcriptional profile of a cell can be classified
into three
categories. The primary effect would be the loss of
expression of the
transcription factor itself. The secondary effect
would be the loss of
expression of genes that are directly dependent
on the transcription
factor for their expression. The tertiary
effect would involve genes
whose expression is responding to the
altered physiology of the cell
resulting from the primary and
secondary effects. When classifying
genes into sets, two types
of errors can be made: type 1 errors exclude
genes when they should
be included within the set, and type 2 errors
include genes when
they should be excluded. In the present study, the
primary effect
is the effect on
GCR1 expression itself. The
more interesting
class of genes contains those which are directly
dependent on
Gcr1p for their expression; genes of this class would be
expected
to have common DNA sequence motifs through which Gcr1p
acts.
Common sequence motif.
We analyzed the upstream DNA region of
the 53 ORFs identified for common sequence motifs by using the
algorithm AlignACE (43). The algorithm successfully
identified a sequence motif that closely resembled the sequence
previously proposed as the consensus Gcr1p DNA-binding site (2,
29), known as the CT box (9). The motif shown in Fig.
1B identifies the T at position 10 as having a high informational
content. The significance of this position was not appreciated
previously. This sequence motif was found at least once within 600 nucleotides of the translational start of 38 of the 53 genes identified
above, as indicated in Fig. 1. Genes strongly dependent on
GCR1 tended to have multiple copies of the CT-box motif in
their regulatory regions. Conversely, the 10 genes in which CT boxes
were not found tended to exhibit less dependence on GCR1
than the others did. This latter class of genes without CT boxes is
most probably responding to tertiary effects of the gcr1
mutation. Six ORFs having CT boxes in their 5' noncoding region or
within the ORF itself were identified as GCR1 dependent when
they had not previously been recognized as such. They are YML032C,
YLR256W (HAP1), YEL035C (UTR5), YER172C
(BRR2), YMR055C (BUB2), and YPL277C.
CT boxes that score better than the average of the aligned CT boxes
identified in front of the
GCR1-dependent genes occur
elsewhere in the genome. In total, 854 CT boxes were identified,
and
most of these sites (670 of 854) occur within coding regions.
CT boxes
are found scattered throughout the yeast genome resident
on the long
terminal repeats of Ty1 and Ty2 (
12). Altogether,
158 CT
boxes occur within 600 nucleotides of the initiation codon
of ORFs. In
some cases, these motifs were found in front of genes
that were not
identified as
GCR1-dependent genes by the above
criteria.
Two possibilities exist: either these sites are not
Gcr1p-binding
sites, and the cognate genes are not
GCR1-dependent
genes,
or they are but not under the physiological conditions
tested here. We
showed previously, in the context of two glycolytic
enzyme gene UAS
elements, that a CT box alone is not sufficient
for Gcr1p binding in
vivo (
18). In the context of the
TPI1 and
PYK1 UAS elements, Gcr1p binding in vivo requires Rap1p
bound
at an appropriately spaced Rap1p-binding site. There are genetic
and biochemical indications that regions adjacent to Gcr1p-binding
sites in Ty elements are important for
GCR1-mediated
expression
(
19,
51). The observations with Ty elements
suggest that Gcr1p
may have other binding partners in addition to
Rap1p. Thus, it
is possible to envision that there may be
GCR1-dependent genes
for which the binding of Gcr1p is
regulated by another DNA-binding
protein whose expression itself is
regulated. If the hypothetical
binding partner of Gcr1p was not
expressed under the experimental
conditions used, the requirement for
Gcr1p would be
imperceptible.
The sequence-aligning algorithm AlignACE (
43) did not
identify a sequence motif that closely resembled the consensus Rap1p
DNA-binding site among the
GCR1-dependent genes, even though
Rap1p-binding
sites are essential features of several glycolytic enzyme
gene
UAS
elements.
Expression levels of GCR1-dependent genes.
We
compared the hybridization intensities obtained with samples from the
wild-type strain, S150-2B, growing in YPD medium to the results of
serial analysis of gene expression (SAGE), as reported by Velculescu et
al., of yeast strain YPH499 growing in the same medium (54).
In terms of fraction of total hybridization signal for microarray
experiments and occurrences of sequence tags for SAGE experiments,
markers for transcripts encoding Ty Gag and Gag-Pol proteins,
glycolytic enzymes, and ribosomal proteins were in the top 1% of each
list. On the whole, our microarray results are in general agreement
with the SAGE results of Velculescu et al. (54), although
there are some differences between the relative levels of highly
expressed genes, with the most notable being those carried on Ty
elements. The microarray result indicates that Ty transcripts make up
ca. 18% of polyadenylated RNA in YPD-grown cells, whereas the SAGE
results indicate that they make up ca. 0.5% of transcripts
(54). Based on Northern analysis, Curcio et al.
(16) previously estimated that Ty transcripts may make up as
much as 50% of polyadenylated RNA in yeast cells. The reason for the
discrepancy in measurements of Ty transcript abundance is not clear and
may have to do with peculiarities of the three different assay systems
used. However, by whatever measure, Ty transcripts are among the most
abundant in yeast.
Apart from ratio comparisons of genes between conditions or strains,
hybridization intensities of the individual elements
of an array can be
used, within limits, as an indication of the
relative abundance of the
corresponding transcripts within cells.
For the wild-type strain
growing on YPGL, the 53 ORFs identified
above as
GCR1-dependent genes accounted for 27% of the total
hybridization
intensity on the microarray. The same 53 ORFs accounted
for only
6% of the total hybridization when cDNA was prepared from the
gcr1 mutant grown in the same medium. This result indicates
that
as a whole,
GCR1-dependent genes are expressed at very
high levels
in wild-type cells, with 0.9% of the genes (53 of 6,144)
accounting
for 27% of the total hybridization signal. Furthermore, the
residual
level of expression of the
GCR1-dependent genes in
gcr1 mutants,
comprising 6% of the total hybridization
signal, is still quite
high compared to the levels of most genes in the
genome. Thus,
while only a few genes are dependent on Gcr1p for
expression,
together they make up a sizable proportion of the yeast
transcriptome
under these growth
conditions.
Yeast cells growing on YPGL are dependent on gluconeogenesis, which is
required for ribose and cell wall biosynthesis. The
GCR1-independent expression of glycolytic enzyme genes in
gcr1 mutants suggests that the residual glycolytic enzyme
gene expression
is sufficient for gluconeogenesis and
growth.
Glucose induction and repression.
The gcr1 mutant
phenotype afforded us the opportunity to investigate alterations in
gene expression that result from the addition of glucose in the absence
of the rapid growth normally associated with glucose. Therefore, a
series of glucose induction-repression experiments were carried out
with wild-type and gcr1 mutant strains. Due to the severe
growth defect of the gcr1 mutant in the presence of glucose,
we chose the 4-h glucose induction protocol that we had previously used
to monitor glycolytic enzyme gene expression in gcr1 mutants
(1). Figure 1 shows the response of the
GCR1-dependent genes to the addition of glucose to the
growth medium. In the wild-type strain growing under steady-state
conditions in YPD medium, the expression profile of
GCR1-dependent genes was similar to the expression profile
of the wild-type strain after a 4-h glucose induction (Fig. 1, compare
lanes 1 to 3 with lanes 4 to 6). In the gcr1 mutant
background, GCR1-dependent genes usually increase their
level of expression as a result of exposure to glucose (lanes 7 to 9).
However, clear distinctions can be made between the responses of the
GCR1-dependent genes, with two classes emerging. The first
class is composed primarily of genes carried by Ty elements. In
gcr1 mutants, these genes displayed a strong induction ratio
when glucose was added to the medium (lanes 7 to 9), whereas in the
wild-type background, glucose induction of these genes was not observed
(lanes 4 to 6). Accordingly, as can be seen in lanes 10 to 12, the
expression levels of the Ty element genes are similar in the
gcr1 mutant and wild-type strains upon addition of glucose.
The second response class, consisting primarily of glycolytic enzyme
genes, displays a modest level of glucose induction in both wild-type
and mutant backgrounds. Glucose induction of the residual glycolytic
enzyme activities has been noted previously with gcr1 mutant
strains (1, 13), Uemura and Fraenkel (52) have
recently shown that gcr1 mutants are capable of responding
to glucose by increasing their capacity to flux glucose through
glycolysis. The residual expression and glucose induction of
GCR1-dependent genes in the gcr1 mutant
background implies that other regulatory elements are operational in
the absence of Gcr1p.
There are similarities and clear differences in the global response of
the wild-type and
gcr1 mutants to the addition of glucose.
In the wild-type background, 444 genes were induced and 711 genes
were
repressed at the twofold level, whereas in the
gcr1 mutant,
the response to glucose was more restricted, with 211 genes being
induced and 252 genes being repressed. The principal elements
of
glucose repression and induction remained intact in the
gcr1 mutant. Both strains responded to glucose by repressing genes
encoding
respiratory functions. Of the 73 ORFs specifying respiratory
function
present on the arrays used, glucose reduced the expression
of 31 in the
mutant compared to 30 in the wild type by greater
than twofold (Fig.
2A, lanes 4 to 6 and
lanes 1 to 3, respectively).
Likewise, similar patterns of repression
were observed in the
wild type and mutant for genes encoding
tricarboxylic acid pathway
functions; 11 out of 24 genes were reduced
in expression by greater
than twofold in the mutant compared to 13 in
the wild type (Fig.
2B, lanes 4 to 6 and lanes 1 to 3, respectively).
On the other
hand, the expression of glucose transporters appeared to
increase
after the addition of glucose to the cultures; the
hybridization
intensities at the elements specifying
HXT1 to
HXT3 were increased
by more than twofold with cDNA prepared
from both the wild type
and mutant after glucose addition. As a class,
genes subject to
growth rate-dependent regulation were induced in the
wild type
and not in the mutant. The most notable examples of growth
rate-dependent
genes are those encoding ribosomal proteins (
24,
30,
31,
35) (see below). The doubling time of the wild-type
strain decreased
from 280 min in YPGL to 135 min on the addition of
glucose. The
gcr1 mutant, on the other hand, effectively
stopped growing on
the addition of glucose. Its doubling time increased
from 300
to 780 min after glucose was added to the culture medium.

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FIG. 2.
(A) Expression profile of respiratory genes in the
wild type and gcr1 mutants. (B) Expression profile of genes
encoding tricarboxylic acid pathway function in the wild type and
gcr1 mutants. (C) Expression profile of genes encoding
glycerol metabolic enzymes and lactate transporter. Total RNA was
isolated for expression profiling 4 h after the addition of
glucose to wild-type and gcr1 mutant cultures growing in
YPGL. Genes were selected for inclusion in panels A and B based on the
Martinsried Institute of Protein Sciences classification scheme. The
lanes are as described in the legend to Fig. 1. Column headings for
panel C are the same as those for panel B. The ratio of transcript
levels is depicted visually according to the color scale on the figure.
Red indicates increased expression relative to the reference, and green
indicates decreased expression relative to the reference.
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Ribosomal protein gene expression in the wild type and
gcr1 mutants.
Santangelo and Tornow (44)
previously reported that ribosomal protein genes are dependent on Gcr1p
acting through Rap1p-binding sites which are known as
UAS(RPG) sites for ribosomal protein genes. These workers
went on to propose that Gcr1p is recruited to UAS(RPG) via
contacts with Rap1p (57). Figure
3 shows the expression pattern of each of
the 132 ribosomal protein genes represented on the arrays that we used.
Figure 3, lanes 13 to 16, show that as a class, ribosomal protein gene
expression appears to be slightly enhanced in the gcr1
mutant growing in YPGL compared to that in wild-type strains growing in
the same medium. Elements on the array specifying cytoplasmic ribosomal
protein genes accounted for 14% of the total hybridization signal in
the mutant compared to 9% for the wild type in the permissive medium
YPGL. Rap1p-binding sites, as noted in Fig. 3B, are the most prominent
feature in the regulatory regions of ribosomal protein genes
(32). The expression profile of ribosomal protein genes in
YPGL indicates that Gcr1p is not required for the expression of
ribosomal protein genes per se. The apparent increase in expression of
ribosomal protein genes under this condition in the gcr1
mutant may be due to increased availability of the transcriptional
apparatus that otherwise would be engaged in transcribing
GCR1-dependent genes.

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|
FIG. 3.
(A) Expression profile of 132 cytoplasmic ribosomal
protein genes in wild-type and gcr1 mutant strains growing
in the absence and presence of glucose. The lanes are as described in
the legend to Fig. 1. The ratio of transcript levels is depicted
visually according to the color scale on the figure. Red indicates
increased expression relative to the reference, and green indicates
decreased expression relative to the reference. (B) Logos
representation (46) of the common sequence motif found 166 times among the 132 cytoplasmic ribosomal protein genes by using
AlignACE (43). The height of each position represents the
informational content of that position in bits of information. This
sequence motif closely resembles the consensus binding site for Rap1p
as recently refined by Lascaris et al. (32).
|
|
The expression of cytoplasmic ribosomal protein genes in wild-type and
gcr1 mutant strains differed markedly on the addition
of
glucose to the growth medium (Fig.
3, compare lanes 4 to 6
with lanes 7 to 9). With wild-type cells, exposure to glucose
resulted in an
increase in the expression level of ribosomal protein
genes to 21% of
the total hybridization signal, which is in agreement
with the SAGE
results of Velculescu et al. (
54) as noted by
Lascaris et
al. (
32). By contrast, addition of glucose to cultures
of
gcr1 mutants resulted in a reduction in the level of
ribosomal
protein gene expression from 14 to 11% of the total
hybridization
signal. It is important to recall, as noted above, that
addition
of glucose to the growth medium of wild-type cells results in
an increased growth rate whereas a similar addition to mutant
cultures
results in drastic reductions in the growth rate. Both
growth rate
differences and differences in ribosomal protein gene
expression
patterns were magnified in comparisons between wild-type
and mutant
strains after glucose induction (Fig.
3, lanes 10 to
12). The
differences in ribosomal protein gene expression between
the wild type
and the
gcr1 mutant can best be explained by differences
in
growth rates between the two strains. Ribosomal protein gene
expression
is known to be subject to growth rate-dependent regulation
(
24,
30,
31,
35). The expression profiles of ribosomal
protein genes
presented here argue for an important but indirect
role of Gcr1p in
ribosomal protein gene expression. The expression
profile of ribosomal
protein genes in
gcr1 mutants growing in
the presence of
glucose is a manifestation of a tertiary effect
of the
gcr1 mutation.
Gene expression and growth phenotype.
One of the most striking
features of gcr1 mutants is the severe growth defect that
they exhibit when grown on media containing glucose, whereas they grow
relatively normally under gluconeogenic conditions (13). The
gene expression pattern observed with the gcr1 mutant
provides an explanation for the growth phenotypes of the mutant. Figure
2 shows that upon addition of glucose to a medium otherwise permissive
for growth, such as YP supplemented with glycerol plus lactate, the
mutant, like the wild-type strain, responded by repressing the
expression of genes encoding key respiratory enzymes, genes involved in
trichloroacetic acid cycle function, and genes involved in the
metabolism of alternative carbon sources. Repression of
GUT1, GUT2, and JEN1 is relevant to
the discussion here (Fig. 2C). These genes encode functions that are
required for the utilization of glycerol and lactate. GUT1
and GUT2 encode the activities required for the conversion
of glycerol to dihydroxyacetone-phosphate (40, 41), and
JEN1 encodes a lactate-proton symporter required to
transport lactate into the cell (8). In the wild-type and gcr1 mutant strains, each of the aforementioned genes was
repressed by fourfold or greater on addition of glucose. Thus, the
gcr1 mutant responds to glucose, as do wild-type cells, by
reprogramming its gene expression profile to take advantage of the
available glucose; however, the gcr1 mutant does not have
sufficient glycolytic enzyme gene expression to permit normal rates of
growth when utilizing glucose alone. The plight of the mutant is
further aggravated by glucose repression, which suppresses the
expression of genes involved in metabolism of other energy sources,
thereby robbing the cell of any potential of utilizing the available
carbon sources, notably lactate and minor constituents of YP.
 |
ACKNOWLEDGMENTS |
We thank Frank Rosenzweig, Hiroshi Uemura, and Phil Farabaugh for
advice and comments. We thank Eisen et al. (20) and Roth et
al. (43) for making their algorithms freely available.
This work was supported in part by a grant from the National Science
Foundation (MCB 9816990).
 |
FOOTNOTES |
*
Corresponding author. Mailing address: Department of
Molecular Genetics and Microbiology, Box 100266, University of Florida, Gainesville, FL 32610-0266. Phone: (352) 392-0680. Fax: (352) 392-3133. E-mail: hvbaker{at}ufl.edu.
 |
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Deminoff, S. J., Santangelo, G. M.
(2001). Rap1p Requires Gcr1p and Gcr2p Homodimers to Activate Ribosomal Protein and Glycolytic Genes, Respectively. Genetics
158: 133-143
[Abstract]
[Full Text]