Positive regulation in the general amino acid control of Saccharomyces cerevisiae

Regulatory region of the Aspergillus nidulans argB gene

We have constructed a series of deletion plasmids which contain the Aspergillus nidulans argB gene for ornithine carbamoyltransferase (OTC). These deletions comprise the 5' upstream sequence of the argB gene. The pro- arg- strain of A. nidulans was transformed with the above plasmids. Several arg+ transformants of integration types I and II, obtained using each of the deletion plasmids, were studied, and their ability to de-repress OTC level by proline starvation was compared. It was concluded that nucleotides located between -150 and -50 bp upstream of the argB gene are significant for its cross-pathway regulation. This regulatory region contains three copies of the TGACTC hexanucleotide which is a cis-acting regulatory sequence of general amino acid control in yeast.

Molecular characterization of GCD1, a yeast gene required for general control of amino acid biosynthesis and cell-cycle initiation

The GCD1 gene product of Saccharomyces cerevisiae has been implicated in the coordination of the cell cycle with the general control of amino acid biosynthesis (M. Wolfner et al., J. Mol. Biol. 96:273-290, 1975). Strains containing the gcd1-1 allele constitutively express the amino acid biosynthetic genes at the induced levels normally found only during conditions of amino acid starvation. In addition, gcd1-1 strains do not grow at high temperatures because under these conditions they are unable to proceed beyond the START step of the cell division cycle. We have cloned and sequenced the GCD1 gene and examined various aspects of cellular metabolism in order to elucidate its role(s) in regulating gene expression and the cell cycle. GCD1 encodes a 1.7 kb RNA whose expression is not regulated as a function of amino acid starvation. Overexpression of this RNA does not affect the regulation of amino acid biosynthetic genes or cell growth. GCD1 is an essential gene because cells containing a gcd1-HIS3 disruption are unable to grow. The essential function of GCD1 may be involved in protein synthesis because a gcd1-1 strain incorporates low levels of 35S-methionine into protein when cells are shifted to the restrictive temperature. GCD1 encodes a protein of 511 amino acids whose predicted sequence does not exhibit significant homology to any other known proteins and appears too large to be a ribosomal protein. We suggest that GCD1 encodes a component of the normal protein synthesis machinery that is involved in the translational regulation of GCN4, a protein that coordinately activates the transcription of amino acid biosynthetic genes. GCD1 may also be part of a sensing mechanism in which cells monitor the protein synthesis capacity prior to initiating a new cell division cycle.

The cross-pathway control gene of Neurospora crassa, cpc-1, encodes a protein similar to GCN4 of yeast and the DNA-binding domain of the oncogene v-jun-encoded protein

Expression of the gene cpc-1 is required for cross-pathway-mediated regulation of amino acid-biosynthetic genes in Neurospora crassa. We have cloned cpc-1 and present an analysis of its structure and regulation. The cpc-1-encoded transcript contains three open reading frames, two of which are located in the 720-nucleotide leader segment preceding the cpc-1 coding region. The two leader open reading frames, if translated, would produce peptides 20 and 41 residues in length. The deduced amino acid sequence of the cpc-1 polypeptide, CPC1, contains segments similar to the DNA-binding and transcriptional activation domains of GCN4, the major cross-pathway regulatory protein of yeast. The structural and functional similarities of CPC1 and GCN4 proteins suggest that cpc-1 encodes the analogous transcriptional activator of N. crassa. Messenger RNA measurements indicate that cpc-1 is transcriptionally regulated in response to amino acid starvation. The segment of CPC1 similar to the DNA-binding domain of GCN4 also is similar to the DNA-binding domains of the avian sarcoma virus oncogene-encoded v-JUN protein and human c-JUN protein.

crl mutants of Saccharomyces cerevisiae resemble both mutants affecting general control of amino acid biosynthesis and omnipotent translational suppressor mutants

Cyocloheximide resistant lethal (crl) mutants of Saccharomyces cerevisiae, defining 22 unlinked complementation groups, are unable to grow at 37 degrees. They are also highly pleiotropic at their permissive temperature of 25 degrees. The mutants are all unable to arrest at the G1 stage of the cell cycle when grown to stationary phase or when starved for a single amino acid, though they do arrest at G1 when deprived of all nitrogen. The crl mutants are also hypersensitive to various amino acid analogs and to 3-aminotriazole. These mutants also "tighten" leaky auxotrophic mutations that permit wild-type cells to grow in the absence of the appropriate amino acid. All of these phenotypes are also exhibited by gcn mutants affecting general control of amino acid biosynthesis. In addition, the crl mutants are all hypersensitive to hygromycin B, an aminoglycoside antibiotic that stimulates translational misreading. The crl mutations also suppress one nonsense mutation which is phenotypically suppressed by hygromycin B. Many crl mutants are also osmotically sensitive. These are phenotypes which the crl mutations have in common with previously isolated omnipotent suppressors. We suggest that the the crl mutations all affect the fidelity of protein translation.

The HAP3 regulatory locus of Saccharomyces cerevisiae encodes divergent overlapping transcripts

Activation of the CYC1 upstream activation site, UAS2, and transcription of several other genes encoding respiratory functions requires the product of the regulatory gene HAP2. We report here the isolation and characterization of a second UAS2 regulatory gene, HAP3. Like mutations in HAP2, a mutation in HAP3 abolishes the activity of UAS2 and prevents growth on nonfermentable carbon sources. The HAP3 gene was cloned and, surprisingly, was found to encode two divergently transcribed, overlapping transcripts: a 570-base RNA and a 3-kilobase (kb) RNA. Chromosomal disruption experiments defined the critical region for HAP3 function to a 1.3-kb segment in which the two transcripts overlap. Analysis of the HAP3 DNA sequence showed that the 570-base transcript could encode a protein of 144 amino acids. Synthesis of the 144-amino-acid protein under regulatory control in vivo demonstrated that this protein is essential for activity of UAS2 as well as for growth on nonfermentable carbon sources. The largest open reading frame in the critical region of the 3-kb transcript is only 86 amino acids. Using site-directed mutagenesis, we demonstrated that the 86-amino-acid open reading frame was not involved in UAS2 activity. The possible role of this 3-kb antisense RNA in HAP3 expression or function is discussed.

Mutational analysis of upstream activation sequence 2 of the CYC1 gene of Saccharomyces cerevisiae: a HAP2-HAP3-responsive site

We analyzed upstream activation sequence 2 (UAS2), one of two independent UAS elements in the CYC1 gene of Saccharomyces cerevisiae. Deletions and linker scanning mutations across the 87 base pairs previously defined as UAS2 showed two separate functional elements required for full activity. Region 1, from -230 to -200, contains the principal activation site and responds to the trans-acting regulatory loci HAP2 and HAP3. A portion of region 1 is homologous to two other HAP2-HAP3-responsive UASs and includes the G----A transition mutation UP1, which increases UAS2 activity. This consensus sequence TNATTGGT bears striking similarity to several CAAT box sequences of higher cells. Region 2, from -192 to -178, substantially enhances the activity of region 1, yet has little activity by itself. These regions bind distinct proteins found in crudely fractionated yeast extracts.

ATR1, a Saccharomyces cerevisiae gene encoding a transmembrane protein required for aminotriazole resistance

In Saccharomyces cerevisiae, 3-amino-1,2,4-triazole (aminotriazole) competitively inhibits the activity of imidazoleglycerolphosphate dehydratase, the product of the HIS3 gene. Wild-type strains are able to grow in the presence of 10 mM aminotriazole because they induce the level of imidazoleglycerolphosphate dehydratase. However, strains containing gcn4 mutations are unable to grow in medium containing aminotriazole because they lack the GCN4 transcriptional activator protein necessary for the coordinate induction of HIS3 and other amino acid biosynthetic genes. Here, we isolated a new gene, designated ATR1, which when present in multiple copies per cell allowed gcn4 mutant strains to grow in the presence of aminotriazole. In wild-type strains, multiple copies of ATR1 permitted growth at extremely high concentrations of aminotriazole (80 mM), whereas a chromosomal deletion of ATR1 caused growth inhibition at very low concentrations (5 mM). When radioactive aminotriazole was added exogenously, cells with multiple copies of ATR1 accumulated less aminotriazole than wild-type cells, whereas cells with the atr1 deletion mutation retained more aminotriazole. Unlike the mammalian mdr or yeast PDR genes that confer resistance to many drugs, ATR1 appears to confer resistance only to aminotriazole. Genetic analysis, mRNA mapping, and DNA sequencing revealed that (i) the primary translation product of ATR1 contains 547 amino acids, (ii) ATR1 transcription is induced by aminotriazole, and (iii) the ATR1 promoter region contains a binding site for the GCN4 activator protein. The deduced amino acid sequence suggests that ATR1 protein is very hydrophobic with many membrane-spanning regions, has several potential glycosylation sites, and may contain an ATP-binding site. We suggest that ATR1 encodes a membrane-associated component of the machinery responsible for pumping aminotriazole (and possibly other toxic compounds) out of the cell.

Sequence and expression of NUC1, the gene encoding the mitochondrial nuclease in Saccharomyces cerevisiae

The DNA sequence and studies on the expression of the NUC1 gene from Saccharomyces cerevisiae are presented. The NUC1 locus is located in the distal portion of the left arm of Chromosome X and encodes the major nuclease found in mitochondria. The inferred amino acid sequence of NUC1 predicts that the nuclease is basic, rich in prolines, of average hydrophobicity, and has a molecular weight for the primary translation product of 37,209 daltons. NUC1 is very poorly expressed, consistent with the codon usage bias determined from the DNA sequence and our previous determination of the number of enzyme molecules per cell. Mapping of the 5' terminus of the NUC1 mRNA reveals that the mRNA has a long 400 base untranslated leader in which are found three open reading frames, each initiated by an AUG. The possibility that these upstream open reading frames contribute to the poor expression of the NUC1 gene is discussed.

The protein factor which binds to the upstream activating sequence of Saccharomyces cerevisiae ENO1 gene

Using a gel retardation assay it was shown that the 87 bp DNA fragment (UAS87) containing the upstream activating sequence (UAS) of S. cerevisiae EN01 gene and a nuclear extract gave rise to three migration-retarded species specific to UAS87. Heat- or proteinase-treatment of the nuclear extract revealed that these species were protein-DNA complexes. The precise binding region of the protein identified by DNaseI protection analysis was found to include a CCAAACA sequence which forms a dyad-symmetrical structure. The amount of one of the three migration-retarded species significantly increased when cells were grown in medium containing a gluconeogenic carbon source. The introduction of pGCR8, a multicopy plasmid containing GCR1 gene, a regulatory gene controlling the expression of several glycolytic enzymes, showed no effect on the amount of three migration-retarded species.

ATR1, a Saccharomyces cerevisiae gene encoding a transmembrane protein required for aminotriazole resistance

In Saccharomyces cerevisiae, 3-amino-1,2,4-triazole (aminotriazole) competitively inhibits the activity of imidazoleglycerolphosphate dehydratase, the product of the HIS3 gene. Wild-type strains are able to grow in the presence of 10 mM aminotriazole because they induce the level of imidazoleglycerolphosphate dehydratase. However, strains containing gcn4 mutations are unable to grow in medium containing aminotriazole because they lack the GCN4 transcriptional activator protein necessary for the coordinate induction of HIS3 and other amino acid biosynthetic genes. Here, we isolated a new gene, designated ATR1, which when present in multiple copies per cell allowed gcn4 mutant strains to grow in the presence of aminotriazole. In wild-type strains, multiple copies of ATR1 permitted growth at extremely high concentrations of aminotriazole (80 mM), whereas a chromosomal deletion of ATR1 caused growth inhibition at very low concentrations (5 mM). When radioactive aminotriazole was added exogenously, cells with multiple copies of ATR1 accumulated less aminotriazole than wild-type cells, whereas cells with the atr1 deletion mutation retained more aminotriazole. Unlike the mammalian mdr or yeast PDR genes that confer resistance to many drugs, ATR1 appears to confer resistance only to aminotriazole. Genetic analysis, mRNA mapping, and DNA sequencing revealed that (i) the primary translation product of ATR1 contains 547 amino acids, (ii) ATR1 transcription is induced by aminotriazole, and (iii) the ATR1 promoter region contains a binding site for the GCN4 activator protein. The deduced amino acid sequence suggests that ATR1 protein is very hydrophobic with many membrane-spanning regions, has several potential glycosylation sites, and may contain an ATP-binding site. We suggest that ATR1 encodes a membrane-associated component of the machinery responsible for pumping aminotriazole (and possibly other toxic compounds) out of the cell.

The HAP3 regulatory locus of Saccharomyces cerevisiae encodes divergent overlapping transcripts

Activation of the CYC1 upstream activation site, UAS2, and transcription of several other genes encoding respiratory functions requires the product of the regulatory gene HAP2. We report here the isolation and characterization of a second UAS2 regulatory gene, HAP3. Like mutations in HAP2, a mutation in HAP3 abolishes the activity of UAS2 and prevents growth on nonfermentable carbon sources. The HAP3 gene was cloned and, surprisingly, was found to encode two divergently transcribed, overlapping transcripts: a 570-base RNA and a 3-kilobase (kb) RNA. Chromosomal disruption experiments defined the critical region for HAP3 function to a 1.3-kb segment in which the two transcripts overlap. Analysis of the HAP3 DNA sequence showed that the 570-base transcript could encode a protein of 144 amino acids. Synthesis of the 144-amino-acid protein under regulatory control in vivo demonstrated that this protein is essential for activity of UAS2 as well as for growth on nonfermentable carbon sources. The largest open reading frame in the critical region of the 3-kb transcript is only 86 amino acids. Using site-directed mutagenesis, we demonstrated that the 86-amino-acid open reading frame was not involved in UAS2 activity. The possible role of this 3-kb antisense RNA in HAP3 expression or function is discussed.

Mutational analysis of upstream activation sequence 2 of the CYC1 gene of Saccharomyces cerevisiae: a HAP2-HAP3-responsive site

We analyzed upstream activation sequence 2 (UAS2), one of two independent UAS elements in the CYC1 gene of Saccharomyces cerevisiae. Deletions and linker scanning mutations across the 87 base pairs previously defined as UAS2 showed two separate functional elements required for full activity. Region 1, from -230 to -200, contains the principal activation site and responds to the trans-acting regulatory loci HAP2 and HAP3. A portion of region 1 is homologous to two other HAP2-HAP3-responsive UASs and includes the G----A transition mutation UP1, which increases UAS2 activity. This consensus sequence TNATTGGT bears striking similarity to several CAAT box sequences of higher cells. Region 2, from -192 to -178, substantially enhances the activity of region 1, yet has little activity by itself. These regions bind distinct proteins found in crudely fractionated yeast extracts.

Isolation and expression analysis of two yeast regulatory genes involved in the derepression of glucose-repressible enzymes

Yeast strains carrying one of the two regulatory mutations cat1 and cat3 are defective in derepression of several glucose-repressible enzymes that are necessary for utilizing non-fermentable carbon sources. Hence, these strains fail to grow on ethanol, glycerol or acetate. The synthesis of isocitrate lyase, malate synthase, malate dehydrogenase and fructose-1,6-bisphosphatase is strongly affected in cat1 and cat3 strains. Genes CAT1 and CAT3 have been isolated by complementation of the cognate mutations after transformation with an episomal plasmid gene library. The restriction map of CAT1 proved its allelism to the earlier isolated SNF1 gene. Both genes appear to exist as single-copy genes per haploid genome as indicated by Southern hybridization. Northern analysis has shown that the 1.35 kb CAT3 mRNA is constitutively expressed, independent of the carbon source in the medium. Derepression studies with CAT3 transformants using a multi-copy plasmid showed over-expression of glyoxylate cycle enzymes. This result would be consistent with a direct effector function for the CAT3 gene product.

A segment of GCN4 mRNA containing the upstream AUG codons confers translational control upon a heterologous yeast transcript

GCN4 encodes a transcriptional activator in Saccharomyces cerevisiae that is regulated at the translational level. We show that an approximately 240-nucleotide segment from the GCN4 mRNA leader containing four AUG codons is sufficient to confer translational control typical of GCN4 upon a GAL1-lacZ fusion transcript. Regulation of the hybrid transcript is dependent upon multiple positive (GCN) and negative (GCD) trans-acting factors shown to regulate GCN4 expression post-transcriptionally. This result limits the target sequences for these factors to a small internal segment of the GCN4 mRNA leader. The elimination of AUG codons within this segment substantially reduces the usual derepressing effect of mutations in five GCD genes upon GCN4-lacZ expression. This supports the idea that the products of these negative regulatory genes act by modulating the effects of the upstream AUG codons on translation of GCN4 mRNA.

Homology between the DNA-binding domain of the GCN4 regulatory protein of yeast and the carboxyl-terminal region of a protein coded for by the oncogene jun

The product of the recently described oncogene jun shows significant amino acid sequence homology with the GCN4 yeast transcriptional activator protein. The similarity is restricted to the 66 carboxyl-terminal amino acids, thought to be the DNA-binding domain of the GCN4 protein. In these alpha-helix-permissive regions of the jun and GCN4 products there is also a lesser but still significant amino acid resemblance to the fos protein and a marginal degree of similarity to myc proteins. The amino acid sequence homology between GCN4 and jun gene products suggests that the jun protein may bind to DNA in a sequence-specific way and exert a regulatory function.

The membrane-associated enzyme phosphatidylserine synthase is regulated at the level of mRNA abundance

To precisely define the functional sequence of the CHO1 gene from Saccharomyces cerevisiae, encoding the regulated membrane-associated enzyme phosphatidylserine synthase (PSS), we subcloned the original 4.5-kilobase (kb) CHO1 clone. In this report a 2.8-kb subclone was shown to complement the ethanolamine-choline auxotrophy and to repair the defect in the synthesis of phosphatidylserine, both of which are characteristic of cho1 mutants. When this subclone was used as a hybridization probe of Northern and slot blots of RNA from wild-type cells, the abundance of a 1.2-kb RNA changed in response to alterations in the levels of the soluble phospholipid precursors inositol and choline. The addition of inositol led to a 40% repression of the 1.2-kb RNA level, while the addition of choline and inositol led to an 85% repression. Choline alone had little repressive effect. The level of 1.2-kb RNA closely paralleled the level of PSS activity found in the same cells as determined by enzyme assays. Disruption of the CHO1 gene resulted in the simultaneous disappearance of 1.2-kb RNA and PSS activity. Cells bearing the ino2 or ino4 regulatory mutations, which exhibit constitutively repressed levels of a number of phospholipid biosynthetic enzymes, had constitutively repressed levels of 1.2-kb RNA and PSS activity. Another regulatory mutation, opi1, which causes the constitutive derepression of PSS and other phospholipid biosynthetic enzymes, caused the constitutive derepression of the 1.2-kb RNA. When cho1 mutant cells were transformed with the 2.8-kb subclone on a single-copy plasmid, the 1.2-kb RNA and PSS activity levels were regulated in a wild-type fashion. The presence of the 2.8-kb subclone on a multicopy plasmid, however, led to the constitutive overproduction of 1.2-kb RNA and PSS in cho1 cells.

Two related regulatory sequences are required for maximal induction of Saccharomyces cerevisiae his3 transcription

In Saccharomyces cerevisiae, the coordinate induction of his3 and other amino acid biosynthesis genes is mediated by the binding of GCN4 activator protein to specific promoter sequences. The his3 regulatory region contains the sequence TGACTC, which with some variation is repeated six times upstream of the mRNA initiation site. The requirements for maximal his3 induction were examined with a series of sequential 5' deletion mutations as well as a set of small internal deletions. Deletions encroaching as far downstream as position -142 behave indistinguishably from the wild-type gene, thus indicating that the two proximal copies of the regulatory sequence are sufficient for maximal induction. Deletions with breakpoints between -137 and -99 confer inducibility, but not to the normal wild-type level. A deletion ending immediately upstream of the proximal TGACTC sequence (position -99) shows some constitutive expression that is independent of the gcn4 gene product. Deletions extending to -94 or beyond do not produce detectable levels of his3 mRNA. Small internal deletions that only remove the proximal regulatory sequence and a 1-base-pair deletion of the thymine residue at -99 abolish induction, but do not affect the basal level of transcription. These results indicate that the proximal copy between -99 and -94 is absolutely required for his3 induction, whereas the copy between -142 and -137 is required only for the maximal level of induction and is inactive by itself. From these and other observations, we suggest the possibility that these related regulatory sequences may be targets for two distinct proteins.

Sequence and nuclear localization of the Saccharomyces cerevisiae HAP2 protein, a transcriptional activator

Activation of the CYC1 upstream activation site (UAS2) and other Saccharomyces cerevisiae genes encoding respiratory functions requires the products of the regulatory loci HAP2 and HAP3. We present here the DNA sequence of the yeast HAP2 gene and an initial investigation into the function of its product. The DNA sequence indicated that HAP2 encoded a 265-amino-acid protein whose carboxyl third was highly basic. Also found in the sequence was a polyglutamine tract spanning residues 120 to 133. Several experiments described herein suggest that HAP2 encodes a direct activator of transcription. First, a bifunctional HAP2-beta-galactosidase fusion gene was localized to the yeast nucleus. Second, a lexA-HAP2 fusion gene was capable of activating transcription when bound to a lexA operator site. The additional requirement for the HAP3 product in activation is discussed.

MMS sensitivity of all amino acid-requiring mutants in aspergillus and its suppression by mutations in a single gene

All available amino acid-requiring mutants of Aspergillus nidulans were found to be hypersensitive to MMS (methyl methanesulfonate) to various degrees. On MMS media, secondary mutations could be selected which suppress this MMS sensitivity but do not affect the requirement. Many such mutations were analyzed and found to be alleles of one gene, smsA (= suppressor of MMS sensitivity), which mapped distal on the right arm of chromosome V. This gene is more likely to be involved in general regulation of amino acid biosynthesis than MMS uptake, since a variety of pathway interactions were clearly modified by smsA suppressors in the absence of MMS.

Multiple GCD genes required for repression of GCN4, a transcriptional activator of amino acid biosynthetic genes in Saccharomyces cerevisiae

GCN4 encodes a positive regulator of multiple unlinked genes encoding amino acid biosynthetic enzymes in Saccharomyces cerevisiae. Expression of GCN4 is coupled to amino acid availability by a control mechanism involving GCD1 as a negative effector and GCN1, GCN2, and GCN3 as positive effectors of GCN4 expression. We used reversion of a gcn2 gcn3 double mutation to isolate new alleles of GCD1 and mutations in four additional GCD genes which we designate GCD10, GCD11, GCD12, and GCD13. All of the mutations lead to constitutive derepression of HIS4 transcription in the absence of the GCN2+ and GCN3+ alleles. By contrast, the gcd mutations require the wild-type GCN4 allele for their derepressing effect, suggesting that each acts by influencing the level of GCN4 activity in the cell. Consistent with this interpretation, mutations in each GCD gene lead to constitutive derepression of a GCN4::lacZ gene fusion. Thus, at least five gene products are required to maintain the normal repressed level of GCN4 expression in nonstarvation conditions. Interestingly, the gcd mutations are pleiotropic and also affect growth rate in nonstarvation conditions. In addition, certain alleles lead to a loss of M double-stranded RNA required for the killer phenotype. This pleiotropy suggests that the GCD gene products contribute to an essential cellular function, in addition to, or in conjunction with, their role in GCN4 regulation.

Expression of the Saccharomyces cerevisiae inositol-1-phosphate synthase (INO1) gene is regulated by factors that affect phospholipid synthesis

The INO1 gene of Saccharomyces cerevisiae encodes the regulated enzyme inositol-1-phosphate synthase, which catalyzes the first committed step in the synthesis of inositol-containing phospholipids. The expression of this gene was analyzed under conditions known to regulate phospholipid synthesis. RNA blot hybridization with a genomic clone for INO1 detected two RNA species of 1.8 and 0.6 kb. The abundance of the 1.8-kb RNA was greatly decreased when the cells were grown in the presence of the phospholipid precursor inositol, as was the enzyme activity of the synthase. Complementation analysis showed that this transcript encoded the INO1 gene product. The level of INO1 RNA was repressed 12-fold when the cells were grown in medium containing inositol, and it was repressed 33-fold when the cells were grown in the presence of inositol and choline together. The INO1 transcript was present at a very low level in cells containing mutations (ino2 and ino4) in regulatory genes unlinked to INO1 that result in inositol auxotrophy. The transcript was constitutively overproduced in cells containing a mutation (opi1) that causes constitutive expression of inositol-1-phosphate synthase and results in excretion of inositol. The expression of INO1 RNA was also examined in cells containing a mutation (cho2) affecting the synthesis of phosphatidylcholine. In contrast to what was observed in wild-type cells, growth of cho2 cells in medium containing inositol did not result in a significant decrease in INO1 RNA abundance. Inositol and choline together were required for repression of the INO1 transcript in these cells, providing evidence for a regulatory link between the synthesis of inositol- and choline-containing lipids. The level of the 0.6-kb RNA was affected, although to a lesser degree, by many of the same factors that influence INO1 expression.

Sequence and nuclear localization of the Saccharomyces cerevisiae HAP2 protein, a transcriptional activator

Activation of the CYC1 upstream activation site (UAS2) and other Saccharomyces cerevisiae genes encoding respiratory functions requires the products of the regulatory loci HAP2 and HAP3. We present here the DNA sequence of the yeast HAP2 gene and an initial investigation into the function of its product. The DNA sequence indicated that HAP2 encoded a 265-amino-acid protein whose carboxyl third was highly basic. Also found in the sequence was a polyglutamine tract spanning residues 120 to 133. Several experiments described herein suggest that HAP2 encodes a direct activator of transcription. First, a bifunctional HAP2-beta-galactosidase fusion gene was localized to the yeast nucleus. Second, a lexA-HAP2 fusion gene was capable of activating transcription when bound to a lexA operator site. The additional requirement for the HAP3 product in activation is discussed.

Saccharomyces cerevisiae PHO5 promoter region: location and function of the upstream activation site

Saccharomyces cerevisiae repressible acid phosphatase (PHO5) is induced when inorganic phosphate in the culture medium is depleted. To study the mechanism of this regulation, we constructed various deletions in the 5'-flanking region of the PHO5 gene. Two elements were revealed by this analysis: an upstream activation site (UAS) and a downstream element, both playing parts in the expression of this gene. The UAS is located between -384 and -292 upstream of the initiation codon and activates expression of the gene when inorganic phosphate is depleted. It consists of two homologous regions (UAS I and UAS II) that contain CTGCACAAATG and an adenine-plus-thymine-rich sequence, either one of which suffices for the effect. The downstream element includes a putative TATA box at -100 from the ATG codon and is necessary for efficient transcription and expression of the normal-sized PHO5 transcript. The distance between the UAS and the downstream element can be altered without causing loss of expression efficiency, and the action of the UAS is not affected by its orientation. These results are consistent with a model wherein UAS acts as a site of activation for transcription by interaction with a protein factor(s) that becomes active when inorganic phosphate is depleted from the culture medium.

Two related regulatory sequences are required for maximal induction of Saccharomyces cerevisiae his3 transcription

In Saccharomyces cerevisiae, the coordinate induction of his3 and other amino acid biosynthesis genes is mediated by the binding of GCN4 activator protein to specific promoter sequences. The his3 regulatory region contains the sequence TGACTC, which with some variation is repeated six times upstream of the mRNA initiation site. The requirements for maximal his3 induction were examined with a series of sequential 5' deletion mutations as well as a set of small internal deletions. Deletions encroaching as far downstream as position -142 behave indistinguishably from the wild-type gene, thus indicating that the two proximal copies of the regulatory sequence are sufficient for maximal induction. Deletions with breakpoints between -137 and -99 confer inducibility, but not to the normal wild-type level. A deletion ending immediately upstream of the proximal TGACTC sequence (position -99) shows some constitutive expression that is independent of the gcn4 gene product. Deletions extending to -94 or beyond do not produce detectable levels of his3 mRNA. Small internal deletions that only remove the proximal regulatory sequence and a 1-base-pair deletion of the thymine residue at -99 abolish induction, but do not affect the basal level of transcription. These results indicate that the proximal copy between -99 and -94 is absolutely required for his3 induction, whereas the copy between -142 and -137 is required only for the maximal level of induction and is inactive by itself. From these and other observations, we suggest the possibility that these related regulatory sequences may be targets for two distinct proteins.

The membrane-associated enzyme phosphatidylserine synthase is regulated at the level of mRNA abundance

To precisely define the functional sequence of the CHO1 gene from Saccharomyces cerevisiae, encoding the regulated membrane-associated enzyme phosphatidylserine synthase (PSS), we subcloned the original 4.5-kilobase (kb) CHO1 clone. In this report a 2.8-kb subclone was shown to complement the ethanolamine-choline auxotrophy and to repair the defect in the synthesis of phosphatidylserine, both of which are characteristic of cho1 mutants. When this subclone was used as a hybridization probe of Northern and slot blots of RNA from wild-type cells, the abundance of a 1.2-kb RNA changed in response to alterations in the levels of the soluble phospholipid precursors inositol and choline. The addition of inositol led to a 40% repression of the 1.2-kb RNA level, while the addition of choline and inositol led to an 85% repression. Choline alone had little repressive effect. The level of 1.2-kb RNA closely paralleled the level of PSS activity found in the same cells as determined by enzyme assays. Disruption of the CHO1 gene resulted in the simultaneous disappearance of 1.2-kb RNA and PSS activity. Cells bearing the ino2 or ino4 regulatory mutations, which exhibit constitutively repressed levels of a number of phospholipid biosynthetic enzymes, had constitutively repressed levels of 1.2-kb RNA and PSS activity. Another regulatory mutation, opi1, which causes the constitutive derepression of PSS and other phospholipid biosynthetic enzymes, caused the constitutive derepression of the 1.2-kb RNA. When cho1 mutant cells were transformed with the 2.8-kb subclone on a single-copy plasmid, the 1.2-kb RNA and PSS activity levels were regulated in a wild-type fashion. The presence of the 2.8-kb subclone on a multicopy plasmid, however, led to the constitutive overproduction of 1.2-kb RNA and PSS in cho1 cells.

A rapid method for determining tRNA charging levels in vivo: analysis of yeast mutants defective in the general control of amino acid biosynthesis

We describe a simple method to quantitate the intracellular levels of charged tRNA species representing all 20 amino acids. Small RNA species are isolated from yeast cells under conditions where amino acids remain bound to their cognate tRNAs. After chromatographic removal of free amino acids, the tRNAs are discharged, and the amounts of the released amino acids are then quantitated. This method was applied to yeast cells from a wild type strain and from three mutant strains that are defective both in the general control of amino acid biosynthesis and in protein synthesis. Two of these mutant strains, previously shown to be defective in the methionine or isoleucine tRNA synthetases, respectively contain undetectable amounts of charged methionine or isoleucine although their levels of the remaining 19 amino acids are similar to a wild type strain. In contrast, a gcd1 mutant strain has normal levels of all 20 amino-acyl tRNA species. Thus, gcd1 strains are defective in general control of amino acid biosynthesis for reasons other than artifactual starvation of an amino acid due to a failure in tRNA changing.

Negative regulatory gene for general control of amino acid biosynthesis in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, many amino acid biosynthetic pathways are coregulated by a complex general control system: starvation for a single amino acid results in the derepression of amino acid biosynthetic genes in multiple pathways. Derepression of these genes is mediated by positive (GCN) and negative (GCD) regulatory genes. In this paper we describe the isolation and characterization of a previously unreported negative regulatory gene, GCD3. A gcd3 mutation is recessive to wild type, confers resistance to multiple amino acid analogs, and results in overproduction and partially constitutive elevation of mRNA levels for amino acid biosynthetic genes. Furthermore, a gcd3 mutation can overcome the derepression-deficient phenotype of mutations in the positive regulatory GCN1, GCN2, and GCN3 genes. However, the gcd3 mutation cannot overcome the derepression-deficient phenotype of a gcn4 mutation, suggesting that GCD3 acts as a negative regulator of the important GCN4 gene. Northern blot analysis confirmed this conclusion, in that the steady-state levels of GCN4 mRNA are greatly increased in a gcd3 mutant. Thus, the negative regulatory gene GCD3 plays a central role in derepression of amino acid biosynthetic genes.

GCN4 protein, a positive transcription factor in yeast, binds general control promoters at all 5' TGACTC 3' sequences

The GCN4 gene is required for the general amino acid control derepression response in yeast. GCN4 protein protects a repeated sequence motif in the 5'-untranslated region of HIS4, HIS3, ILV1, and ILV2 genes subject to general control. At low concentrations of GCN4, only certain repeats in these genes are bound. The repeats differ slightly from the 5' TGACTC 3' consensus core sequence, and the selective binding of some sites at low GCN4 concentrations is related to the relative affinity of these sites to GCN4. Using purified GCN4 protein obtained from an overproducing strain of Escherichia coli, we were able to obtain complete protection of all of the repeat elements in these four genes at high GCN4 concentrations. Analysis of the relative binding constant to the 15 repeated sequences protected by GCN4 shows that the optimal binding site for GCN4 is 5' RRTGACTC 3' followed by a short stretch of thymidines. Another protein, present mostly in yeast nuclear extracts, binds to the HIS4 promoter at a site overlapping one of the GCN4 binding sites. This protein is displaced from its binding site at high GCN4 concentrations.

Multiple GCD genes required for repression of GCN4, a transcriptional activator of amino acid biosynthetic genes in Saccharomyces cerevisiae

GCN4 encodes a positive regulator of multiple unlinked genes encoding amino acid biosynthetic enzymes in Saccharomyces cerevisiae. Expression of GCN4 is coupled to amino acid availability by a control mechanism involving GCD1 as a negative effector and GCN1, GCN2, and GCN3 as positive effectors of GCN4 expression. We used reversion of a gcn2 gcn3 double mutation to isolate new alleles of GCD1 and mutations in four additional GCD genes which we designate GCD10, GCD11, GCD12, and GCD13. All of the mutations lead to constitutive derepression of HIS4 transcription in the absence of the GCN2+ and GCN3+ alleles. By contrast, the gcd mutations require the wild-type GCN4 allele for their derepressing effect, suggesting that each acts by influencing the level of GCN4 activity in the cell. Consistent with this interpretation, mutations in each GCD gene lead to constitutive derepression of a GCN4::lacZ gene fusion. Thus, at least five gene products are required to maintain the normal repressed level of GCN4 expression in nonstarvation conditions. Interestingly, the gcd mutations are pleiotropic and also affect growth rate in nonstarvation conditions. In addition, certain alleles lead to a loss of M double-stranded RNA required for the killer phenotype. This pleiotropy suggests that the GCD gene products contribute to an essential cellular function, in addition to, or in conjunction with, their role in GCN4 regulation.

Saturation mutagenesis of the yeast his3 regulatory site: requirements for transcriptional induction and for binding by GCN4 activator protein

Expression of the yeast his3 and other amino acid biosynthetic genes is induced during conditions of amino acid starvation. The coordination of this response is mediated by a positive regulatory protein called GCN4, which binds specifically to regulatory sites upstream of all coregulated genes and stimulates their transcription. The nucleotide sequence requirements of the his3 regulatory site were determined by analysis of numerous point mutations obtained by a novel method of cloning oligonucleotides. Almost all single base pair mutations within the nine base pair sequence ATGACTCTT significantly reduce his3 induction in vivo and GCN4 binding in vitro, whereas changes outside this region have minimal effects. One mutation, which generates a sequence that most closely resembles the consensus for 15 coregulated genes, increases both the level of induction and the affinity for GCN4 protein. The palindromic nature of the optimal sequence, ATGACTCAT, suggest that GCN4 protein binds as a dimer to adjacent half-sites that possibly overlap.

Analysis of the Kluyveromyces lactis positive regulatory gene LAC9 reveals functional homology to, but sequence divergence from, the Saccharomyces cerevisiae GAL4 gene

The galactose metabolism positive regulatory gene from Kluyveromyces lactis, LAC9, has been isolated through its ability to activate expression of galactose metabolism enzyme genes in Saccharomyces cerevisiae. The LAC9 gene also activates expression of the S. cerevisiae alpha-galactosidase (MEL1) and K. lactis beta-galactosidase (LAC4) genes in S. cerevisiae. Although LAC9-activated gene expression in K. lactis is not glucose repressed, activation of MEL1 gene expression by LAC9 in S. cerevisiae is. The LAC9 gene is expressed at an extremely low level as a approximately 2.9-kb mRNA, and encodes a protein of 865 amino acids. Although the LAC9 gene is functionally analogous to the S. cerevisiae GAL4 gene, the bulk of its protein sequence shows little homology to that of GAL4. Two of the three regions of homology that do exist, however, are restricted to areas of GAL4 protein already implicated in nuclear localization, DNA binding, and transcriptional activation.

Expression of the Saccharomyces cerevisiae inositol-1-phosphate synthase (INO1) gene is regulated by factors that affect phospholipid synthesis

The INO1 gene of Saccharomyces cerevisiae encodes the regulated enzyme inositol-1-phosphate synthase, which catalyzes the first committed step in the synthesis of inositol-containing phospholipids. The expression of this gene was analyzed under conditions known to regulate phospholipid synthesis. RNA blot hybridization with a genomic clone for INO1 detected two RNA species of 1.8 and 0.6 kb. The abundance of the 1.8-kb RNA was greatly decreased when the cells were grown in the presence of the phospholipid precursor inositol, as was the enzyme activity of the synthase. Complementation analysis showed that this transcript encoded the INO1 gene product. The level of INO1 RNA was repressed 12-fold when the cells were grown in medium containing inositol, and it was repressed 33-fold when the cells were grown in the presence of inositol and choline together. The INO1 transcript was present at a very low level in cells containing mutations (ino2 and ino4) in regulatory genes unlinked to INO1 that result in inositol auxotrophy. The transcript was constitutively overproduced in cells containing a mutation (opi1) that causes constitutive expression of inositol-1-phosphate synthase and results in excretion of inositol. The expression of INO1 RNA was also examined in cells containing a mutation (cho2) affecting the synthesis of phosphatidylcholine. In contrast to what was observed in wild-type cells, growth of cho2 cells in medium containing inositol did not result in a significant decrease in INO1 RNA abundance. Inositol and choline together were required for repression of the INO1 transcript in these cells, providing evidence for a regulatory link between the synthesis of inositol- and choline-containing lipids. The level of the 0.6-kb RNA was affected, although to a lesser degree, by many of the same factors that influence INO1 expression.

Random-clone strategy for genomic restriction mapping in yeast

An approach to global restriction mapping is described that is applicable to any complex source DNA. By analyzing a single restriction digest for each member of a redundant set of lambda clones, a data base is constructed that contains fragment-size lists for all the clones. The clones are then grouped into subsets, each member of which is related to at least one other member by a significant overlap. Finally, a tree-searching algorithm seeks restriction maps that are consistent with the fragment-size lists for all the clones in each subset. The feasibility of the approach has been demonstrated by collecting data on 5000 lambda clones containing random 15-kilobase inserts of yeast DNA. It is shown that these data can be analyzed to produce regional maps of the yeast genome, extending in some cases for over 100 kilobases. In combination with hybridization probes to previously cloned genes, these local maps are already useful for defining the physical arrangement of closely linked genes. They may in the future serve as building blocks for the construction of a continuous global map.

General amino acid control and specific arginine repression in Saccharomyces cerevisiae: physical study of the bifunctional regulatory region of the ARG3 gene

To characterize further the regulatory mechanism modulating the expression of the Saccharomyces cerevisiae ARG3 gene, i.e., the specific repression by arginine and the general amino acid control, we analyzed by deletion the region upstream of that gene, determined the nucleotide sequence of operator-constitutive-like mutations affecting the specific regulation, and examined the behavior of an ARG3-galK fusion engineered at the initiating codon of ARG3. Similarly to what was observed in previous studies on the HIS3 and HIS4 genes, our data show that the general regulation acts as a positive control and that a sequence containing the nucleotide TGACTC, between positions -364 and -282 upstream of the transcription start, functions as a regulatory target site. This sequence contains the most proximal of the two TGACTC boxes identified in front of ARG3. While the general control appears to modulate transcription efficiency, the specific repression by arginine displays a posttranscriptional component (F. Messenguy and E. Dubois, Mol. Gen. Genet. 189:148-156, 1983). Our deletion and gene fusion analyses confirm that the specific and general controls operate independently of each other and assign the site responsible for arginine-specific repression to between positions -170 and +22. In keeping with this assignment, the two operator-constitutive-like mutations were localized at positions -80 and -46, respectively, and thus in a region which is not transcribed. We discuss a hypothesis accounting for the involvement of untranscribed DNA in a posttranscriptional control.

A hierarchy of trans-acting factors modulates translation of an activator of amino acid biosynthetic genes in Saccharomyces cerevisiae

The GCN4 gene encodes a positive effector of amino acid biosynthetic genes in Saccharomyces cerevisiae. Genetic analysis has suggested that GCN4 is regulated by a hierarchy of interacting positive and negative effectors in response to amino acid starvation. Results presented here for a GCN4-lacZ gene fusion support this regulatory model and suggest that the regulators of GCN4 exert their effects primarily at the level of translation of GCN4 mRNA. Both the GCN2 and GCN3 products appear to stimulate translation of GCN4 mRNA in response to amino acid starvation, because a recessive mutation in either gene blocked derepression of GCN4-lacZ fusion enzyme levels but did not reduce the fusion transcript level relative to that in wild-type cells grown in the same conditions. The GCD1 product appears to inhibit translation of GCN4 mRNA because under certain growth conditions, the gcd1-101 mutation led to derepression of the GCN4-lacZ fusion enzyme level in the absence of any increase in the fusion transcript level. In addition, the gcd1-101 mutation suppressed the low translational efficiency of GCN4-lacZ mRNA observed in gcn2- and gcn3- cells. A deletion of four small open reading frames in the 5' leader of GCN4-lacZ mRNA mimicked the effect of a gcd1 mutation and derepressed translation of the fusion transcript in the absence of either starvation conditions or the GCN2 and GCN3 products. By contrast, in a gcd1- strain, the deletion resulted in little additional increase in the translational efficiency of the fusion transcript. These results suggest that GCD1 mediates the translational repression normally exerted by the GCN4 leader sequences and that GCN2 and GCN3 antagonize these negative elements in response to amino acid starvation. The effects of the trans-acting mutations on the translation of GCN4-lacZ mRNA remained intact even when transcription of the fusion gene was placed under the control of the S. cerevisiae GAL1 transcriptional control element.

Functional domains of the yeast regulatory protein GAL4

In the yeast Saccharomyces cerevisiae regulation of the galactose/melibiose regulon rests on a dosage-dependent functional interplay between the positive regulator of transcription, the GAL4 protein, and the negative regulator, GAL80 protein. We have used this interplay to select in vitro generated fusions between the yeast ADH1 promoter and GAL4 coding sequence that overproduce GAL4 protein, allowing the identification of GAL4 protein in crude extracts from yeast. One type of these constructions produces a GAL4 protein that lacks its normal NH2 terminus. This protein is unable to complement a gal4 lesion but still retains a domain that functionally antagonizes the negative regulatory protein. One defect in this truncated protein is its inability to be concentrated in the nucleus. However, the nuclear localization defect is complemented by full-length GAL4 protein. The truncated protein also appears to effect changes in the relative transcriptional levels of the structural genes. These observations imply that GAL4 protein consists of several domains, including ones for nuclear localization, interaction with the negative regulatory protein, and, possibly, separable transcriptional activation domains for the structural genes.

Negative regulatory gene for general control of amino acid biosynthesis in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, many amino acid biosynthetic pathways are coregulated by a complex general control system: starvation for a single amino acid results in the derepression of amino acid biosynthetic genes in multiple pathways. Derepression of these genes is mediated by positive (GCN) and negative (GCD) regulatory genes. In this paper we describe the isolation and characterization of a previously unreported negative regulatory gene, GCD3. A gcd3 mutation is recessive to wild type, confers resistance to multiple amino acid analogs, and results in overproduction and partially constitutive elevation of mRNA levels for amino acid biosynthetic genes. Furthermore, a gcd3 mutation can overcome the derepression-deficient phenotype of mutations in the positive regulatory GCN1, GCN2, and GCN3 genes. However, the gcd3 mutation cannot overcome the derepression-deficient phenotype of a gcn4 mutation, suggesting that GCD3 acts as a negative regulator of the important GCN4 gene. Northern blot analysis confirmed this conclusion, in that the steady-state levels of GCN4 mRNA are greatly increased in a gcd3 mutant. Thus, the negative regulatory gene GCD3 plays a central role in derepression of amino acid biosynthetic genes.

Multiple cis-acting elements modulate the translational efficiency of GCN4 mRNA in yeast

The expression of the GCN4 gene in yeast is regulated at the translational level: growth of yeast cells under amino acid starvation conditions results in an increase in the translational efficiency of GCN4 mRNA. A sequence within the 5' untranslated region of this mRNA, which contains four small open reading frames, acts in cis to suppress translation when growth occurs in rich media. In this report, we have analyzed the effects on translation of a series of deletion, insertion, and substitution mutations in the 5' untranslated region of GCN4 mRNA. This analysis showed that at least two distinct cis elements located within the region of the small upstream open reading frames are required, in conjunction with trans positive elements, for the translational activation of GCN4 mRNA. We propose that the translational efficiency of GCN4 mRNA is modulated by the rate of translation initiation at the upstream AUG codons.

Saccharomyces cerevisiae PHO5 promoter region: location and function of the upstream activation site

Saccharomyces cerevisiae repressible acid phosphatase (PHO5) is induced when inorganic phosphate in the culture medium is depleted. To study the mechanism of this regulation, we constructed various deletions in the 5'-flanking region of the PHO5 gene. Two elements were revealed by this analysis: an upstream activation site (UAS) and a downstream element, both playing parts in the expression of this gene. The UAS is located between -384 and -292 upstream of the initiation codon and activates expression of the gene when inorganic phosphate is depleted. It consists of two homologous regions (UAS I and UAS II) that contain CTGCACAAATG and an adenine-plus-thymine-rich sequence, either one of which suffices for the effect. The downstream element includes a putative TATA box at -100 from the ATG codon and is necessary for efficient transcription and expression of the normal-sized PHO5 transcript. The distance between the UAS and the downstream element can be altered without causing loss of expression efficiency, and the action of the UAS is not affected by its orientation. These results are consistent with a model wherein UAS acts as a site of activation for transcription by interaction with a protein factor(s) that becomes active when inorganic phosphate is depleted from the culture medium.

Kinetics of the intracellular availability of heme after supplementing a heme-deficient yeast mutant with 5-aminolevulinate

The 5-aminolevulinic acid synthetase-deficient yeast mutant ole3 (Bard, M., Woods, R.A. & Haslam, J.M. (1974) Biochem. Biophys. Res. Commun. 56, 324-330), pregrown in a 5-aminolevulinic acid containing medium, can grow on glucose or galactose medium in the absence of heme for about 13 generations. When supplemented with 5-aminolevulinate before their 9th cell division, the cells can be induced to full respiratory competence. From the measurement of heme-dependent parameters (e.g. respiration, transcription of iso-1-cytochrome c mRNA and the post-translational proteolytic processing of the heme-free intermediate precursor of cytochrome c1 to its heme-containing mature form) it can be judged that heme is available in the cell about one hour after the addition of 5-aminolevulinate. The onset of respiration, however, does not occur to an appreciable extent before the 3rd hour of induction. The heme analogue deuteroporphyrin IX prevents respiratory adaptation but yet effects the transcription of heme-controlled genes.

Cloning of the arg-12 gene of Neurospora crassa and regulation of its transcript via cross-pathway amino acid control

The arg-12 locus of Neurospora crassa encodes ornithine carbamoyl transferase, which is one of many amino acid synthetic enzymes whose activity is regulated through cross-pathway (or general) amino acid control. We report here the use of probes derived from the functionally equivalent arg-B gene of Aspergillus nidulans to identify and clone a 10 kb Neurospora DNA fragment carrying the arg-12 gene. Short Neurospora DNA probes derived from this fragment were used to identify a 1.5 kb polyA+ transcript of the arg-12 region. Arg-12 transcript levels increased approximately 20 fold under conditions of arginine or histidine limitation in strains having normal cross-pathway regulation (cpc-1+) but showed no such response in a cpc-1 mutant strain impaired in this regulation. Time course studies in cpc-1+ strains revealed a rapid response (within 10 m) of arg-12 transcript levels following inhibition of histidine synthesis by 3 amino 1,2,4 triazole, but a delayed response following arginine deprivation of an arginine requiring strain. In contrast to the behaviour of arg-12 mRNA, the level of the Neurospora am gene transcript (specifying NADP dependent glutamate dehydrogenase) was unaffected either by amino acid limitation or by the cpc-1 mutation. A possible role for the cpc-1+ product as a positive regulator of transcription of genes subject to cross-pathway control is discussed.

A new negative control gene for amino acid biosynthesis in Saccharomyces cerevisiae

Enzyme levels in multiple amino acid biosynthetic pathways in yeast are coregulated. This control is effected largely at the transcriptional level by a number of regulatory genes. We report the isolation and characterization of a new negative regulatory gene, GCD4, for this general control system. GCD4 mutations are recessive and define a single Medelian gene on chromosome III. A gcd4 mutation results in resistance to different amino acid analogs and elevated, but fully inducible, mRNA levels of genes under general control. Epistasis analysis indicates that GCD4 acts more directly than the positive regulators GCN1, GCN2, GCN3 and GCN5, but less directly than GCN4, on the transcription of the amino acid biosynthetic genes. These data imply that GCD4 is a negative regulator of the positive effector, GCN4. Although GCD4 occupies the same position relative to the GCN genes as other GCD genes, it produces a unique phenotype. These results illustrate the diversity of function of negative regulators in general control.

The general control of amino acid biosynthetic genes in the yeast Saccharomyces cerevisiae

Enzymes in diverse amino acid biosynthetic pathways in Saccharomyces cerevisiae are subject to a general amino acid control in which starvation for any amino acid leads to increased levels of the mRNAs encoding these enzymes. The short nucleotide sequence TGACTC, found nontandemly repeated upstream from the coregulated structural genes, serves as a cis-acting site for positive regulation of transcription. Multiple trans-acting repressors and activators have been identified. Most of these factors act indirectly by regulating the level of an activator encoded by the GCN4 gene. This regulation occurs at the level of GCN4 translation and is mediated by sequences in the long 5' leader of GCN4 mRNA. The GCN4 protein is the most likely candidate for the transcriptional activator that interacts with the TGACTC sequences at the structural genes.