Concerted repression of the synthesis of the arginine biosynthetic enzymes by aminoacids: a comparison between the regulatory mechanisms controlling aminoacid biosyntheses in bacteria and in yeast

Characterization and kinetic mechanism of mono- and bifunctional ornithine acetyltransferases from thermophilic microorganisms

The argJ gene coding for N2-acetyl-L-ornithine: L-glutamate N-acetyltransferase, the key enzyme involved in the acetyl cycle of L-arginine biosynthesis, has been cloned from thermophilic procaryotes: the archaeon Methanoccocus jannaschii, and the bacteria Thermotoga neapolitana and Bacillus stearothermophilus. Archaeal argJ only complements an Escherichia coli argE mutant (deficient in acetylornithinase, which catalyzes the fifth step in the linear biosynthetic pathway), whereas bacterial genes additionally complement an argA mutant (deficient in N-acetylglutamate synthetase, the first enzyme of the pathway). In keeping with these in vivo data the purified His-tagged ArgJ enzyme of M. jannaschii only catalyzes N2-acetylornithine conversion to ornithine, whereas T. neapolitana and B. stearothermophilus ArgJ also catalyze the conversion of glutamate to N-acetylglutamate using acetylCoA as the acetyl donor. M. jannaschii ArgJ is therefore a monofunctional enzyme, whereas T. neapolitana and B. stearothermophilus encoded ArgJ are bifunctional. Kinetic data demonstrate that in all three thermophilic organisms ArgJ-mediated catalysis follows ping-pong bi-bi kinetic mechanism. Acetylated ArgJ intermediates were detected in semireactions using [14C]acetylCoA or [14C]N2-acetyl-L-glutamate as acetyl donors. In this catalysis L-ornithine acts as an inhibitor; this amino acid therefore appears to be a key regulatory molecule in the acetyl cycle of L-arginine synthesis. Thermophilic ArgJ are synthesized as protein precursors undergoing internal cleavage to generate alpha and beta subunits which appear to assemble to alpha2beta2 heterotetramers in E. coli. The cleavage occurs between alanine and threonine residues within the highly conserved PXM-ATML motif detected in all available ArgJ sequences.

Biological role of the general control of amino acid biosynthesis in Saccharomyces cerevisiae

The biological role of the "general control of amino acid biosynthesis" has been investigated by analyzing growth and enzyme levels in wild-type, bradytrophic, and nonderepressing mutant strains of Saccharomyces cerevisiae. Amino acid limitation was achieved by using either bradytrophic mutations or external amino acid imbalance. In the wild-type strain noncoordinate derepression of enzymes subject to the general control has been found. Derepressing factors were in the order of 2 to 4 in bradytrophic mutant strains grown under limiting conditions and only in the order of 1.5 to 2 under the influence of external amino acid imbalance. Nonderepressing mutations led to slower growth rates under conditions of amino acid limitation, and no derepression of enzymes under the general control was observed. The amino acid pools were found to be very similar in the wild type and in nonderepressing mutant strains under all conditions tested. Our results indicate that the general control affects all branched amino acid biosynthetic pathways, namely, those of the aromatic amino acids and the aspartate family, the pathways for the basic amino acids lysine, histidine, and arginine, and also the pathways of serine and valine biosyntheses.

A segment of mRNA encoding the leader peptide of the CPA1 gene confers repression by arginine on a heterologous yeast gene transcript

The expression of the yeast gene CPA1, which encodes the small subunit of the arginine pathway carbamoylphosphate synthetase, is repressed by arginine at a translational level. CPA1 mRNA contains a 250-nucleotide-long leader which includes a 25-codon upstream open reading frame (uORF). Oligonucleotide site-directed mutagenesis of this uORF as well as sequencing of constitutive cis-dominant mutations has suggested that the leader peptide product of the CPA1 uORF is an essential negative element for repression of the CPA1 gene by arginine. In this work, a series of deletions affecting the regions 5' and 3' to the uORF in the leader sequence was constructed. The arginine-dependent repression of CPA1 was little affected in these constructions, indicating that these regions are not essential for the regulatory response. This conclusion was further supported by the finding that inserting the mRNA segment encoding the leader peptide sequence of CPA1 in the leader sequence of another gene, namely, GCN4, places this gene under arginine repression. Similarly, the behavior of fusions of the leader sequence of CPA1 with those of ARG4 or GAL10 confirmed that the regions of this leader located upstream and downstream from the uORF are dispensable for the regulation by arginine. Finally, a set of substitution mutations which modify the uORF nucleotide sequence while leaving unchanged the corresponding amino acid sequence was constructed. The mutations did not affect the repression of CPA1 by arginine. The data presented in this paper consequently agree with the conclusion that the leader peptide itself is the main element required for the translational repression of CPA1.

A segment of mRNA encoding the leader peptide of the CPA1 gene confers repression by arginine on a heterologous yeast gene transcript

The expression of the yeast gene CPA1, which encodes the small subunit of the arginine pathway carbamoylphosphate synthetase, is repressed by arginine at a translational level. CPA1 mRNA contains a 250-nucleotide-long leader which includes a 25-codon upstream open reading frame (uORF). Oligonucleotide site-directed mutagenesis of this uORF as well as sequencing of constitutive cis-dominant mutations has suggested that the leader peptide product of the CPA1 uORF is an essential negative element for repression of the CPA1 gene by arginine. In this work, a series of deletions affecting the regions 5' and 3' to the uORF in the leader sequence was constructed. The arginine-dependent repression of CPA1 was little affected in these constructions, indicating that these regions are not essential for the regulatory response. This conclusion was further supported by the finding that inserting the mRNA segment encoding the leader peptide sequence of CPA1 in the leader sequence of another gene, namely, GCN4, places this gene under arginine repression. Similarly, the behavior of fusions of the leader sequence of CPA1 with those of ARG4 or GAL10 confirmed that the regions of this leader located upstream and downstream from the uORF are dispensable for the regulation by arginine. Finally, a set of substitution mutations which modify the uORF nucleotide sequence while leaving unchanged the corresponding amino acid sequence was constructed. The mutations did not affect the repression of CPA1 by arginine. The data presented in this paper consequently agree with the conclusion that the leader peptide itself is the main element required for the translational repression of CPA1.

Induction of "General Control" and thermotolerance in cdc mutants of Saccharomyces cerevisiae

In Saccharomyces cerevisiae starvation for a single amino acid activates the transcription of a set of genes belonging to different amino acid biosynthetic pathways (General Control, GC). We show that mutants affected in GC regulation are also affected in their response to thermal stress. Moreover, growth conditions that are known to induce heat shock proteins induce the GC response. However, unlike heat shock proteins, the transcriptional activator of GC, GCN4, is not induced after a short exposure to heat, and in gcn mutant strains induction of heat resistance is normal.

Deletion analysis of the ARG4 promoter of Saccharomyces cerevisiae: a poly(dAdT) stretch involved in gene transcription

Transcription of the ARG4 gene of Saccharomyces cerevisiae is regulated by general control of amino acid biosynthesis but not by a specific regulatory mechanism. Three deletion mutants (delta I, delta II, delta III) successively removing DNA sequences upstream from the coding sequence have been phenotypically analyzed after insertion into a single copy plasmid. As expected, delta I, which lacks the sequences upstream to -155, including the two putative upstream activation sequences (UAS), was unable to derepress argininosuccinate lyase biosynthesis under conditions of amino acid starvation. In delta II (deleted up to -126) the enzyme activity was very low and cells harbouring this allele were arginine dependent. These drastic phenotypic changes can be attributed to the loss of 12 out of 14 dA residues from positions -124 to -137. This poly (dAdT) sequence most likely serves as an upstream promoter element for constitutive expression of ARG4. The delta III deletion removes all 5' sequences including the putative TATA box. This inactive allele has been successfully used for selecting yeast promoters of unknown origin.

Arginine-specific repression in Saccharomyces cerevisiae: kinetic data on ARG1 and ARG3 mRNA transcription and stability support a transcriptional control mechanism

A specific repression mechanism regulates arginine biosynthesis in Saccharomyces cerevisiae. The involvement of regulatory proteins displaying DNA-binding features and the location of an operator region between the TATA box and the transcription start of the structural gene ARG3 suggest that this mechanism operates at the level of transcription. A posttranscriptional mechanism has, however, been proposed to account for the conspicuous lack of proportionality between ARG3 mRNA steady-state levels (as determined by Northern [RNA] assays; F. Messenguy and E. Dubois, Mol. Gen. Genet. 189:148-156, 1983) and the cognate enzyme activities. In this work, we have analyzed the time course of the incorporation of radioactive precursors into ARG1 and ARG3 mRNAs and the kinetics of their decay under different regulatory statuses. The results (expressed in terms of relative mRNA levels, relative transcription rates, and mRNA half-lives) give the picture expected from a purely transcriptional control. A similar analysis of expression of the gene CPA1, for which a translational regulation by arginine has been clearly demonstrated (M. Werner, A. Feller, F. Messenguy, and A. Piérard, Cell 49:805-813, 1987), indicates that this gene is also partly regulated at the transcriptional level by the ARGR repressor system. Moreover, the half-life of CPA1 mRNA is reduced twofold in the presence of excess arginine; we suggest that this could be inherent in the mechanism of translational regulation of CPA1.

Arginine-specific repression in Saccharomyces cerevisiae: kinetic data on ARG1 and ARG3 mRNA transcription and stability support a transcriptional control mechanism

A specific repression mechanism regulates arginine biosynthesis in Saccharomyces cerevisiae. The involvement of regulatory proteins displaying DNA-binding features and the location of an operator region between the TATA box and the transcription start of the structural gene ARG3 suggest that this mechanism operates at the level of transcription. A posttranscriptional mechanism has, however, been proposed to account for the conspicuous lack of proportionality between ARG3 mRNA steady-state levels (as determined by Northern [RNA] assays; F. Messenguy and E. Dubois, Mol. Gen. Genet. 189:148-156, 1983) and the cognate enzyme activities. In this work, we have analyzed the time course of the incorporation of radioactive precursors into ARG1 and ARG3 mRNAs and the kinetics of their decay under different regulatory statuses. The results (expressed in terms of relative mRNA levels, relative transcription rates, and mRNA half-lives) give the picture expected from a purely transcriptional control. A similar analysis of expression of the gene CPA1, for which a translational regulation by arginine has been clearly demonstrated (M. Werner, A. Feller, F. Messenguy, and A. Piérard, Cell 49:805-813, 1987), indicates that this gene is also partly regulated at the transcriptional level by the ARGR repressor system. Moreover, the half-life of CPA1 mRNA is reduced twofold in the presence of excess arginine; we suggest that this could be inherent in the mechanism of translational regulation of CPA1.

Arginine restriction induced by delta-N-(phosphonacetyl)-L-ornithine signals increased expression of HIS3, TRP5, CPA1, and CPA2 in Saccharomyces cerevisiae

delta-N-(Phosphonacetyl)-L-ornithine (PALO), a transition state analog inhibitor of ornithine transcarbamylase, induced arginine limitation in vivo in Saccharomyces cerevisiae. Arginine restriction caused increased expression of HIS3 and TRP5, measured by the beta-galactosidase activity in strains carrying chromosomally integrated fusions of the promoter regions of each gene with the lacZ gene of Escherichia coli. The increase in beta-galactosidase activity induced by PALO was reversed by the addition of arginine and was dependent on GCN4 protein. These results indicate that PALO, like 3-amino-1,2,4-triazole DL-5-methyltryptophan, can be used to study the effect of limitation of a single amino acid, arginine, on the expression of genes under the general amino acid control regulatory system. Arginine deprivation imposed by PALO also caused increased expression of CPA1 and CPA2, coding respectively for the small and large subunits of arginine-specific carbamyl-phosphate synthetase. The observed increase was GCN4 dependent and was genetically separable from arginine-specific repression of CPA1 mRNA translation. The 5'-flanking regions of CPA1 (reported previously) and CPA2 determined in this study each contained at least two copies of the sequence TGACTC, shown to bind GCN4 protein. The beta-galactosidase activities expressed from CPA1- and CPA2-lacZ fusions integrated into the nuclear DNA of gcn4 mutant strains were five to six times less than in the wild type, when both strains were grown under depressed conditions. The gcn4 mutation reduced basal expression of both CPA1 and CPA2. The addition of arginine to strains containing the CPA1-lacZ fusion further reduced beta-galactosidase activity of the gcn4 mutant, indicating independent regulation of the CPA1 gene by the general amino acid control and by arginine-specific repression. In strains overproducing GCN4 protein, the translational control completely overrode transcriptional activation of CPA1 by general amino acid control.

Arginine restriction induced by delta-N-(phosphonacetyl)-L-ornithine signals increased expression of HIS3, TRP5, CPA1, and CPA2 in Saccharomyces cerevisiae

delta-N-(Phosphonacetyl)-L-ornithine (PALO), a transition state analog inhibitor of ornithine transcarbamylase, induced arginine limitation in vivo in Saccharomyces cerevisiae. Arginine restriction caused increased expression of HIS3 and TRP5, measured by the beta-galactosidase activity in strains carrying chromosomally integrated fusions of the promoter regions of each gene with the lacZ gene of Escherichia coli. The increase in beta-galactosidase activity induced by PALO was reversed by the addition of arginine and was dependent on GCN4 protein. These results indicate that PALO, like 3-amino-1,2,4-triazole DL-5-methyltryptophan, can be used to study the effect of limitation of a single amino acid, arginine, on the expression of genes under the general amino acid control regulatory system. Arginine deprivation imposed by PALO also caused increased expression of CPA1 and CPA2, coding respectively for the small and large subunits of arginine-specific carbamyl-phosphate synthetase. The observed increase was GCN4 dependent and was genetically separable from arginine-specific repression of CPA1 mRNA translation. The 5'-flanking regions of CPA1 (reported previously) and CPA2 determined in this study each contained at least two copies of the sequence TGACTC, shown to bind GCN4 protein. The beta-galactosidase activities expressed from CPA1- and CPA2-lacZ fusions integrated into the nuclear DNA of gcn4 mutant strains were five to six times less than in the wild type, when both strains were grown under depressed conditions. The gcn4 mutation reduced basal expression of both CPA1 and CPA2. The addition of arginine to strains containing the CPA1-lacZ fusion further reduced beta-galactosidase activity of the gcn4 mutant, indicating independent regulation of the CPA1 gene by the general amino acid control and by arginine-specific repression. In strains overproducing GCN4 protein, the translational control completely overrode transcriptional activation of CPA1 by general amino acid control.

Biosynthetic and regulatory role of lys9 mutants of Saccharomyces cerevisiae

Derepression of lysine biosynthetic enzymes of Saccharomyces cerevisiae was investigated in lys9 auxotrophs which lack saccharopine reductase activity. Five enzymes (homocitrate synthase, homoisocitrate dehydrogenase, alpha-aminoadipate aminotransferase, alpha-aminoadipate reductase and saccharopine dehydrogenase) were constitutively derepressed in all lys9 mutants with up to eight-fold higher enzyme levels than in isogenic wild-type cells. Levels of these enzymes in lys2, lys14, and lys15 mutants were the same or lower than those in wild-type cells. The regulatory property of lys9 mutants exhibited recessiveness to the wild-type gene in heterozygous diploids. Unlike the mating type effect, homozygous diploids resulting from crosses between lys9 auxotrophs exhibited even higher levels of derepressed enzymes than the haploid mutants. Addition of a higher concentration of lysine to the growth medium resulted in reduction of enzyme levels although they were still derepressed. These results suggest that lys9 mutants represent a lesion for the saccharopine reductase and may represent a repressor mutation which in the wild-type cells simultaneously represses unlinked structural genes that encode for five of the lysine biosynthetic enzymes.

New positive and negative regulators for general control of amino acid biosynthesis in Saccharomyces cerevisiae

The biosynthesis of most amino acids in Saccharomyces cerevisiae is coregulated. Starvation for a single amino acid results in the derepression of amino acid biosynthetic enzymes in many unrelated pathways. This phenomenon, known as general control, is mediated by both positive (GCN) and negative (GCD) regulatory genes. In this paper we describe the identification and characterization of several new regulatory genes for this system, GCN6, GCN7, GCN8, GCN9, and GCD5. A mutation in the negative regulator GCD5 was isolated on the basis of its suppression of a gcn2 mutation. The effect of gcd5 is a posttranscriptional increase in histidine biosynthetic enzyme activity. Suppressors of gcd5 which are deficient in derepression were in turn isolated. Eight such mutations, defining four new positive regulatory genes (GCN6 through GCN9), were obtained. These mutations are recessive, confer sensitivity to multiple amino acid analogs, and result in decreased mRNA levels for genes under general control. The GCN6 and GCN7 gene products were shown to be positive regulators for transcription of the GCN4 gene, the most direct-acting positive regulator thus far identified. The interaction of GCN6 and GCN7 with GCN4 is fundamentally different from that of previously isolated GCN genes. It should also be noted that these gcn selections gave a completely different nonoverlapping set of mutations from earlier selections which relied on analog sensitivity. Thus, we may have identified a new class of GCN genes which are functionally distinct from GCN1 through GCN5.

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.

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.

New positive and negative regulators for general control of amino acid biosynthesis in Saccharomyces cerevisiae

The biosynthesis of most amino acids in Saccharomyces cerevisiae is coregulated. Starvation for a single amino acid results in the derepression of amino acid biosynthetic enzymes in many unrelated pathways. This phenomenon, known as general control, is mediated by both positive (GCN) and negative (GCD) regulatory genes. In this paper we describe the identification and characterization of several new regulatory genes for this system, GCN6, GCN7, GCN8, GCN9, and GCD5. A mutation in the negative regulator GCD5 was isolated on the basis of its suppression of a gcn2 mutation. The effect of gcd5 is a posttranscriptional increase in histidine biosynthetic enzyme activity. Suppressors of gcd5 which are deficient in derepression were in turn isolated. Eight such mutations, defining four new positive regulatory genes (GCN6 through GCN9), were obtained. These mutations are recessive, confer sensitivity to multiple amino acid analogs, and result in decreased mRNA levels for genes under general control. The GCN6 and GCN7 gene products were shown to be positive regulators for transcription of the GCN4 gene, the most direct-acting positive regulator thus far identified. The interaction of GCN6 and GCN7 with GCN4 is fundamentally different from that of previously isolated GCN genes. It should also be noted that these gcn selections gave a completely different nonoverlapping set of mutations from earlier selections which relied on analog sensitivity. Thus, we may have identified a new class of GCN genes which are functionally distinct from GCN1 through GCN5.

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.

General and specific controls of lysine biosynthesis in Saccharomyces cerevisiae

Six of the eight enzymes of the alpha-aminoadipate pathway for the biosynthesis of lysine in Saccharomyces cerevisiae were examined for repressibility to lysine and for susceptibility to the general control of amino acid biosynthesis. All of the enzymes exhibited a 2 to 4 fold lower level of specific activity in the wildtype strain X2180 when grown in lysine supplemented medium as compared to minimal medium. However, levels of only three of the enzymes, alpha-aminoadipate reductase, saccharopine reductase, and saccharopine dehydrogenase, were derepressed in the leaky lysine mutant 7305d and leaky arginine mutant 7853-6c when grown in minimal medium. These observations are characteristic of enzymes under general control of amino acid biosynthesis. The remaining three enzymes, homocitrate synthetase, homoaconitase and homoisocitrate dehydrogenase were repressed in 7305d cells grown in minimal or lysine supplemented medium.

alpha-Aminoadipate pathway for the biosynthesis of lysine in lower eukaryotes

Bacteria and green plants use the diaminopimelate pathway for the biosynthesis of the essential amino acid, lysine; however, yeast and other higher fungi use the alpha-aminoadipate (AA) pathway. The AA pathway has been investigated in detail biochemically, genetically, and in terms of regulatory mechanisms in the baker's yeast Saccharomyces cerevisiae. The genetic analysis of lysine auxotrophs of S. cerevisiae revealed that there are more than 12 lysine genes for 8 enzyme-catalyzed steps. Lysine genes are not linked to each other and seven of the genes are mapped on six different linkage groups (chromosomes). The gene-enzyme relationships have been determined for ten of the lysine loci which include two unlinked gene functions required for each of AA reductase (LYS2 and LYS5) and Saccharopine reductase (LYS9 and LYS14). Five of the lysine enzymes are localized in mitochondria and three in cytosol. The lysine pathway of S. cerevisiae is regulated by feedback inhibition and end product repression. Two, and possibly three, of the enzymes exhibit general control of amino acid biosynthesis and at least five of the enzymes coded for, by unlinked genes, are simultaneously depressed in a regulatory (repressor) gene-mutant.

Temporal analysis of general control of amino acid biosynthesis in Saccharomyces cerevisiae: role of positive regulatory genes in initiation and maintenance of mRNA derepression

In Saccharomyces cerevisiae, starvation for a single amino acid results in the derepression of enzyme activities in multiple amino acid biosynthetic pathways. Derepression is a consequence of increased transcription of the genes encoding these enzymes. Analysis of the kinetics of mRNA elevation established that derepression occurs within 5 min of a shift of the culture from rich medium to starvation medium. Any starvation condition was sufficient to trigger an initial high mRNA elevation; however, it was the severity of starvation which determined the steady-state mRNA levels that were subsequently established. The products of the positive regulatory genes AAS101, AAS103, and AAS2 were shown to be required in the initiation phase of this response, whereas the AAS102 gene product was required to maintain the new elevated steady-state mRNA levels. The AAS101 and AAS102 genes were cloned. Consistent with their respective roles in initiation and maintenance of derepression. AAS101 mRNA was found to be expressed at high levels in both rich and starvation media, whereas AAS102 mRNA was derepressed only under starvation conditions. The derepression of AAS102 mRNA is dependent on the AAS101 gene product.

Temporal analysis of general control of amino acid biosynthesis in Saccharomyces cerevisiae: role of positive regulatory genes in initiation and maintenance of mRNA derepression

In Saccharomyces cerevisiae, starvation for a single amino acid results in the derepression of enzyme activities in multiple amino acid biosynthetic pathways. Derepression is a consequence of increased transcription of the genes encoding these enzymes. Analysis of the kinetics of mRNA elevation established that derepression occurs within 5 min of a shift of the culture from rich medium to starvation medium. Any starvation condition was sufficient to trigger an initial high mRNA elevation; however, it was the severity of starvation which determined the steady-state mRNA levels that were subsequently established. The products of the positive regulatory genes AAS101, AAS103, and AAS2 were shown to be required in the initiation phase of this response, whereas the AAS102 gene product was required to maintain the new elevated steady-state mRNA levels. The AAS101 and AAS102 genes were cloned. Consistent with their respective roles in initiation and maintenance of derepression. AAS101 mRNA was found to be expressed at high levels in both rich and starvation media, whereas AAS102 mRNA was derepressed only under starvation conditions. The derepression of AAS102 mRNA is dependent on the AAS101 gene product.

Identification of AAS genes and their regulatory role in general control of amino acid biosynthesis in yeast

In yeast, most amino acid biosynthetic pathways are coregulated: starvation for a single amino acid results in derepression of enzyme activities for many different biosynthetic pathways. This phenomenon is referred to as "general control of amino acid biosynthesis." In this paper we describe the isolation and characterization of 43 amino acid analog-sensitive (aas-) mutants that are perturbed in this general regulatory system. These 43 mutations define four unlinked complementation groups, AAS101, AAS102, AAS103, and AAS104, two of which identify previously unreported genes involved in general control. These aas mutants are unable to derepress a number of amino acid biosynthetic genes, resulting in increased sensitivity to amino acid analogs, reduced growth rates, and reduced enzyme activity levels under amino acid starvation conditions. Thus, the AAS+ gene products function as positive regulatory elements for this system. We show that the AAS genes mediate these effects by regulating the mRNA levels of genes under their control.

Regulation of HIS4-lacZ fusions in Saccharomyces cerevisiae

The beginning of the Saccharomyces cerevisiae HIS4 gene has been fused to the structural gene for Escherichia coli beta-galactosidase. This construction, which contains HIS4 DNA from -732 to +30 relative to the translation initiation codon, has been integrated into the yeast genome at two chromosomal locations, HIS4 and URA3. At both locations, this 762-base-pair stretch of DNA is sufficient for initiating expression of beta-galactosidase activity in S. cerevisiae and confers upon this activity the regulatory response normally found for HIS4.

Participation of transcriptional and post-transcriptional regulatory mechanisms in the control of arginine metabolism in yeast

In yeast, as in other organisms, amino acid biosynthetic pathways share a common regulatory control. The manifestation of this control is that derepression of the enzymes belonging to several amino acid biosynthetic pathways follows amino acid starvation or tRNA discharging. The arginine anabolic and catabolic pathways are, in addition, regulated specifically by arginine in opposite ways by common regulators. We have measured the mRNA levels for four genes subject to the general amino acid control: HIS4, ARG3, ARG4 and CPAII and compared them to the corresponding enzyme levels. Similarly we have measured the mRNA levels for two genes subject to the arginine specific regulation: ARG3 and CAR1, the former gene belongs to the arginine anabolic pathway and the latter to the arginine catabolic one. HIS4, ARG4 and CPAII enzyme and messenger amounts are perfectly coordinated in all the conditions of general repression or derepression tested. However, arginine does not reduce the level of the ARG3 mRNA enough to explain the reduction of ornithine carbamoyltransferase activity nor does it increase the level of the CAR1 mRNA enough to explain the extent of induction of arginase. Coordination of enzyme and ARG3 mRNA is achieved only when the specific control is eliminated. The half-lives of the ARG3 and CAR1 messengers are enhanced in mutants leading to constitutive expression of ornithine carbamoyltransferase and arginase. These data suggest that the control that coordinates the synthesis of all the amino acids in the yeast cell operates at the level of transcription while the arginine specific regulatory mechanism seems to operate at a post-transcriptional level.

A short nucleotide sequence required for regulation of HIS4 by the general control system of yeast

We have made deletions of the HIS4 5' noncoding region in vitro and inserted these deletions into the yeast genome by transformation. Deletions that extend from -588 to -235 have no detectable effects on either promoter or regulatory functions. Deletions that extend to -138 affect promoter function, but are still regulated by the general control of amino acid biosynthesis. A deletion that extends to -136 cannot derepress HIS4 mRNA in response to the general control. This deletion removes all copies of the sequence 5'-TGACTC-3', which appears at positions -194, -182 and -138 in strains without the deletion. The importance of at least one copy of this repeat for regulation of HIS4 is shown by the reappearance of this sequence in revertants of the -136 deletion that have regained the regulatory response. The fact that deletion of this sequence leads to the inability to derepress suggests that HIS4 is under positive control.

Regulation of HIS4-lacZ fusions in Saccharomyces cerevisiae

The beginning of the Saccharomyces cerevisiae HIS4 gene has been fused to the structural gene for Escherichia coli beta-galactosidase. This construction, which contains HIS4 DNA from -732 to +30 relative to the translation initiation codon, has been integrated into the yeast genome at two chromosomal locations, HIS4 and URA3. At both locations, this 762-base-pair stretch of DNA is sufficient for initiating expression of beta-galactosidase activity in S. cerevisiae and confers upon this activity the regulatory response normally found for HIS4.

Mutants affecting amino acid cross-pathway control in Neurospora crassa

Arginine-requiring mutants ofNeurospora crassawere isolated using a strain partially impaired in an enzyme of the arginine pathway (bradytroph). Among these, five strains were found which carry mutations at a new locus,cpc-1+. The recessivecpc-1alleles interfere with the cross-pathway control of amino acid biosynthetic enzymes. The enzymes studied, three of arginine and one each of histidine and lysine biosynthesis, fail to derepress under conditions which normally result in elevation of enzyme concentration, namely arginine, histidine or tryptophan limitation. Enzymes not involved in amino acid biosynthesis are still able to derepress in the presence ofcpc-1. In wild-type backgound, i.e. with the bradytroph replaced,cpc-1strains lose the original arginine-requirement.cpc-1mutations confer sensitivity of growth to 3-amino-1,2,4-triazole.

Biological role of the general control of amino acid biosynthesis in Saccharomyces cerevisiae

The biological role of the "general control of amino acid biosynthesis" has been investigated by analyzing growth and enzyme levels in wild-type, bradytrophic, and nonderepressing mutant strains of Saccharomyces cerevisiae. Amino acid limitation was achieved by using either bradytrophic mutations or external amino acid imbalance. In the wild-type strain noncoordinate derepression of enzymes subject to the general control has been found. Derepressing factors were in the order of 2 to 4 in bradytrophic mutant strains grown under limiting conditions and only in the order of 1.5 to 2 under the influence of external amino acid imbalance. Nonderepressing mutations led to slower growth rates under conditions of amino acid limitation, and no derepression of enzymes under the general control was observed. The amino acid pools were found to be very similar in the wild type and in nonderepressing mutant strains under all conditions tested. Our results indicate that the general control affects all branched amino acid biosynthetic pathways, namely, those of the aromatic amino acids and the aspartate family, the pathways for the basic amino acids lysine, histidine, and arginine, and also the pathways of serine and valine biosyntheses.

[General control of amino acid biosynthesis in Hansenula henricii]

The general control of amino acid biosynthesis was investigated in Hansenula henricii. By limitation for single amino acids in wild type strain and mutants no derepression of enzymes was caused. In prototrophic revertants however, obtained from auxotrophic mutants (his, pdx) enzyme activities were 2 - 17 times higher than in the wild type. In these revertants enzymes of the biosynthetic pathways for tryptophan, phenylalanine, tyrosine, arginine, histidine, cysteine, methionine, asparagine, threonine, and isoleucine were derepressed. The amino acid pool pattern of the revertants if completely different from that in the wild type strain. A discussion of possible mechanisms of general control of amino acid biosynthesis in H. henricii is presented.

Map locations of some mutations conferring resistance to arginine hydroxamate in Bacillus subtilis 168

Mutations conferring resistance to arginine hydroxamate in Bacillus subtilis 168 have been located on the genetic map by PBS1-mediated transduction. The majority of these mutations, belonging to classes 1, 2 and 4 of Harwood and Baumberg (1977) and affecting only expression of arginine catabolic enzymes, map at a locus designated ahr A cotransducible with cysA, purA and sacA. The order of markers in this region appears to be sacA-ahrA-purA-cysA. Certain anomalies were observed in the properties of Pur+ transductants from crosses with an Ahr donor and a purA recipient. A single ahr mutation (class 3), also affecting only arginine catabolism, maps between ctrA and sacA at a locus designated ahrB. Two others (class 6), affecting simultaneously enzymes of both arginine biosynthesis and catabolism, map between lys and aroD at a locus designated ahrC. Preliminary attempts to define the nature of functional products specified by these ahr loci suggest that a protein is encoded at ahrA.

Regulation of compartmentation of amino acid pools in Saccharomyces cerevisiae and its effects on metabolic control

Compartmentation of intracellular amino acid pools has been studied under various growth conditions in wild-type strains as well as in mutants. Aspartate, glutamate, leucine and isoleucine pools are present in high concentrations in the cytoplasm, while all the other amino acids are more vacuolar. The nature of the nitrogen source for growth, the effectiveness of nitrogen assimilation, the rate of protein synthesis and the presence of high internal basic amino acid pools are important factors in the repartition of amino acid pools between the cytoplasm and the vacuole.

Regulation of activity and synthesis of N-acetylglutamate synthase from Saccharomyces cerevisiae

Feedback inhibition of N-acetylgutamate synthase in a particulate fraction from Saccharomyces cerevisiae by L-arginine was synergistically enhanced by N-actylglutamate, whereas coenzyme A let to an additive enhancement of arginine inhibition. N-acetylglutamate synthase was not inhibited by polyamines, nor was the enzyme inactivated by incubation in the presence of coenzyme A and zinc ions. Evidence was obtained for the involvement of at least three different regulatory mechanisms in the expression of N-acetylglutamate synthase: arginine-specific repression, glucose repression and general amino acid control. The combined action of these control mechanisms led to a 90-fold variation in the specific activity of the enzyme.

Dual regulation of the synthesis of the arginine pathway carbamoylphosphate synthase of Saccharomyces cerevisiae by specific and general controls of amino acid biosynthesis

The synthesis of the arginine pathway carbamoylphosphate synthase (CPSase A) of Saccharomyces cerevisiae is subject to two control mechanisms. One mechanism is specific for CPSase A and is exerted by arginine; it probably involves a repressor-operator type of interaction. This "specific" mechanism regulates the expression of gene cpaI coding for the small "glutaminase" subunit of CPSase A but has little influence on the production of the large subunit of the enzyme, a product of gene cpaII. This large component, which alone has no biological significance, accumulates freely under conditions of arginine repression. The second mechanism is general: it controls enzyme synthesis in a number of amino acid biosynthetic pathways in addition to the arginine sequence. Two types of evidence that this "general" mechanism participates in the control of CPSase A synthesis are presented: (1) Derepression upon starvation for any amino acid of which the synthesis is subject to this general control; and (2) repression during growth in amino acid-rich medium. In contrast to the specific mechanism, the "general" mechanism regulates the expression of both the cpaI and cpaII genes.