Positive regulation in the general amino acid control of Saccharomyces cerevisiae

Control of cell growth and division in Saccharomyces cerevisiae

Considerable advances have been made in recent years in our understanding of the biochemistry of protein and nucleic acid synthesis and, particularly, the molecular biology of gene expression in eukaryotes. The yeast Saccharomyces cerevisiae, and to a lesser extent Schizosaccharomyces pombe, has had a preeminent role as a focus for these studies, principally because of the facility with which these organisms can be experimentally manipulated biochemically and genetically. This review will be designed to critically examine and integrate recent advances in several vital areas of regulatory control of enzyme synthesis in yeast: structure and organization of DNA, transcriptional regulation, post-transcriptional modification, control of translation, post-translational modification and secretion, and cell-cycle modulation. It will attempt to emphasize and illustrate, where detailed information is available, principal underlying molecular mechanisms, and it will attempt to make relevant comparisons of this material to inferred and demonstrated facets of regulatory control of enzyme and protein synthesis in higher eukaryotes.

Molecular characterization of chromosomal genes affecting double-stranded RNA replication in Saccharomyces cerevisiae

We cloned MAK11, MAK18, and MKT1 utilizing their genetic map positions. The MAK11 gene is close to CDC16 on chromosome XI. Both genes were cloned on a single 7-kb fragment, and both have now been sequenced. The MAK18 gene is located close to PET3 on chromosome VIII. A large plasmid carrying PET3 was obtained from R. Elder and R.E. Esposito and was found to also have the MAK18 gene. The MAK16 gene has been subcloned and sequenced starting with a clone provided by J. Crowley and D. Kaback. The MKT1 gene was mapped near the gene for topoisomerase II. The topoisomerase II clone was used as the starting point for chromosome-walking to isolate MKT1. A deletion-insertion mutation (disruption) of MKT1 results in an inability to maintain M2, but does not affect M1 or L-A maintenance. Clones of SKI3 and SKI8 were selected using the cold sensitivity for cell growth of ski- M1 strains. The SKI8 gene was disrupted and found to be nonessential for cell growth in the absence of M double-stranded RNA (dsRNA). The SKI3 and SKI8 genes were mapped using these clones. We have also obtained other clones suppressing the pathology caused by the high M titer in ski- strains. These clones are not the SKI genes themselves but somehow avoid the growth defect without repressing M copy number.

SUM1, an apparent positive regulator of the cryptic mating-type loci in Saccharomyces cerevisiae

The mating-type information residing at the HML and HMR loci in Saccharomyces cerevisiae is kept unexpressed by the action of at least four MAR (or SIR) loci. To determine possible interactions between the MAR/SIR gene products and to find new regulatory loci, we sought extragenic suppressors of the mar1-1 mutation. A strain with the genotype HMLa MAT alpha HMRa mar1-1 is unable to mate because of the simultaneous expression of a and alpha information. A mutant of this strain was isolated that exhibits an alpha phenotype and, therefore, presumably fails to express the HML and HMR loci. We designate the new locus SUM1 (suppressor of mar). The mutation is recessive, centromere unlinked and does not correspond to the MAT, HML, HMR, SIR1, MAR1, MAR2 (SIR3) or SIR4 loci. The sum1 mutation affects expression of both a and alpha information at the HM loci. Suppression by sum1-1 is neither allele specific nor locus specific as it suppresses a deletion mutation of the MAR1 locus and mutations in SIR3 and SIR4. The sum1-1 mutation has no discernible phenotype in a Mar+ strain. We propose that the MAR/SIR gene products negatively regulate the SUM1 locus, the gene product of which is necessary for expression of the HM loci.

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.

Effect of alpha-isopropylmalate on the synthesis of RNA and protein in Neurospora

The leu-3/alpha-IPM (alpha-isopropylmalate) regulatory system, previously shown to control several genes of leucine, isoleucine, valine, and histidine biosynthesis, appears likely to be involved also in the regulation of overall RNA and protein synthesis in Neurospora. Upon addition of alpha-IPM the synthesis of all major species of stable RNA was found to be transiently inhibited by approximately 50%. A similar reduction was observed in overall protein synthesis. The inhibition was dependent in both cases on a functional leu-3 gene product, in conformance with previously established patterns of alpha-IPM dependent gene regulation. The overt resemblance of the phenomenon described here to the 'stringent response' of bacteria is noted but neither the mechanism of inhibition nor the precise role of alpha-IPM in the process has been established.

Changes in gene expression elicited by amino acid limitation in Neurospora crassa strains having normal or mutant cross-pathway amino acid control

The effects of amino acid limitation on gene expression have been investigated in Neurospora crassa strains carrying normal (cpc-1+) or mutant (cpc-1) alleles at a locus implicated in cross-pathway amino acid control. Electrophoresis and fluorography were used to reveal the patterns of label incorporation into polypeptides in vivo, or after in vitro translation of extracted mRNAs. In a cpc-1+ strain at least 20% of detectable in vitro translation products showed relative increases in incorporation when RNA was obtained from mycelium grown under conditions of arginine limitation, by comparison with conditions of arginine sufficiency. A cpc-1 mutation, which impairs derepression of a variety of amino acid synthetic enzymes following amino acid limitation, had little detectable effect on in vivo polypeptide synthesis during amino acid sufficient growth or following pyrimidine limitation. However the mutation substantially altered the response to arginine or histidine limitation. The majority of in vitro translation products that showed increased expression in arginine limited cpc-1+ failed to increase in cpc-1 strains, but arginine limitation of cpc-1 also resulted in increases that did not occur in cpc-1+ strains. This may reflect both direct and indirect consequences of the impairment of cross-pathway control.

Point mutation generates constitutive expression of an inducible eukaryotic gene

We describe the analysis of two cis-dominant mutations that result in constitutive expression of the inducible CAR1 gene from yeast. One mutation results from insertion of a Ty element just upstream from the point where CAR1-specific transcription begins. The other mutation is a C-to-G transversion at position -153. Isolation of this point mutation, outside of the transcribed region of CAR1, suggests that expression of this gene is regulated at transcription. It also demonstrates the feasibility and usefulness of analyzing the regulatory sequences of eukaryotic genes on a nucleotide-by-nucleotide basis.

The INO2 and INO4 loci of Saccharomyces cerevisiae are pleiotropic regulatory genes

We isolated a mutant of Saccharomyces cerevisiae defective in the formation of phosphatidylcholine via methylation of phosphatidylethanolamine. The mutant synthesized phosphatidylcholine at a reduced rate and accumulated increased amounts of methylated phospholipid intermediates. It was also found to be auxotrophic for inositol and allelic to an existing series of ino4 mutants. The ino2 and ino4 mutants, originally isolated on the basis of an inositol requirement, are unable to derepress the cytoplasmic enzyme inositol-1-phosphate synthase (myo-inositol-1-phosphate synthase; EC 5.5.1.4). The INO4 and INO2 genes were, thus, previously identified as regulatory genes whose wild-type product is required for expression of the INO1 gene product inositol-1-phosphate synthase (T. Donahue and S. Henry, J. Biol. Chem. 256:7077-7085, 1981). In addition to the identification of a new ino4-allele, further characterization of the existing series of ino4 and ino2 mutants, reported here, demonstrated that they all have a reduced capacity to convert phosphatidylethanolamine to phosphatidylcholine. The pleiotropic phenotype of the ino2 and ino4 mutants described in this paper suggests that the INO2 and INO4 loci are involved in the regulation of phospholipid methylation in the membrane as well as inositol biosynthesis in the cytoplasm.

Isolation and characterization of the RNA2, RNA3, and RNA11 genes of Saccharomyces cerevisiae

Temperature-sensitive mutations in the genes RNA2 through RNA11 cause accumulation of intervening sequence containing precursor mRNAs in Saccharomyces cerevisiae. Three different plasmids have been isolated which complement both the temperature-sensitive lethality and precursor mRNA accumulation when introduced into rna2, rna3, and rna11 mutant strains. The yeast sequences on these plasmids have been shown by Southern transfer hybridization and genetic mapping to be derived from the RNA2, RNA3, and RNA11 genomic loci. Part of the RNA2 gene is homologous to more than one region of the yeast genome, whereas the RNA3 and RNA11 genes are single copy. RNAs homologous to these loci have been identified by RNA transfer hybridization, and the specific RNAs which are associated with the Rna+ phenotype have been mapped. This was done by a combination of transcript mapping, subcloning, and in vitro mutagenesis. The transcripts are found to be enriched in polyadenylated RNA and are of very low abundance (0.01-0.001% polyadenylated RNA).

High-level expression and molecular cloning of genes encoding Candida tropicalis peroxisomal proteins

The development of peroxisomes in the cells of Candida tropicalis grown on oleic acid was accompanied by a markedly high expression of peroxisomal proteins. On the basis of this finding, the nuclear DNA library of this yeast was screened by differential hybridization, and 102 clones of oleic acid-inducible sequences were isolated. Seven coding regions were found to form clusters in three stretches of the genomic DNA. Five of the regions were identified as genes for peroxisomal polypeptides (PXPs). The coding sequence for PXP-2 hybrid selected an additional mRNA for PXP-4, the subunit of long-chain acyl coenzyme A oxidase, which was the most abundant PXP. PXP-2 and PXP-4 were close in apparent molecular weight and generated similar peptides when digested with a protease. The gene for PXP-4 was adjacent to that for PXP-2 on the genome and also hybridized to the mRNA coding for PXP-5. These and other similar results suggest that the genes for the peroxisomal proteins of this organism arose by duplication of a few ancestral genes.

Positive regulatory interactions of the HIS4 gene of Saccharomyces cerevisiae

The role of cis- and trans-acting elements in the expression of HIS4 has been examined by using HIS4-lacZ fusions in which lacZ expression is dependent upon the HIS4 5' noncoding region. The cis-acting sequences involved in regulation were defined by studying the effects of the wild-type and various deletions and their revertants on regulation via the general control of amino acid biosynthesis. The role of trans-acting genes was analyzed by studying the regulation of the HIS4-lacZ fusions in strains carrying mutations in the GCN (AAS) or GCD (TRA) genes and in strains carrying the GCN genes on high-copy-number plasmids. These studies have led to the following conclusions. (i) HIS4 is positively regulated by the general control. (ii) At least one copy of the 5'TGACTC3' repeat at -136 is required in cis for this regulation. (iii) Both the GCN4 gene and at least one copy of the repeated sequence are required for expression at the repressed level. (iv) The open reading frames in the 5' noncoding region are not required in either cis or trans for the regulation of HIS4.

A synthetic HIS4 regulatory element confers general amino acid control on the cytochrome c gene (CYC1) of yeast

Hybrid promoters constructed from upstream sequences of the yeast HIS4 promoter and the downstream element of the yeast CYC1 promoter place iso-1-cytochrome c (CYC1) expression under the general amino acid control, typical of HIS4. HIS4 fragments that confer regulation contain at least one copy of the sequence T-G-A-C-T-C that is repeated at HIS4 and other genes subject to the general control. A 14-base-pair synthetic oligonucleotide containing a single copy of the HIS4 repeat places CYC1 under the general control. Two copies of this oligonucleotide produce a derepressed level of expression nearly equivalent to that conferred by the largest HIS4 5' noncoding fragments we examined and direct regulated expression of a set of transcripts with 5' ends typical of the CYC1 promoter. Comparison of the expression levels conferred by the short synthetic repeat and larger HIS4 5' fragments reveals additional promoter elements required for maintaining efficient gene expression under repressing growth conditions.

Isolation and characterization of the RNA2+, RNA4+, and RNA11+ genes of Saccharomyces cerevisiae

We used genetic complementation to isolate DNA fragments that encode the Saccharomyces cerevisiae genes RNA2+, RNA4+, and RNA11+ and to localize the genes on the cloned DNA fragments. RNA blot-hybridization analyses coupled with genetic analyses indicated the RNA2+ is coded by a 3.0-kilobase (kb) transcript, RNA4+ is coded by a 1.6-kb transcript, and RNA11+ is coded by a 1.3-kb or a 1.7-kb transcript or both; none of the cloned genes contains detectable introns. All three genes were transcribed into messages of very low abundance (approximately 20 times lower than a ribosomal protein message). DNA blot-hybridization revealed that all cloned genes are represented only once in the yeast chromosome. mRNA for RNA2+ and RNA4+ is produced in approximate proportion to gene dosage, whereas RNA11+ transcription appears to be not nearly so dependent on gene dosage. On a medium-copy plasmid (5 to 10 copies per cell), each cloned gene complemented mutations only in its own gene, indicating that each gene encodes a unique function. Genetic analysis by integrative transformation indicated that we cloned the RNA2+, RNA4+, and RNA11+ structural genes and not second-site suppressors.

Isolation and characterization of the RNA2, RNA3, and RNA11 genes of Saccharomyces cerevisiae

Temperature-sensitive mutations in the genes RNA2 through RNA11 cause accumulation of intervening sequence containing precursor mRNAs in Saccharomyces cerevisiae. Three different plasmids have been isolated which complement both the temperature-sensitive lethality and precursor mRNA accumulation when introduced into rna2, rna3, and rna11 mutant strains. The yeast sequences on these plasmids have been shown by Southern transfer hybridization and genetic mapping to be derived from the RNA2, RNA3, and RNA11 genomic loci. Part of the RNA2 gene is homologous to more than one region of the yeast genome, whereas the RNA3 and RNA11 genes are single copy. RNAs homologous to these loci have been identified by RNA transfer hybridization, and the specific RNAs which are associated with the Rna+ phenotype have been mapped. This was done by a combination of transcript mapping, subcloning, and in vitro mutagenesis. The transcripts are found to be enriched in polyadenylated RNA and are of very low abundance (0.01-0.001% polyadenylated RNA).

The INO2 and INO4 loci of Saccharomyces cerevisiae are pleiotropic regulatory genes

We isolated a mutant of Saccharomyces cerevisiae defective in the formation of phosphatidylcholine via methylation of phosphatidylethanolamine. The mutant synthesized phosphatidylcholine at a reduced rate and accumulated increased amounts of methylated phospholipid intermediates. It was also found to be auxotrophic for inositol and allelic to an existing series of ino4 mutants. The ino2 and ino4 mutants, originally isolated on the basis of an inositol requirement, are unable to derepress the cytoplasmic enzyme inositol-1-phosphate synthase (myo-inositol-1-phosphate synthase; EC 5.5.1.4). The INO4 and INO2 genes were, thus, previously identified as regulatory genes whose wild-type product is required for expression of the INO1 gene product inositol-1-phosphate synthase (T. Donahue and S. Henry, J. Biol. Chem. 256:7077-7085, 1981). In addition to the identification of a new ino4-allele, further characterization of the existing series of ino4 and ino2 mutants, reported here, demonstrated that they all have a reduced capacity to convert phosphatidylethanolamine to phosphatidylcholine. The pleiotropic phenotype of the ino2 and ino4 mutants described in this paper suggests that the INO2 and INO4 loci are involved in the regulation of phospholipid methylation in the membrane as well as inositol biosynthesis in the cytoplasm.

Evidence for translational regulation of the activator of general amino acid control in yeast

The GCN4 gene encodes a positive regulator of unlinked amino acid biosynthetic genes in yeast. I present evidence that the GCN4 gene is itself regulated by amino acid availability and that the regulation occurs at the translational level. A GCN4-lacZ fusion was used as a measure of the expression of GCN4 gene product. Starvation for histidine leads to derepression of the fusion enzyme in the wild type but not in a gcn2- strain. The gcn2- mutation does not reduce fusion transcript levels relative to wild type, suggesting that the product of GCN2 functions as an activator of GCN4 translation. The GCN4 transcript has a 5' leader that is approximately equal to 600 nucleotides long and contains four small open reading frames. A deletion of the small open reading frames results in constitutive derepression of fusion enzyme levels as the result of an approximately equal to 10-fold increase in the efficiency of translation of the fusion transcript. The deletion suppresses the requirement for GCN2 function. These results suggest that the GCN4 5' leader acts in cis to repress GCN4 translation and that GCN4 translation increases in response to amino acid starvation as the result of GCN2 antagonism of the repressing sequences in the GCN4 5' leader.

High-level expression and molecular cloning of genes encoding Candida tropicalis peroxisomal proteins

The development of peroxisomes in the cells of Candida tropicalis grown on oleic acid was accompanied by a markedly high expression of peroxisomal proteins. On the basis of this finding, the nuclear DNA library of this yeast was screened by differential hybridization, and 102 clones of oleic acid-inducible sequences were isolated. Seven coding regions were found to form clusters in three stretches of the genomic DNA. Five of the regions were identified as genes for peroxisomal polypeptides (PXPs). The coding sequence for PXP-2 hybrid selected an additional mRNA for PXP-4, the subunit of long-chain acyl coenzyme A oxidase, which was the most abundant PXP. PXP-2 and PXP-4 were close in apparent molecular weight and generated similar peptides when digested with a protease. The gene for PXP-4 was adjacent to that for PXP-2 on the genome and also hybridized to the mRNA coding for PXP-5. These and other similar results suggest that the genes for the peroxisomal proteins of this organism arose by duplication of a few ancestral genes.

Factors permitting prolonged translation by isolated pea chloroplasts

The following parameters were found to prolong the time-course of translation by isolated pea (Pisum sativum, cv Progress No. 9) chloroplasts: addition of other amino acids (an effect synergistic with sufficient free Mg(2+)), use of lower light intensities, and additions of inorganic phosphate and ATP. In a chloroplast system which includes these parameters, active translation usually extends to almost an hour. The total amount of leucine incorporated is routinely 60 to 100 nanomoles/milligram chlorophyll and often 200 nanomoles/milligram chlorophyll. Accurate estimation of the amount of amino acid incorporated depends on supplying the labeled amino acid at a concentration sufficient to overcome isotope dilution effects from endogenous pools. Approximately 39 thylakoid and 60 stroma polypeptides were visible on autoradiographs after labeling with [(35)S]methionine. Label in a few of the polypeptide bands was increased or decreased by specific changes in the reaction conditions. Due to the long period of activity and the large number of labeled products, this chloroplast system should be useful for future studies of chloroplast translation.

5' untranslated sequences are required for the translational control of a yeast regulatory gene

In yeast, many genes encoding amino acid biosynthetic enzymes are subject to a common regulatory system called the general control of amino acid biosynthesis. The product of the regulatory gene GCN4 is required for an increase in transcription of general control-regulated genes when yeast are grown under amino acid-starvation conditions. In this report, we show that the expression of the GCN4 gene is regulated at the translational level: the efficiency of translation of the GCN4 mRNA is dramatically increased during growth under amino acid-starvation conditions. The complete nucleotide sequence of the GCN4 gene, presented here, reveals the existence of an unusually long 5' untranslated region in the corresponding mRNA. In vivo analysis of the effects of a deletion in this 5' leader has enabled us to define a region required for the translational regulation of the GCN4 mRNA.

Positive regulatory interactions of the HIS4 gene of Saccharomyces cerevisiae

The role of cis- and trans-acting elements in the expression of HIS4 has been examined by using HIS4-lacZ fusions in which lacZ expression is dependent upon the HIS4 5' noncoding region. The cis-acting sequences involved in regulation were defined by studying the effects of the wild-type and various deletions and their revertants on regulation via the general control of amino acid biosynthesis. The role of trans-acting genes was analyzed by studying the regulation of the HIS4-lacZ fusions in strains carrying mutations in the GCN (AAS) or GCD (TRA) genes and in strains carrying the GCN genes on high-copy-number plasmids. These studies have led to the following conclusions. (i) HIS4 is positively regulated by the general control. (ii) At least one copy of the 5'TGACTC3' repeat at -136 is required in cis for this regulation. (iii) Both the GCN4 gene and at least one copy of the repeated sequence are required for expression at the repressed level. (iv) The open reading frames in the 5' noncoding region are not required in either cis or trans for the regulation of HIS4.

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.