Characterization of transposable element-associated mutations that alter yeast alcohol dehydrogenase II expression

Lucky, times ten: A career in Texas science

On January 21, 2017, I received an E-mail from Herb Tabor that I had been simultaneously hoping for and dreading for several years: an invitation to write a "Reflections" article for the Journal of Biological Chemistry On the one hand, I was honored to receive an invitation from Herb, a man I have admired for over 40 years, known for 24 years, and worked with as a member of the Editorial Board and Associate Editor of the Journal of Biological Chemistry for 17 years. On the other hand, the invitation marked the waning of my career as an academic scientist. With these conflicting emotions, I wrote this article with the goals of recording my career history and recognizing the many mentors, trainees, and colleagues who have contributed to it and, perhaps with pretension, with the desire that students who are beginning a career in research will find inspiration in the path I have taken and appreciate the importance of luck.

Identification of three genes required for the glucose-dependent transcription of the yeast transcriptional activator ADR1

Glucose repression of the ADH2 gene from Saccharomyces cerevisiae is mediated by the synthesis and activity of the transcriptional activator ADR1. In this study, we isolated mutations in three new genes (SAF1, SAF2 and SAF3) that suppressed the glucose-insensitive expression of ADH2 caused by the ADR1-5c allele. The mechanism by which the SAF genes maintain ADR1-5c function was investigated. Each of the mutated SAF genes was found to suppress ADR1-5c activity by lowering ADR1-5c steady state mRNA levels 5- to 8-fold under glucose growth conditions. ADR1 mRNA levels were similarly affected by the saf mutations. In contrast, mutations in the SAF genes had little or no effect on ADR1-5c or ADR1 mRNA levels under ethanol growth conditions. The stability of ADR1-5c mRNA was unaffected by mutations in each of the SAF genes, implying that the SAF genes are required for the transcription of ADR1 mRNA under glucose growth conditions. The possible function of the three SAF genes in ADR1 expression is discussed.

Glucose repression of the yeast ADH2 gene occurs through multiple mechanisms, including control of the protein synthesis of its transcriptional activator, ADR1

The rate of ADH2 transcription increases dramatically when Saccharomyces cerevisiae cells are shifted from glucose to ethanol growth conditions. Since ADH2 expression under glucose growth conditions is strictly dependent on the dosage of the transcriptional activator ADR1, we investigated the possibility that regulation of the rate of ADR1 protein synthesis plays a role in controlling ADR1 activation of ADH2 transcription. We found that the rate of ADR1 protein synthesis increased 10- to 16-fold within 40 to 60 min after glucose depletion, coterminous with initiation of ADH2 transcription. Changes in ADR1 mRNA levels contributed only a twofold effect on ADR1 protein synthetic differences. The 510-nt untranslated ADR1 mRNA leader sequence was found to have no involvement in regulating the rate of ADR1 protein synthesis. In contrast, sequences internal to ADR1 coding region were determined to be necessary for controlling ADR1 translation. The ADR1c mutations which enhance ADR1 activity under glucose growth conditions did not affect ADR1 protein translation. ADR1 was also shown to be multiply phosphorylated in vivo under both ethanol and glucose growth conditions. Our results indicate that derepression of ADH2 occurs through multiple mechanisms involving the ADR1 regulatory protein.

Glucose repression of the yeast ADH2 gene occurs through multiple mechanisms, including control of the protein synthesis of its transcriptional activator, ADR1

The rate of ADH2 transcription increases dramatically when Saccharomyces cerevisiae cells are shifted from glucose to ethanol growth conditions. Since ADH2 expression under glucose growth conditions is strictly dependent on the dosage of the transcriptional activator ADR1, we investigated the possibility that regulation of the rate of ADR1 protein synthesis plays a role in controlling ADR1 activation of ADH2 transcription. We found that the rate of ADR1 protein synthesis increased 10- to 16-fold within 40 to 60 min after glucose depletion, coterminous with initiation of ADH2 transcription. Changes in ADR1 mRNA levels contributed only a twofold effect on ADR1 protein synthetic differences. The 510-nt untranslated ADR1 mRNA leader sequence was found to have no involvement in regulating the rate of ADR1 protein synthesis. In contrast, sequences internal to ADR1 coding region were determined to be necessary for controlling ADR1 translation. The ADR1c mutations which enhance ADR1 activity under glucose growth conditions did not affect ADR1 protein translation. ADR1 was also shown to be multiply phosphorylated in vivo under both ethanol and glucose growth conditions. Our results indicate that derepression of ADH2 occurs through multiple mechanisms involving the ADR1 regulatory protein.

A spontaneous chromosomal amplification of the ADH2 gene in Saccharomyces cerevisiae

A spontaneous antimycin A-resistant mutant carrying approximately four extra copies of ADH2 on chromosome XII was isolated from yeast strain 315-1D which lacks a functional copy of ADH1 and thus is antimycin A-sensitive. The additional copies of the normally glucose-repressed ADH2 are expressed during growth on glucose accounting for the antimycin A resistance. These extra copies are inserted into nonadjacent ribosomal DNA sequences (rDNA) near the recombination stimulating sequence HOT1. Each extra copy of the ADH2 gene (1548 bp) replaces most of the 37S transcript (approximately 7400 bp) in one of the approximately 200 copies of the rDNA present in the yeast genome. All four extra copies of ADH2 are lost at a rate of approximately 1 x 10(-5) deletions per cell per generation. One of the joints between the rDNA and ADH2 DNA is located 7 nucleotides downstream from 20 adenine residues in the normal copy of ADH2. This joint occurs at the end of a stretch of 16-29 thymidines in the rDNA which has been expanded to 57-59 thymidines. The other novel joint is located in a short region of sequence similarity between ADH2 and the rDNA. These observations suggest that amplification of ADH2 was a two step process: first the ADH2 gene was inserted into the rDNA, then multiple copies were generated by unequal crossing over or gene conversion within the rDNA.

TEC1, a gene involved in the activation of Ty1 and Ty1-mediated gene expression in Saccharomyces cerevisiae: cloning and molecular analysis

Ty and Ty-mediated gene expression observed in haploid cells of Saccharomyces cerevisiae depends on several determinants, some of which are required for the expression of haploid-specific genes. We report here the cloning and molecular analysis of TEC1. TEC1 encodes a 486-amino-acid protein that is a trans-acting factor required for full Ty1 expression and Ty1-mediated gene activation. However, mutation or deletion of the TEC1 gene had little effect on total Ty2 transcript levels. Our analysis provides clear evidence that TEC1 is not involved in mating or sporulation processes. Unlike most of the proteins involved in Ty and adjacent gene expression, the product of TEC1 has no known cellular function. Although there was no mating-type effect on TEC1 expression, our results indicate that the TEC1 and the a/alpha diploid controls on Ty1 expression are probably not cumulative.

TEC1, a gene involved in the activation of Ty1 and Ty1-mediated gene expression in Saccharomyces cerevisiae: cloning and molecular analysis

Ty and Ty-mediated gene expression observed in haploid cells of Saccharomyces cerevisiae depends on several determinants, some of which are required for the expression of haploid-specific genes. We report here the cloning and molecular analysis of TEC1. TEC1 encodes a 486-amino-acid protein that is a trans-acting factor required for full Ty1 expression and Ty1-mediated gene activation. However, mutation or deletion of the TEC1 gene had little effect on total Ty2 transcript levels. Our analysis provides clear evidence that TEC1 is not involved in mating or sporulation processes. Unlike most of the proteins involved in Ty and adjacent gene expression, the product of TEC1 has no known cellular function. Although there was no mating-type effect on TEC1 expression, our results indicate that the TEC1 and the a/alpha diploid controls on Ty1 expression are probably not cumulative.

A region internal to the coding sequences is essential for transcription of the yeast Ty-D15 element

The major transcript of the yeast transposable element Ty1 has its 5' end in one delta and the 3' end in the opposite delta, the direct repeats of about 335 base pairs (bp) at each end of the element. The transcriptional initiation signals of the Ty-D15 element that give rise to this transcript were found to have a number of unusual characteristics. The 5' delta by itself, which contained the initiation site for Ty transcription, gave no detectable transcription. A region internal to the transcript in a translated part of the element and about 140 bp downstream of the 5' delta was essential for initiation of the major Ty transcript. This internal activating region (IAR) had several interesting properties. When the portion of the delta upstream of the initiation site was replaced with DNA fragments that did not by themselves act as promoters, initiation directed by the IAR still occurred at about the same position, 200 to 400 bp upstream of the IAR. If fragments containing the IAR were inverted, transcription could still occur. When 468 or 636 bp was inserted between the delta and the IAR, initiations occurred near the normal delta initiation site and in the inserted DNA. Therefore, the location and properties of transcription signals for Ty-D15 differ considerably from those expected for a yeast gene transcribed by RNA polymerase II.

A region internal to the coding sequences is essential for transcription of the yeast Ty-D15 element

The major transcript of the yeast transposable element Ty1 has its 5' end in one delta and the 3' end in the opposite delta, the direct repeats of about 335 base pairs (bp) at each end of the element. The transcriptional initiation signals of the Ty-D15 element that give rise to this transcript were found to have a number of unusual characteristics. The 5' delta by itself, which contained the initiation site for Ty transcription, gave no detectable transcription. A region internal to the transcript in a translated part of the element and about 140 bp downstream of the 5' delta was essential for initiation of the major Ty transcript. This internal activating region (IAR) had several interesting properties. When the portion of the delta upstream of the initiation site was replaced with DNA fragments that did not by themselves act as promoters, initiation directed by the IAR still occurred at about the same position, 200 to 400 bp upstream of the IAR. If fragments containing the IAR were inverted, transcription could still occur. When 468 or 636 bp was inserted between the delta and the IAR, initiations occurred near the normal delta initiation site and in the inserted DNA. Therefore, the location and properties of transcription signals for Ty-D15 differ considerably from those expected for a yeast gene transcribed by RNA polymerase II.

Control of yeast gene expression by transposable elements: maximum expression requires a functional Ty activator sequence and a defective Ty promoter

Integration of a transposable element adjacent to a gene frequently results in an alteration in expression of the nearby gene. The purpose of our experiments was to identify cis-acting sequences within a yeast transposon (Ty) that are important for expression of the adjacent gene. The role of these sequences in Ty transcription was also analyzed in order to examine the relationship between Ty and adjacent gene expression. Three naturally occurring Ty elements located at the HIS4 locus were examined. These Ty elements differed by multiple sequence changes and had different effects on HIS4 expression. To determine which sequences were important to Ty and HIS4 expression, Ty::lacZ and Ty::HIS4::lacZ fusion genes were constructed and analyzed. Results of these experiments indicated that a sequence element is present in the Ty epsilon region that is necessary for HIS4 expression but which has only a modest effect on Ty transcription. Additionally, a mutation in the Ty promoter region decreased Ty transcription and increased HIS4 expression. The opposite effects of this mutation on Ty and adjacent gene expression were probably caused by promoter competition.

Control of yeast gene expression by transposable elements: maximum expression requires a functional Ty activator sequence and a defective Ty promoter

Integration of a transposable element adjacent to a gene frequently results in an alteration in expression of the nearby gene. The purpose of our experiments was to identify cis-acting sequences within a yeast transposon (Ty) that are important for expression of the adjacent gene. The role of these sequences in Ty transcription was also analyzed in order to examine the relationship between Ty and adjacent gene expression. Three naturally occurring Ty elements located at the HIS4 locus were examined. These Ty elements differed by multiple sequence changes and had different effects on HIS4 expression. To determine which sequences were important to Ty and HIS4 expression, Ty::lacZ and Ty::HIS4::lacZ fusion genes were constructed and analyzed. Results of these experiments indicated that a sequence element is present in the Ty epsilon region that is necessary for HIS4 expression but which has only a modest effect on Ty transcription. Additionally, a mutation in the Ty promoter region decreased Ty transcription and increased HIS4 expression. The opposite effects of this mutation on Ty and adjacent gene expression were probably caused by promoter competition.

DNA sequence analysis of spontaneous mutations in the SUP4-o gene of Saccharomyces cerevisiae

A collection of 196 spontaneous mutations in the SUP4-o gene of the yeast Saccharomyces cerevisiae was analyzed by DNA sequencing. The classes of mutation identified included all possible types of base-pair substitution, deletions of various lengths, complex alterations involving multiple changes, and insertions of transposable elements. Our findings demonstrate that at least several different mechanisms are responsible for spontaneous mutagenesis in S. cerevisiae.

DNA sequence analysis of spontaneous mutations in the SUP4-o gene of Saccharomyces cerevisiae

A collection of 196 spontaneous mutations in the SUP4-o gene of the yeast Saccharomyces cerevisiae was analyzed by DNA sequencing. The classes of mutation identified included all possible types of base-pair substitution, deletions of various lengths, complex alterations involving multiple changes, and insertions of transposable elements. Our findings demonstrate that at least several different mechanisms are responsible for spontaneous mutagenesis in S. cerevisiae.

The yeast ROAM mutation--identification of the sequences mediating host gene activation and cell-type control in the yeast retrotransposon, Ty

When the yeast retrotransposon, Ty, integrates into the 5' flanking region of a gene it can activate the expression of that gene. At the same time the activated gene is brought under cell-type specific control such that expression is high in haploid a or alpha cells but low in a/alpha diploids. These Ty mediated mutations are known as ROAM mutations. In this study we have used a ROAM mutation created in vitro to identify the sequences within Ty that mediate this phenomenon. We show that a single activator located within the coding region of the Ty element is responsible for ROAM activation. This sequence, which is regulated by the mating type of the cell, differs from classical enhancer elements in that its activity is strictly orientation dependent. An independent activator located downstream of the ROAM sequence activated transcription only in the non-ROAM orientation. This sequence may be part of an internal promoter that controls expression of the sub-genomic 5.0kb Ty transcript.

Primary structure requirements for correct sorting of the yeast mitochondrial protein ADH III to the yeast mitochondrial matrix space

Alcohol dehydrogenase isoenzyme III (ADH III) in Saccharomyces cerevisiae, the product of the ADH3 gene, is located in the mitochondrial matrix. The ADH III protein was synthesized as a larger precursor in vitro when the gene was transcribed with the SP6 promoter and translated with a reticulocyte lysate. A precursor of the same size was detected when radioactively pulse-labeled proteins were immunoprecipitated with anti-ADH antibody. This precursor was rapidly processed to the mature form in vivo with a half-time of less than 3 min. The processing was blocked if the mitochondria were uncoupled with carbonyl cyanide m-chlorophenylhydrazone. Mutant enzymes in which only the amino-terminal 14 or 16 amino acids of the presequence were retained were correctly targeted and imported into the matrix. A mutant enzyme that was missing the amino-terminal 17 amino acids of the presequence produced an active enzyme, but the majority of the enzyme activity remained in the cytoplasmic compartment on cellular fractionation. Random amino acid changes were produced in the wild-type presequence by bisulfite mutagenesis of the ADH3 gene. The resulting ADH III protein was targeted to the mitochondria and imported into the matrix in all of the mutants tested, as judged by enzyme activity. Mutants containing amino acid changes in the carboxyl-proximal half of the ADH3 presequence were imported and processed to the mature form at a slower rate than the wild type, as judged by pulse-chase studies in vivo. The unprocessed precursor appeared to be unstable in vivo. It was concluded that only a small portion of the presequence contains the necessary information for correct targeting and import. Furthermore, the information for correct proteolytic processing of the presequence appears to be distinct from the targeting information and may involve secondary structure information in the presequence.

Ty1 sequence with enhancer and mating-type-dependent regulatory activities

Some insertion mutations in Saccharomyces cerevisiae activate the expression of adjacent structural genes. The CYC7-H2 mutation is a Ty1 insertion 5' to the iso-2-cytochrome c coding region of CYC7. The Ty1 insertion causes a 20-fold increase in CYC7 expression in a and alpha haploid cell types of S. cerevisiae. This activation is repressed in the a/alpha diploid cell type. Previous computer analysis of the CYC7-H2 Ty1 activator region identified two related sequences with homology both to mammalian enhancers and to a yeast a/alpha control site. A 112-base-pair (bp) DNA fragment encompassing one of these blocks of homology functioned as one component of the Ty1 activator. A 28-bp synthetic oligonucleotide with the wild-type homology block sequence was also functional. A single base pair mutation within the enhancer core of the synthetic 28-bp regulatory element reduced its activation ability to near background amounts. In addition, the 112-bp Ty1 fragment by itself functioned as a target for repression of adjacent gene expression in a/alpha diploid cells.

Isolation of a Saccharomyces cerevisiae centromere DNA-binding protein, its human homolog, and its possible role as a transcription factor

A protein that binds specifically to Saccharomyces cerevisiae centromere DNA element I was purified on the basis of a nitrocellulose filter-binding assay. This protein, termed centromere-binding protein 1 (CP1), was heat stable and renaturable from sodium dodecyl sulfate (SDS), and assays of eluates from SDS gels indicated a molecular weight of 57,000 to 64,000. An activity with similar specificity and stability was detected in human lymphocyte extracts, and analysis in SDS gels revealed a molecular weight of 39,000 to 49,000. CP1-binding sites occurred not only at centromeres but also near many transcription units, for example, adjacent to binding sites for the GAL4-positive regulatory protein upstream of the GAL2 gene in S. cerevisiae and adjacent to the TATA element of the adenovirus major late promoter. A factor (termed USF) that binds to the latter site and stimulates transcription has been isolated from HeLa cells by others.

Structure of the Saccharomyces cerevisiae HO gene and analysis of its upstream regulatory region

The HO gene product of Saccharomyces cerevisiae is a site-specific endonuclease that initiates mating type interconversion. We have determined the nucleotide sequence of a 3,129-base-pair (bp) segment containing HO. The segment contains a single long open reading frame encoding a polypeptide of 586 amino acids, which has unusual (unbiased) codon usage and is preceded by 762 bp of upstream region. The predicted HO protein is basic (16% lysine and arginine) and is calculated to have a secondary structure that is 30% helical. The corresponding transcript is initiated approximately 50 nucleotides prior to the presumed initiation codon. Insertion of an Escherichia coli lacZ gene fragment into the putative HO coding segment inactivated HO and formed a hybrid HO-lacZ gene whose beta-galactosidase activity was regulated by the mating type locus in the same manner as HO (repressed by a 1-alpha 2). Upstream regions of 1,360 and 762 bp conferred strong repression; 436 bp led to partial constitutivity and 301 bp to full constitutivity. Thus, DNA sequences that confer repression of HO by a1-alpha 2 are at least 250 nucleotides upstream of the transcription start point and are within 436 nucleotides of the HO initiation codon. The progressive loss of repression suggests that both the -762 to -436 and the -436 to -301 intervals contain sites for regulation by a1-alpha 2. The HO gene contains two distinct regions that promote autonomous replication of plasmids in S. cerevisiae. These regions contain sequences that are homologous to the two conserved sequences that are associated with ARS activity.

Ty insertions at two loci account for most of the spontaneous antimycin A resistance mutations during growth at 15 degrees C of Saccharomyces cerevisiae strains lacking ADH1

The mutation rate to antimycin A resistance was determined for strains of Sacchromyces cerevisiae lacking a functional copy of the structural gene for alcohol dehydrogenase I (ADH1). One type of mutation that can cause antimycin A resistance in these strains is insertion of the transposable element Ty 5' to ADH2, the structural gene for the glucose-repressed isozyme of alcohol dehydrogenase, resulting in expression of this gene during growth on glucose. Here we show that after growth at 15 or 20 degrees C on glucose, 30% of the antimycin A resistance mutations are Ty insertions at ADH2 and another 65% of the mutations are Ty insertions at ADH4, a new locus identified and cloned as described in this paper. At 30 degrees C only 6% of the mutations are Ty insertions at either of these two loci. In addition, we show that the transposition rate is lower in mating-incompetent (a/alpha) cells than in either haploid or diploid mating-competent cells. Our results suggest that under certain conditions Ty transposition may be a major cause of spontaneous mutations in S. cerevisiae.

Primary structure requirements for correct sorting of the yeast mitochondrial protein ADH III to the yeast mitochondrial matrix space

Alcohol dehydrogenase isoenzyme III (ADH III) in Saccharomyces cerevisiae, the product of the ADH3 gene, is located in the mitochondrial matrix. The ADH III protein was synthesized as a larger precursor in vitro when the gene was transcribed with the SP6 promoter and translated with a reticulocyte lysate. A precursor of the same size was detected when radioactively pulse-labeled proteins were immunoprecipitated with anti-ADH antibody. This precursor was rapidly processed to the mature form in vivo with a half-time of less than 3 min. The processing was blocked if the mitochondria were uncoupled with carbonyl cyanide m-chlorophenylhydrazone. Mutant enzymes in which only the amino-terminal 14 or 16 amino acids of the presequence were retained were correctly targeted and imported into the matrix. A mutant enzyme that was missing the amino-terminal 17 amino acids of the presequence produced an active enzyme, but the majority of the enzyme activity remained in the cytoplasmic compartment on cellular fractionation. Random amino acid changes were produced in the wild-type presequence by bisulfite mutagenesis of the ADH3 gene. The resulting ADH III protein was targeted to the mitochondria and imported into the matrix in all of the mutants tested, as judged by enzyme activity. Mutants containing amino acid changes in the carboxyl-proximal half of the ADH3 presequence were imported and processed to the mature form at a slower rate than the wild type, as judged by pulse-chase studies in vivo. The unprocessed precursor appeared to be unstable in vivo. It was concluded that only a small portion of the presequence contains the necessary information for correct targeting and import. Furthermore, the information for correct proteolytic processing of the presequence appears to be distinct from the targeting information and may involve secondary structure information in the presequence.

Ty1 sequence with enhancer and mating-type-dependent regulatory activities

Some insertion mutations in Saccharomyces cerevisiae activate the expression of adjacent structural genes. The CYC7-H2 mutation is a Ty1 insertion 5' to the iso-2-cytochrome c coding region of CYC7. The Ty1 insertion causes a 20-fold increase in CYC7 expression in a and alpha haploid cell types of S. cerevisiae. This activation is repressed in the a/alpha diploid cell type. Previous computer analysis of the CYC7-H2 Ty1 activator region identified two related sequences with homology both to mammalian enhancers and to a yeast a/alpha control site. A 112-base-pair (bp) DNA fragment encompassing one of these blocks of homology functioned as one component of the Ty1 activator. A 28-bp synthetic oligonucleotide with the wild-type homology block sequence was also functional. A single base pair mutation within the enhancer core of the synthetic 28-bp regulatory element reduced its activation ability to near background amounts. In addition, the 112-bp Ty1 fragment by itself functioned as a target for repression of adjacent gene expression in a/alpha diploid cells.

Isolation of a Saccharomyces cerevisiae centromere DNA-binding protein, its human homolog, and its possible role as a transcription factor

A protein that binds specifically to Saccharomyces cerevisiae centromere DNA element I was purified on the basis of a nitrocellulose filter-binding assay. This protein, termed centromere-binding protein 1 (CP1), was heat stable and renaturable from sodium dodecyl sulfate (SDS), and assays of eluates from SDS gels indicated a molecular weight of 57,000 to 64,000. An activity with similar specificity and stability was detected in human lymphocyte extracts, and analysis in SDS gels revealed a molecular weight of 39,000 to 49,000. CP1-binding sites occurred not only at centromeres but also near many transcription units, for example, adjacent to binding sites for the GAL4-positive regulatory protein upstream of the GAL2 gene in S. cerevisiae and adjacent to the TATA element of the adenovirus major late promoter. A factor (termed USF) that binds to the latter site and stimulates transcription has been isolated from HeLa cells by others.

ADR1-mediated regulation of ADH2 requires an inverted repeat sequence

DNA sequence analysis of wild-type and mutant ADH2 loci suggested that two unusual features 5' of the promoter, a 22-base-pair perfect dyad sequence and a (dA)20 tract, were important for regulation of this gene (D. W. Russell, M. Smith, D. Cox, V. M. Williamson, and E. T. Young, Nature [London] 304:652-654, 1983). Oligonucleotide-directed mutagenesis was used to construct ADH2 genes lacking the 22-base-pair dyad or the (dA)20 tract (V.-L. Chan and M. Smith, Nucleic Acids Res. 12:2407-2419, 1984). These mutant genes and other ADH2 deletions constructed by BAL 31 endonuclease digestion were studied after replacing the wild-type chromosomal locus with the altered alleles by the technique of gene transplacement (T. L. Orr-Weaver, J. W. Szostak, and R. S. Rothstein, Proc. Natl. Acad. Sci. USA 78:6354-6358, 1981), using canavanine resistance as the selectable marker. Deletions lacking the dyad failed to derepress normally and did not respond to mutations at the ADR1 locus, which encodes a protein necessary to activate ADH2. Deletions of the (dA)20 tract did not have a detectable phenotype. A small deletion located just 3' to the (dA)20 tract (between positions -164 and -146) had a low amount of ADR1-dependent transcription during repressed growth conditions, indicating that the regulatory protein encoded by ADR1 is present in a potentially active form during repression and that alterations of a DNA sequence in the promoter region can unmask its latent activity.

Structure of the Saccharomyces cerevisiae HO gene and analysis of its upstream regulatory region

The HO gene product of Saccharomyces cerevisiae is a site-specific endonuclease that initiates mating type interconversion. We have determined the nucleotide sequence of a 3,129-base-pair (bp) segment containing HO. The segment contains a single long open reading frame encoding a polypeptide of 586 amino acids, which has unusual (unbiased) codon usage and is preceded by 762 bp of upstream region. The predicted HO protein is basic (16% lysine and arginine) and is calculated to have a secondary structure that is 30% helical. The corresponding transcript is initiated approximately 50 nucleotides prior to the presumed initiation codon. Insertion of an Escherichia coli lacZ gene fragment into the putative HO coding segment inactivated HO and formed a hybrid HO-lacZ gene whose beta-galactosidase activity was regulated by the mating type locus in the same manner as HO (repressed by a 1-alpha 2). Upstream regions of 1,360 and 762 bp conferred strong repression; 436 bp led to partial constitutivity and 301 bp to full constitutivity. Thus, DNA sequences that confer repression of HO by a1-alpha 2 are at least 250 nucleotides upstream of the transcription start point and are within 436 nucleotides of the HO initiation codon. The progressive loss of repression suggests that both the -762 to -436 and the -436 to -301 intervals contain sites for regulation by a1-alpha 2. The HO gene contains two distinct regions that promote autonomous replication of plasmids in S. cerevisiae. These regions contain sequences that are homologous to the two conserved sequences that are associated with ARS activity.

Transcriptional analysis of Ty1 deletion and inversion derivatives at CYC7

One class of Ty insertion mutation in Saccharomyces cerevisiae activates expression of adjacent structural genes. The CYC7-H2 mutation, in which a Ty1 element is inserted 5' to the iso-2-cytochrome c coding region of CYC7, causes a 20-fold increase in CYC7 expression. Deletion analysis of CYC7-H2 has shown that distal regions of the Ty1 element are not essential for the transcriptional activation at CYC7. In this report, we have analyzed Ty1 and CYC7 RNA from two CYC7-H2 deletion derivative genes to determine whether a direct correlation exists between transcription of Ty1 and transcription of the adjacent gene. Assuming that all Ty1 elements in the genome are transcribed equally, amounts of CYC7-H2 deletion derivative Ty1 RNA were found to be at least fivefold lower than the amount estimated for the average Ty1 element. These same Ty1 deletion derivatives caused a 20-fold increase in adjacent CYC7 expression. This finding suggests that the mechanism by which Ty1 activates adjacent gene expression does not require normal levels of Ty1 transcription. Two inversion derivatives of the CYC7-H2 Ty1 have also been analyzed. These derivatives did not produce any iso-2-cytochrome c or any normal CYC7 mRNA. Instead they were found to produce a Tyl-CYC7 fusion RNA. Consistent with our findings on CYC7-H2 Ty1 transcription, the amount of the fusion RNA was very low. In addition, the Ty1 inversion derivatives produced a new RNA that mapped to sequences upstream from the inverted Ty1 segment. Similar to Ty1 insertions that activate transcription, the new RNA was found to be transcribed away from Ty1.

Isolation and DNA sequence of ADH3, a nuclear gene encoding the mitochondrial isozyme of alcohol dehydrogenase in Saccharomyces cerevisiae

The Saccharomyces cerevisiae nuclear gene, ADH3, that encodes the mitochondrial alcohol dehydrogenase isozyme ADH III was cloned by virtue of its nucleotide homology to ADH1 and ADH2. Both chromosomal and plasmid-encoded ADH III isozymes were repressed by glucose and migrated heterogeneously on nondenaturing gels. Nucleotide sequence analysis indicated 73 and 74% identity for ADH3 with ADH1 and ADH2, respectively. The amino acid identity between the predicted ADH III polypeptide and ADH I and ADH II was 79 and 80%, respectively. The open reading frame encoding ADH III has a highly basic 27-amino-acid amino-terminal extension relative to ADH I and ADH II. The nucleotide sequence of the presumed leader peptide has a high degree of identity with the untranslated leader regions of ADH1 and ADH2 mRNAs. A strain containing a null allele of ADH3 did not have a detectably altered phenotype. The cloned gene integrated at the ADH3 locus, indicating that this is the structural gene for ADH III.

ADR1-mediated regulation of ADH2 requires an inverted repeat sequence

DNA sequence analysis of wild-type and mutant ADH2 loci suggested that two unusual features 5' of the promoter, a 22-base-pair perfect dyad sequence and a (dA)20 tract, were important for regulation of this gene (D. W. Russell, M. Smith, D. Cox, V. M. Williamson, and E. T. Young, Nature [London] 304:652-654, 1983). Oligonucleotide-directed mutagenesis was used to construct ADH2 genes lacking the 22-base-pair dyad or the (dA)20 tract (V.-L. Chan and M. Smith, Nucleic Acids Res. 12:2407-2419, 1984). These mutant genes and other ADH2 deletions constructed by BAL 31 endonuclease digestion were studied after replacing the wild-type chromosomal locus with the altered alleles by the technique of gene transplacement (T. L. Orr-Weaver, J. W. Szostak, and R. S. Rothstein, Proc. Natl. Acad. Sci. USA 78:6354-6358, 1981), using canavanine resistance as the selectable marker. Deletions lacking the dyad failed to derepress normally and did not respond to mutations at the ADR1 locus, which encodes a protein necessary to activate ADH2. Deletions of the (dA)20 tract did not have a detectable phenotype. A small deletion located just 3' to the (dA)20 tract (between positions -164 and -146) had a low amount of ADR1-dependent transcription during repressed growth conditions, indicating that the regulatory protein encoded by ADR1 is present in a potentially active form during repression and that alterations of a DNA sequence in the promoter region can unmask its latent activity.

Identification of the DNA sequences controlling the expression of the MAT alpha locus of yeast

We have excised a 28-base-pair DNA fragment from the MAT alpha intergenic region and tested its ability to direct diploid-specific transcriptional repression. This fragment (1643-1671, 5'-GCTTCCCAATGTAGAAAAGTACA-TCATA-3') lies within a region required for the normal diploid-specific repression of the MAT alpha transcripts. First, the fragment was inserted into a 53-base-pair MAT alpha deletion that expresses alpha 1 and alpha 2 constitutively. Insertion of the fragment restores proper diploid regulation to the MAT alpha transcripts: alpha 1 mRNA is strongly repressed and alpha 2 mRNA is reduced by a factor of approximately equal to 10 from its haploid level. The fragment works equally well in either orientation, and two copies of the fragment do not lead to stronger repression than a single copy. We also inserted the fragment at three sites upstream of the CYC1-lacZ fusion gene. Insertions placing the regulatory fragment between the CYC1 upstream activator sequence (UAS) and the coding region make beta-galactosidase efficiently in alpha haploids but produce 1/40th the enzyme in a/alpha diploids. This diploid-specific repression requires functional MATa-1 gene product. Insertion of the MAT fragment on the opposite side of the UAS (37 base pairs upstream of the UAS) also caused diploid repression of the fusion gene, but only by a factor of 7. When the regulatory fragment is inserted at a large distance on the far side of the UAS (375 base pairs), it has little if any effect on beta-galactosidase expression. We postulate that this sequence is the operator recognized by the diploid-specific repressor.

X-linked mutations that give rise to overproduction of glucose-6-phosphate dehydrogenase in Drosophila melanogaster

Two X-linked mutations that give rise to overproduction of glucose-6-phosphate dehydrogenase (G6PD) were found among the progenies of isogenic strains which had been subjected to selection for high G6PD activity. Mapping of the high-activity factor in these mutants was carried out using car ZwB sw males of low G6PD activity. As a result, the factor mapped 0.02-0.04 unit to the left of the Zw locus. The amount of the G6PD gene was also quantitated utilizing a cloned G6PD gene as a probe, but no significant difference was found between the mutants and low-G6PD activity flies which shared the same X, second, and third chromosomes with the mutants. These findings are consistent with our notion that the mutations might be regulatory mutations, possibly resulting from the insertion of a novel class of transposable genetic elements.

Ty insertions at two loci account for most of the spontaneous antimycin A resistance mutations during growth at 15 degrees C of Saccharomyces cerevisiae strains lacking ADH1

The mutation rate to antimycin A resistance was determined for strains of Sacchromyces cerevisiae lacking a functional copy of the structural gene for alcohol dehydrogenase I (ADH1). One type of mutation that can cause antimycin A resistance in these strains is insertion of the transposable element Ty 5' to ADH2, the structural gene for the glucose-repressed isozyme of alcohol dehydrogenase, resulting in expression of this gene during growth on glucose. Here we show that after growth at 15 or 20 degrees C on glucose, 30% of the antimycin A resistance mutations are Ty insertions at ADH2 and another 65% of the mutations are Ty insertions at ADH4, a new locus identified and cloned as described in this paper. At 30 degrees C only 6% of the mutations are Ty insertions at either of these two loci. In addition, we show that the transposition rate is lower in mating-incompetent (a/alpha) cells than in either haploid or diploid mating-competent cells. Our results suggest that under certain conditions Ty transposition may be a major cause of spontaneous mutations in S. cerevisiae.

The relationship between the "TATA" sequence and transcription initiation sites at the HIS4 gene of Saccharomyces cerevisiae

Transcription of the HIS4 gene begins at a single site (I) at position -60 from the ATG that begins translation. We have made linker insertions/deletions in the 5' noncoding region to identify the elements required for the specificity of transcription initiation. Although there are four sequences that begin TATA and are near the start of transcription (-170, -132, -123, and -102) only the sequence at -123 (TATA-123) is required for transcription initiation. By inserting synthetic oligonucleotides into a mutant from which TATA-123 had been deleted, we found that just TATA or TATAA does not work but that TATAAA functions almost as well as the wild-type sequence. This hexamer does not work in the opposite orientation (TTTATA). When a synthetic TATA sequence is placed upstream from the normal site, the site of initiation also moves upstream in a roughly cometric way even when TATA-123 is present. Analysis of transcripts in strains where the distance between the TATA sequence and the wild-type site of transcription initiation (I site) has been altered shows that in yeast, unlike higher cells, transcription does not initiate at a strictly defined distance from the TATA sequence. Constructions that alter the distance between the TATA and the I site or remove the I site change the pattern of transcription initiation without affecting the level of HIS4 expression. Deletions that eliminate the I site produce heterogeneous transcripts and deletions that substantially shorten the distance between TATA-123 and the I site initiate at multiple sites downstream from the I site. Thus, both the TATA and the sequences downstream from it determine the pattern of transcription initiation.

Isolation and DNA sequence of ADH3, a nuclear gene encoding the mitochondrial isozyme of alcohol dehydrogenase in Saccharomyces cerevisiae

The Saccharomyces cerevisiae nuclear gene, ADH3, that encodes the mitochondrial alcohol dehydrogenase isozyme ADH III was cloned by virtue of its nucleotide homology to ADH1 and ADH2. Both chromosomal and plasmid-encoded ADH III isozymes were repressed by glucose and migrated heterogeneously on nondenaturing gels. Nucleotide sequence analysis indicated 73 and 74% identity for ADH3 with ADH1 and ADH2, respectively. The amino acid identity between the predicted ADH III polypeptide and ADH I and ADH II was 79 and 80%, respectively. The open reading frame encoding ADH III has a highly basic 27-amino-acid amino-terminal extension relative to ADH I and ADH II. The nucleotide sequence of the presumed leader peptide has a high degree of identity with the untranslated leader regions of ADH1 and ADH2 mRNAs. A strain containing a null allele of ADH3 did not have a detectably altered phenotype. The cloned gene integrated at the ADH3 locus, indicating that this is the structural gene for ADH III.

Nucleotide sequence characterization of Ty 1-17, a class II transposon from yeast

We have determined the nucleotide sequence of a class II yeast transposon (Ty 1-17) which is found just centromere-distal to the LEU2 structural gene on chromosome III of Saccharomyces cerevisiae. The complete element is 5961 bp long and is bounded by two identical, directly repeated, delta sequences of 332 bp each. The sequence organization indicates that Ty 1-17 is a retrotransposon, like the class I elements characterized previously. It contains two long open reading-frames, TyA (439 amino acids) and TyB (1349 amino acids). In this paper, the sequences of the two classes of yeast transposon are compared with one another and with analogous elements, such as retroviral proviruses, cauliflower mosaic virus and copia sequences. Features of the Ty 1-17 sequence which may be important to its mechanism of transposition and its genetic action are discussed.

Deletion analysis identifies a region, upstream of the ADH2 gene of Saccharomyces cerevisiae, which is required for ADR1-mediated derepression

Deletion analysis was used to identify sequences upstream of the ADH2 gene of Saccharomyces cerevisiae that are required for its regulation. 5' and 3' internal deletions of the ADH2 control region were created in vitro, and the fragments were ligated adjacent to the ADH1 promoter and structural gene. Hybrid genes with 3' deletions extending from -119 to -216 (the start site of ADH2 transcription is designated +1) were fully repressed and derepressed to high levels. Hybrid genes with 3' deletions extending from -119 to -257 were repressed but failed to significantly derepress. Hybrid genes lacking the -216 to -257 region also failed to respond to ADR1-5c, a mutant allele of the unlinked regulatory gene ADR1, which confers constitutive expression on ADH2. This implies that the region between these deletion endpoints, which includes a 22-base-pair sequence of dyad symmetry, is required for efficient derepression of an adjacent promoter. Internal deletions extending in the 3' direction from position -1141 confirmed these results. Deletion mutants lacking the region -1141 to -259 were normally regulated, whereas deletions extending from -1141 to -115 were not derepressible. These results support the hypotheses that the ADH2 promoter may normally be in an inactive conformation in the yeast chromosome and that derepression of ADH2 requires positive activation mediated through an upstream activation sequence located between 216 and 257 base pairs 5' to the start site of ADH2 transcription. No evidence for a DNA sequence mediating repression was obtained.

Transposable element sequences involved in the enhancement of yeast gene expression

The his4-917 mutation of yeast results from the insertion of a Ty element, Ty917, into the 5' regulatory region of the HIS4 gene. Ty917 prevents HIS4 transcription, thus rendering the cell histidine requiring. Recombination between Ty917 and a Ty element elsewhere in the yeast genome can result in the replacement of part or all of the Ty917 element by sequences from the Ty element. Recombinant derivatives display a variety of phenotypes including His-, weakly His+, and strongly His+. In most of the His+ derivatives, the expression of HIS4 is controlled by genes at the mating type locus. To identify the Ty sequences important in controlling the expression of an adjacent gene, we used Ty elements that have different effects on gene expression to construct hybrid Ty elements in vitro. The effects of these hybrid elements on HIS4 expression were examined. These experiments indicate that the critical sequence differences between Ty elements that permit HIS4 expression and those that prevent its expression lie in the rightmost (HIS4-proximal) 730 base pairs of the element. The DNA sequence of this region was determined for three elements: Ty917, which prevents HIS4 expression; Ty917(467), which confers a weak His+ phenotype; and Ty917(480), which confers a strong His+ phenotype. Within this region, Ty917(467) differs from Ty917 by a single base-pair change that is in the internal (epsilon) region of the Ty element. Ty917(480) differs from Ty917 by this same base-pair change and by 10 changes in the terminal delta sequence. The sequence change common to Ty917(467) and Ty917(480) lies in a region of the Ty element that is homologous to the simian virus 40 enhancer of transcription.

Deletion analysis identifies a region, upstream of the ADH2 gene of Saccharomyces cerevisiae, which is required for ADR1-mediated derepression

Deletion analysis was used to identify sequences upstream of the ADH2 gene of Saccharomyces cerevisiae that are required for its regulation. 5' and 3' internal deletions of the ADH2 control region were created in vitro, and the fragments were ligated adjacent to the ADH1 promoter and structural gene. Hybrid genes with 3' deletions extending from -119 to -216 (the start site of ADH2 transcription is designated +1) were fully repressed and derepressed to high levels. Hybrid genes with 3' deletions extending from -119 to -257 were repressed but failed to significantly derepress. Hybrid genes lacking the -216 to -257 region also failed to respond to ADR1-5c, a mutant allele of the unlinked regulatory gene ADR1, which confers constitutive expression on ADH2. This implies that the region between these deletion endpoints, which includes a 22-base-pair sequence of dyad symmetry, is required for efficient derepression of an adjacent promoter. Internal deletions extending in the 3' direction from position -1141 confirmed these results. Deletion mutants lacking the region -1141 to -259 were normally regulated, whereas deletions extending from -1141 to -115 were not derepressible. These results support the hypotheses that the ADH2 promoter may normally be in an inactive conformation in the yeast chromosome and that derepression of ADH2 requires positive activation mediated through an upstream activation sequence located between 216 and 257 base pairs 5' to the start site of ADH2 transcription. No evidence for a DNA sequence mediating repression was obtained.

Gene conversion and reciprocal exchange in a Ty-mediated translocation in yeast

A haploid yeast mutant carrying a reciprocal translocation was analyzed. Cloning and comparison of sequences involved in the translocation event in wildtype and mutant revealed that the crossover between nonhomologous chromosomes has occurred within Ty sequences. By DNA sequence analysis it could be demonstrated that the reciprocal recombination event is accompanied by a short segment of non-reciprocal exchange (gene conversion) in the immediate vicinity of the crossover. Analysis of the translocation mutant and revertant isolates also indicated that the regulatory effect of Ty elements on adjacent genes can be modified by discrete changes within a Ty element.

Identification of regulatory regions within the Ty1 transposable element that regulate iso-2-cytochrome c production in the CYC7-H2 yeast mutant

The CYC7-H2 mutation in the yeast Saccharomyces cerevisiae was caused by insertion of a Ty1 transposable element in front of the iso-2-cytochrome c structural gene, CYC7. The Ty1 insertion places iso-2-cytochrome c production under control of regulatory signals that are normally required for mating functions in yeast cells. We have investigated the regions of the Ty1 insertion that are responsible for the aberrant production of iso-2-cytochrome c in the CYC7-H2 mutant. Five alterations of the CYC7-H2 gene were obtained by specific restriction endonuclease cleavage of the cloned DNA and ligation of appropriate fragments. The CYC7+, CYC7-H2, and modified CYC7-H2 genes were each inserted into the yeast vector YIp5 and used to transform a cytochrome c-deficient yeast strain. Expression and regulation of each allele integrated at the CYC7 locus have been compared in vivo by determination of the amount of iso-2-cytochrome c produced. These results show that distal regions of the Ty1 element are not essential for the CYC7-H2 overproducing phenotype. In contrast, alterations in the vicinity of the proximal Ty1 junction abolish the CYC7-H2 expression and give rise to different phenotypes.

Identification of regulatory regions within the Ty1 transposable element that regulate iso-2-cytochrome c production in the CYC7-H2 yeast mutant

The CYC7-H2 mutation in the yeast Saccharomyces cerevisiae was caused by insertion of a Ty1 transposable element in front of the iso-2-cytochrome c structural gene, CYC7. The Ty1 insertion places iso-2-cytochrome c production under control of regulatory signals that are normally required for mating functions in yeast cells. We have investigated the regions of the Ty1 insertion that are responsible for the aberrant production of iso-2-cytochrome c in the CYC7-H2 mutant. Five alterations of the CYC7-H2 gene were obtained by specific restriction endonuclease cleavage of the cloned DNA and ligation of appropriate fragments. The CYC7+, CYC7-H2, and modified CYC7-H2 genes were each inserted into the yeast vector YIp5 and used to transform a cytochrome c-deficient yeast strain. Expression and regulation of each allele integrated at the CYC7 locus have been compared in vivo by determination of the amount of iso-2-cytochrome c produced. These results show that distal regions of the Ty1 element are not essential for the CYC7-H2 overproducing phenotype. In contrast, alterations in the vicinity of the proximal Ty1 junction abolish the CYC7-H2 expression and give rise to different phenotypes.

A minor class of 5S rRNA genes in Saccharomyces cerevisiae X2180-1B, one member of which lies adjacent to a Ty transposable element

In Saccharomyces cerevisiae the majority of the genes for 5S rRNA lie within a 9kb rDNA sequence that is present as 100-200 tandemly-repeated copies on Chromosome XII. Following our observations that about 10% of yeast 5S rRNA exists as minor variant sequences, we screened a collection of yeast DNA fragments cloned in lambda gt for 5S rRNA genes whose flanking sequences differed from those adjacent to 5S rRNA genes of the rDNA repeat. Three variant 5S rRNA genes were isolated on the basis of such dissimilarity to rDNA repeat sequences. They display a remarkable conservation of their DNA in the vicinity of the 5S coding region, and are examples of a minor form of 5S rRNA coding sequence present in a small number of copies in the yeast genome. These variant sequences appear to be transcribed as efficiently as 5S rRNA genes of the rDNA repeat. In one of our isolates of the variant sequence a Ty transposable element is inserted 145bp upstream of the initiation point for 5S rRNA synthesis.

Carbon source dependence of transposable element-associated gene activation in Saccharomyces cerevisiae

Seven cis-dominant mutations leading to the overproduction of the glucose-repressible alcohol dehydrogenase isozyme ADHII (structural gene, ADH2) in Saccharomyces cerevisiae have previously been shown to be due to insertion of a transposable element, Ty, in the 5' regulatory region of the ADH2 gene. We showed that although mating-competent cells (a, alpha, a/a, or alpha/alpha cells) overproduced both ADHII enzyme and ADH2 mRNA, mating-incompetent cells (a/alpha or ste-cells) produced much less ADHII enzyme and ADH2 mRNA. This mating type effect on ADH2 expression was greatest in the presence of a normally derepressing carbon source, glycerol, and much less apparent in the presence of a repressing carbon source, glucose. In addition, Ty insertion led to an aberrant carbon source response in mating-incompetent cells--the normally glucose-repressible ADHII becomes glycerol repressible. The mating type effect and aberrant carbon source response in mating-incompetent cells was specific for Ty-associated mutations in the 5' flanking region of the ADH2 gene in that a non-Ty mutation in the same region did not show these effects. Finally, Ty1 RNA levels also showed a/alpha, suppression, which was apparent only during growth on a nonfermentable carbon source such as glycerol. This suggests that Ty-mediated gene expression is subject to regulation by both mating competence and carbon catabolites.

In vitro generation of specific deletions in DNA cloned in M13 vectors using synthetic oligodeoxyribonucleotides: mutants in the 5'-flanking region of the yeast alcohol dehydrogenase II gene

Deletion mutants are particularly useful in defining the boundaries of noncoding genetic functions. Such mutants can be precisely generated using synthetic oligodeoxyribonucleotides as mutagens. In this paper we describe the application of this method to recombinant DNA cloned in a phage M13-derived vector. The mutagenic oligodeoxyribonucleotides, 20 and 21 nucleotides in length, were used to delete a tract of 20 dA-dT base-pairs and an adjacent 22 base-pair perfect dyad from the ADR3 locus, the 5'-flanking regulatory region of the ADR2 gene, of Saccharomyces cerevisiae with high efficiency.

Expression of Ty-lacZ fusions in Saccharomyces cerevisiae

We have determined the nucleotide sequence of about 520 bp spanning the 5' delta regions (Figure 1) of two Tyl elements. There is an open reading frame running out of the deltas for at least 180 nucleotides into the internal region of each element. The functional significance of these open reading frames has been tested by fusing them to a defective E.coli lacZ gene. Expression of B-galactosidase in yeast transformants containing these fusions shows that Tyl elements contain functional translation signals.

Carbon source dependence of transposable element-associated gene activation in Saccharomyces cerevisiae

Seven cis-dominant mutations leading to the overproduction of the glucose-repressible alcohol dehydrogenase isozyme ADHII (structural gene, ADH2) in Saccharomyces cerevisiae have previously been shown to be due to insertion of a transposable element, Ty, in the 5' regulatory region of the ADH2 gene. We showed that although mating-competent cells (a, alpha, a/a, or alpha/alpha cells) overproduced both ADHII enzyme and ADH2 mRNA, mating-incompetent cells (a/alpha or ste-cells) produced much less ADHII enzyme and ADH2 mRNA. This mating type effect on ADH2 expression was greatest in the presence of a normally derepressing carbon source, glycerol, and much less apparent in the presence of a repressing carbon source, glucose. In addition, Ty insertion led to an aberrant carbon source response in mating-incompetent cells--the normally glucose-repressible ADHII becomes glycerol repressible. The mating type effect and aberrant carbon source response in mating-incompetent cells was specific for Ty-associated mutations in the 5' flanking region of the ADH2 gene in that a non-Ty mutation in the same region did not show these effects. Finally, Ty1 RNA levels also showed a/alpha, suppression, which was apparent only during growth on a nonfermentable carbon source such as glycerol. This suggests that Ty-mediated gene expression is subject to regulation by both mating competence and carbon catabolites.