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

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.

Molecular cloning and characterization of the MET6 gene of Saccharomyces cerevisiae

A yeast DNA fragment complementing the met6 mutation in yeast (Saccharomyces cerevisiae) was cloned in a shuttle vector, Yep13, by transforming a yeast host with plasmid DNA prepared from yeast gene bank CV13 of K. Nasmyth. A restriction map of a 6-kb Sau3A insert was constructed. A 2.6-kb fragment (Sau3A-BamHI) complementing the mutation was found by subcloning. Evidence that the DNA fragment contains the yeast MET6 gene was obtained by genomic integration. A 2.5-kb transcript is found both in wild-type (wt) and met6 yeast strains by Northern blotting experiments, indicating that the mutation acts at posttranscriptional level. The rate of transcription for the integrant lies between the values observed for the wt and mutant strains. The functional gene product seems to be involved in negative regulation of transcription of the MET6 gene.

Arrangement of genes TRP1 and TRP3 of Saccharomyces cerevisiae strains

The tryptophan biosynthetic genes TRP1 and TRP3 and partly also TRP2 and TRP4 have been compared by the technique of Southern hybridization and enzyme measurements in twelve wild isolates of Saccharomyces cerevisiae from natural sources of different continents, in the commonly used laboratory strain S. cerevisiae X2180-1A and in a Kluyveromyces marxianus strain. We could classify these strains into four groups, which did not correlate with their geographical distribution. In no case are the TRP3 and TRP1 genes fused as has been found in other ascomycetes. Two strains were found which, in contrast to strain X2180-1A, show derepression of gene TRP1. Two examples are discussed to demonstrate the usefulness of Southern hybridizations for the identification of closely related strains.

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.

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.

Transcriptional regulation of the MET3 gene of Saccharomyces cerevisiae

The MET3 gene, coding for ATP sulfurylase (ATPS), an enzyme implicated in methionine biosynthesis in Saccharomyces cerevisiae, was cloned by functional complementation, after transformation, of a yeast met3 mutant strain. The cloned MET3 gene was used as a probe to measure the specific MET3 messenger RNA in a wild-type strain grown under conditions which promote or fail to promote repression of ATPS synthesis. It was found that the level of MET3 messenger RNA is reduced ten-fold when the strain is grown under conditions where ATPS synthesis is repressed, suggesting that the MET3 expression is regulated transcriptionally. The direction of transcription and the size of the transcript have been determined.

Mode of action of glyphosate in Candida maltosa

The broad-spectrum herbicide glyphosate inhibits the growth of Candida maltosa and causes the accumulation of shikimic acid and shikimate-3-phosphate. Glyphosate is a potent inhibitor of three enzymes of aromatic amino acid biosynthesis in this yeast. In relation to tyrosine-sensitive 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase and dehydroquinate synthase, the inhibitory effect appears at concentrations in the mM range, but 5-enolpyruvylshikimate 3-phosphate (EPSP) synthase is inhibited by micromolar concentrations of glyphosate. Inhibition of partially purified EPSP synthase reaction by glyphosate is competitive with respect to phosphoenolpyruvate (PEP) with a Ki-value of 12 microM. The app. Km for PEP is about 5-fold higher and was 62 microM. Furthermore, the presence of glyphosate leads to derepression of many amino acid biosynthetic enzymes.

The nucleotide sequence of the yeast ARG4 gene

The complete nucleotide sequence of a 2296-bp DNA fragment containing the yeast (Saccharomyces cerevisiae) ARG4 gene has been determined. This gene specifies the synthesis of the arginine biosynthetic enzyme, argininosuccinate lyase (EC 4.3.2.1). The sequence contains one major open reading frame of 463 codons, giving a calculated Mr of 52 010 for the protein, in good agreement with the experimentally determined value of 53 000. The sequence upstream from the ARG4 gene shares structural features in common with other yeast genes subject to general amino acid control.

Expression of an artificial yeast TRP-gene cluster in yeast and Escherichia coli

All five tryptophan biosynthetic genes of Saccharomyces cerevisiae were unified on plasmid pME554, which is based on 2 micrometer DNA and pBR322 sequences allowing for autonomous replication in yeast and E. coli. Homologous and heterologous expression of this artificial yeast TRP-gene cluster was studied. Plasmid pME554 allowed for nearly normal growth of a yeast strain bearing auxotrophic mutations in all five TRP-genes. The plasmid-borne genes TRP2 to TRP5 were expressed and regulated normally in the frame of the general control. Gene TRP1, carried on an EcoRI/Bg/II fragment lacking the ARS1 function, was expressed poorly and did not respond to the general control like the chromosomally-borne TRP1 gene. Plasmid pME554 allowed for poor growth of E. coli strain W3110 tna- delta trpEA2 on minimal medium. Marked stimulation was observed, however, when anthranilic acid or indole were added. Accordingly, poor expression of the first Trp-enzyme anthranilate synthase and the last enzyme tryptophan synthase was found, whereas the other three genes were moderately well expressed in E. coli.

[General control of amino acid biosynthesis in mutants of Candida spec. EH 15/D]

The general control of amino acid biosynthesis was investigated in Candida spec. EH 15/D, using single and double mutant auxotrophic strains and prototrophic revertants starved for their required amino acids. These experiments show that starvation for lysine, histidine, arginine, leucine, threonine, proline, serine, methionine, homoserine, asparagine, glutamic acid or aspartic acid can result in derepression of enzymes. A correlation was found between the degree of derepression, growth of strains, and concentration of required amino acids. The amino acids pool pattern of mutants and revertants is different from that in the wild type strain.

Isolation of Saccharomyces cerevisiae TRP3

Several plasmids, isolated from two plasmid pools, complemented a Saccharomyces cerevisiae trp3 mutant with defective indole-3-glycerol-phosphate synthase activity. Restriction mapping indicated that a 1.2-kilobase StuI segment was common to all complementing plasmids. Southern blot hybridization established that a cloned 5.2-kilobase BamHI fragment was derived intact from chromosomal DNA. A yeast trp3 mutant transformed with trp3-complementing plasmids contained approximately 40-fold elevated indole-3-glycerol-phosphate synthase activity. These plasmids also complemented an Escherichia coli trpC mutant, and transformants exhibited enzyme activity. Yeast trp3 is therefore associated with a 1.2-kilobase StuI DNA segment.

Nucleotide sequence of yeast LEU2 shows 5'-noncoding region has sequences cognate to leucine

The LEU2 structural gene and its regulatory sequences were isolated on a 2200 bp Xho I-Sal I fragment. Sequencing of the 5'-noncoding region showed that at -151 there is an open reading frame of 23 codons of which six are for leucine. The leucine codon usage in this reading frame follows exactly that of other yeast genes. At the carboxy-terminal end and immediately after the peptide reading frame, a 14 bp hairpin (followed by a T-rich segment) can form in the putative mRNA; this arrangement closely resembles an RNA polymerase terminator. These and other features suggest a model for regulation. Preceding this is a gene (which starts at -463) for tRNALeu3, the major tRNALeu isoacceptor. RNA polymerase III transcription start and termination signals flank 5' and 3' ends, respectively, of the structural gene. The features noted above are in the same DNA strand that codes for the LEU2 gene product.

Growth inhibition by alpha-aminoadipate and reversal of the effect by specific amino acid supplements in Saccharomyces cerevisiae

The growth of Saccharomyces cerevisiae wild-type strain X2180 in minimal medium was inhibited by the addition of higher-than-supplementary levels of alpha-aminoadipate. This inhibitory effect was reversed by the addition of arginine, asparagine, aspartate, glutamine, homoserine, methionine, or serine as single amino acid supplements. Mutants belonging to the lys2 and lys14 loci were able to grow in lysine-supplemented alpha-aminoadipate medium, although not as well as when selected amino acids were added. Growth in alpha-aminoadipate medium by all strains was accompanied by an accumulation of alpha-ketoadipate. Glutamate:keto-adipate transaminase levels were derepressed two- to fivefold in lys2 mutants using alpha-aminoadipate as a nitrogen source. Wild-type strain X2180 growing in amino acid-supplemented AA medium exhibited higher levels of alpha-aminoadipate reductase. Mutants unable to use alpha-aminoadipate without amino acid supplementation were obtained by treatment of lys2 strain MW5-64 and were shown to have glutamate: ketoadipate transaminase activity and to lack alpha-aminoadipate reductase activity. Altered cell morphologies, including increased size, multiple buds, pseudohyphae, and germ tubes, evidenced by cells grown in alpha-aminoadipate medium suggest that higher-than-supplementary levels of alpha-aminoadipate result in an impairment of cell division.

Evidence that alpha-isopropylmalate synthase of Saccharomyces cerevisiae is under the "general" control of amino acid biosynthesis

The specific activity and the immunoreactive amount of alpha-isopropylmalate synthase were more than three times above wild-type values in a Saccharomyces cerevisiae mutant (cdr1) with constitutively derepressed levels of enzymes known to be under the "general" control of amino acid biosynthesis. The specific activity was also higher in lysine- and arginine-leaky strains when these were grown under limiting conditions, and in wild-type cells grown in the presence of 5-methyltryptophan. A low specific activity was found in a mutant (ndr1) unable to derepress enzymes of the general control system. Neither isopropylmalate isomerase nor beta-isopropylmalate dehydrogenase responded to general control signals.

The nucleotide sequence of the HIS4 region of yeast

We have determined the nucleotide sequence of the yeast HIS4 gene and its 5' and 3' flanking sequences. The protein chain has a calculated Mr-value of 87 935. Th 5' end of the HIS4 transcript maps at a position 63 bp upstream from the site of initiation of protein synthesis. The 3' end of the HIS4 transcript maps at a position approx. 118 bp after the UAG termination codon. There is no evidence for intervening sequences within the transcription unit. Functional sub-regions within the HIS4 coding frame have been identified by determining the sequence changes for various his4 mutations.