Biosynthesis of the branched-chain amino acids in yeast: a trifluoroleucine-resistant mutant with altered regulation of leucine uptake

Asp578 in LEU4p Is One of the Key Residues for Leucine Feedback Inhibition Release in Sake Yeast

We identified a new mutation, Asp578Tyr, in alpha-isopropylmalate synthase (a LEU4 gene product) that releases leucine feedback inhibition and causes hyperproduction of isoamyl alcohol (i-AmOH) in sake yeast. Spontaneous sake yeast mutants that express resistance to 5,5,5-trifluoro-DL-leucine (TFL) were isolated, and a mutant strain, TFL20, was characterized at the genetic and biochemical levels. An enzyme assay for alpha-isopropylmalate synthase showed that strain TFL20 was released from feedback inhibition by L-leucine. Furthermore, DNA sequencing of the LEU4 gene for a haploid of the mutant TFL20 revealed that aspartic acid in position 578 changes to tyrosine. A comparison of the three-dimensional structures of wild-type LEU4p and mutant LEU4D578Yp by the homology modeling method showed that Asp578 is important for leucine feedback inhibition. We conclude that the mutation from Asp to Tyr in 578 is a novel change causing release from leucine feedback inhibition.

Six new loci controlling resistance top-fluorophenylalanine inAspergillus nidulans

Twelve FPA-resistant mutants were selected on medium containingp-fluorophenylalanine and ethionine. Dominance tests in heterozygous diploids showed that 8 out of 12 are dominant and 4 recessive to their wild-type alleles. One mutant,fpa60, showed a partial requirement for tyrosine and was found to be allelic to anfpaAmutant described previously. A tyrosine non-requirer,fpa65, was also assigned to this locus. The other 10 mutants did not show any growth requirement and were simultaneously resistant to ethionine and 3-amino-L-tyrosine. Of the 8 dominant mutants, 3 were allelic to the permease-mutants at the locusfpaD.Dominant mutants showed higher degrees of resistance than recessive ones. Six new loci, identified after preliminary genetic analysis, were located on 3 linkage groups: 3 on linkage group VI, and one each on linkage groups I, V, and VIII. The recombinantfpaD11;fpaK69 was found to be sensitive to FPA.

Biochemical and molecular characterization of Saccharomyces cerevisiae strains obtained from sugar-cane juice fermentations and their impact in cachaça production

Saccharomyces cerevisiae strains from different regions of Minas Gerais, Brazil, were isolated and characterized aiming at the selection of starter yeasts to be used in the production of cachaça, the Brazilian sugar cane spirit. The methodology established took into account the screening for biochemical traits desirable in a yeast cachaça producer, such as no H2S production, high tolerance to ethanol and high temperatures, high fermentative capacity, and the abilities to flocculate and to produce mycocins. Furthermore, the yeasts were exposed to drugs such as 5,5',5"-trifluor-D,L-leucine and cerulenin to isolate those that potentially overproduce higher alcohols and esters. The utilization of a random amplified polymorphic DNA-PCR method with primers based on intron splicing sites flanking regions of the COX1 gene, as well as microsatellite analysis, was not sufficient to achieve good differentiation among selected strains. In contrast, karyotype analysis allowed a clear distinction among all strains. Two selected strains were experimentally evaluated as cachaça producers. The results suggest that the selection of strains as fermentation starters requires the combined use of biochemical and molecular criteria to ensure the isolation and identification of strains with potential characteristics to produce cachaça with a higher quality standard.

Isolation of Saccharomyces cerevisiae strains producing higher levels of flavoring compounds for production of "cachaça" the Brazilian sugarcane spirit

In Brazil, spontaneous fermentation and open vessels are still used to produce cachaça (the Brazilian sugarcane spirit) and this fermentation is characterized by mixed cultures with continuous succession of yeast species. This work shows the development of a methodology for isolation of yeasts, particularly Saccharomyces cerevisiae, used in the production of cachaça. According to the proposed strategy, the strains were selected for their ability to adapt to stress conditions encountered during fermentation of the sugarcane juice such as high sucrose concentration; high temperatures and high alcohol concentration; for their capacity to flocculate; and for their higher fermentative ability. For strains with such characteristics, specific procedures were employed to select for 5,5,5-trifluoro-DL-leucine (TFL) and cerulenin-resistant strains, since these characteristics are related to a higher capacity of production of the flavoring compounds isoamyl alcohol and caproic acid, respectively. The effectiveness of such a selection strategy was documented. Taken together, the results obtained present the development of a new strategy to isolate yeast strains with appropriated characteristics to be used in the cachaça industry. Moreover, the results obtained offer an explanation for the great variability in terms of chemical composition found in products obtained even in a single distillery.

Genetic and biochemical characterization of Saccharomyces cerevisiae mutants resistant to trifluoroleucine

Eighteen mutants resistant to 5',5',5'-trifluoroleucine (TFL), a leucine analog, were isolated in Saccharomyces cerevisiae strains YNN281 and YNN282. The mutants were characterized genetically and clustered in two groups, one comprising all the dominant (TFL1) and the other one all the recessive (tfl2) mutations. Genetic and biochemical data suggested that the dominant mutations are located on the LEU4 gene, coding for alpha-isopropylmalate synthase I. These mutations resulted in accumulation of leucine as a consequence of the synthesis of an enzyme insensitive to the feedback inhibition by leucine. Leucine excretion in the TFL1 mutants appeared to be affected by the genetic background of the strain and was greatly influenced by lysine metabolism. The measurement of intra- and extracellular amino acid concentrations in prototrophic strains carrying TFL1 or tfl2 genes showed that both were leucine overproducers. Some of the TFL-resistant mutants were tested in alcoholic fermentation of grape must: analysis of the fermentation secondary metabolites showed that the major effect of the TFL-resistant strains was an increased production of isoamyl alcohol compared to that of the parental strain.

A high-affinity uptake system for branched-chain amino acids in Saccharomyces cerevisiae

In order to isolate mutants with impaired uptake of branched-chain amino acids, mutants were induced which on complex medium were sensitive to an inhibitor of branched-chain amino acid biosynthesis. Eighteen mutants of independent origin were found. Ten of them were assayed for branched-chain amino acid uptake. Three of these were impaired in the uptake of L-valine, L-isoleucine and L-leucine, while the rest were unaffected in uptake of any of the three amino acids. Kinetics of the uptake by one selected mutant and the parental strain S288C were compared to models for one or two systems obeying Michaelis-Menten kinetics. This analysis suggested that a high-affinity system for all three amino acids is absent in the mutant, whereas low-affinity uptake of L-isoleucine and L-leucine by one or more systems remains unaffected. Moreover, medium-affinity uptake components for L-valine and L-leucine, not originally seen in the wild type, were identified in the mutant. In the wild type, 10 mM of any of the amino acids L-alanine, L-cysteine, L-isoleucine, L-leucine, L-tryptophan and L-valine reduce uptake of any of the three branched-chain amino acids. We propose that a permease responsible for high-affinity uptake of the branched-chain amino acids in strain S288C is partially or completely inactive in the mutant. Tetrad analysis shows that the phenotype can be ascribed to a single Mendelian gene. The wild-type allele is denoted BAP1 for branched-chain amino acid permease. The BAP1-dependent system is different from the general amino acid permease.

Molecular analysis of the yeast VPS3 gene and the role of its product in vacuolar protein sorting and vacuolar segregation during the cell cycle

vps3 mutants of the yeast Saccharomyces cerevisiae are impaired in the sorting of newly synthesized soluble vacuolar proteins and in the acidification of the vacuole (Rothman, J. H., and T. H. Stevens. Cell. 47:1041-1051; Rothman, J. H., C. T. Yamashiro, C. K. Raymond, P. M. Kane, and T. H. Stevens. 1989. J. Cell Biol. 109:93-100). The VPS3 gene, which was cloned using a novel selection procedure, encodes a low abundance, hydrophilic protein of 117 kD that most likely resides in the cytoplasm. Yeast strains bearing a deletion of the VPS3 gene (vps3-delta 1) are viable, yet their growth rate is significantly reduced relative to wild-type cells. Temperature shift experiments with strains carrying a temperature conditional vps3 allele demonstrate that cells rapidly lose the capacity to sort the vacuolar protein carboxypeptidase Y upon loss of VPS3 function. Vacuolar morphology was examined in wild-type and vps3-delta 1 yeast strains by fluorescence microscopy. The vacuoles in wild-type yeast cells are morphologically complex, and they appear to be actively partitioned between mother cells and buds during an early phase of bud growth. Vacuolar morphology in vps3-delta 1 mutants is significantly altered from the wild-type pattern, and the vacuolar segregation process seen in wild-type strains is defective in these mutants. With the exception of a vacuolar acidification defect, the phenotypes of vps3-delta 1 strains are significantly different from those of mutants lacking the vacuolar proton-translocating ATPase. These data demonstrate that the acidification defect in vps3-delta 1 cells is not the primary cause of the pleiotropic defects in vacuolar function observed in these mutants.

Regulatory mutants at the his1 locus of yeast

The his1 gene in Saccharomyces cerevisiae codes for phosphoribosyl transferase, an allosteric enzyme that catalyzes the initial step in histidine biosynthesis. Mutants that specifically alter the feedback regulatory function were isolated by selecting his1 prototrophic revertants that overproduce and excrete histidine. The prototrophs were obtained from diploids homoallelic for his1--7 and heterozygous for the flanking markers thr3 and arg6. Among six independently derived mutant isolates, three distinct levels of histidine excretion were detected. The mutants were shown to be second-site alterations mapping at the his1 locus by recovery of the original auxotrophic parental alleles. The double mutants, HIS1--7e, are dominant with respect to catalytic function but recessive in regulatory function. When removed from this his1--7 background, the mutant regulatory site (HIS1-e) still confers prototrophy but not histidine excretion. To yield the excretion phenotype, the primary and altered secondary sites are required in cis array. Differences in histidine excretion levels correlate with resistance to the histidine analogue, triazoalanine.

Regulation of tryptophan biosynthesis in Saccharomyces cerevisiae: mode of action of 5-methyl-tryptophan and 5-methyl-tryptophan-sensitive mutants

In a wild-type strain of Saccharomyces cerevisiae the tryptophan analogue dl-5-methyl-tryptophan (5MT) causes only a slight reduction of the growth rate. Uptake experiments indicate that the limited inhibition is partly due to low levels of 5MT inside the cell. On the other hand, this low concentration of 5MT leads to an increase in the activity of the tryptophan-biosynthetic enzymes. Evidence is presented that suggests that 5MT acts primarily through feedback inhibition of anthranilate synthase, the first enzyme of the pathway. A number of 5MT-sensitive mutants have been isolated, characterized, and assigned to one of the following three classes: class I, strains with altered activity and/or feedback sensitivity of anthranilate synthase; class II, strains with elevated uptake of 5MT; class III, mutants with altered regulation of the tryptophan-biosynthetic enzymes, which do not exhibit increases in activity in the presence of 5MT. This failure to exhibit increased enzyme activities in mutants of class III can also be observed after tryptophan starvation. Two mutants of class III show high sensitivity towards 3-amino-1,2,4-triazole. They can not exhibit derepression of some histidine- and arginine-biosynthetic enzymes under conditions that lead to an increase in these same enzymes in the wild-type strain.

Biosynthesis of the branched-chain amino acids in yeast: a leucine-binding component and regulation of leucine uptake

Use of an ion-exchange resin assay has shown that leucine is bound to a component of a dialyzed extract of yeast. Leucine binding may be related to in vivo uptake of the amino acid. A yeast strain with a 30-fold lower affinity for leucine uptake in vivo has a parallel reduction in affinity for in vitro leucine binding; the rate of leucine uptake in wild-type yeast can be increased four- to fivefold by growth on leucine as a sole nitrogen source. Under these conditions, the specific activity of the leucine-binding component also increases over threefold. Regulation of leucine uptake was studied by using wild-type strain 60615 and a mutant 60615/fl(2) with a constitutively elevated leucine uptake system. Leucine pool formation in the mutant was accompanied by an overshoot, leading to a loss of leucine from the pool. The phenomenon could be observed in the wild type under certain conditions. The mechanism of this process was examined. The leucine uptake system was found to be stable in the absence of protein synthesis. The rate of leucine uptake increased on reduction of the pool of amino acids, and in strain 60615/fl(2) the ability to overshoot was rapidly recovered on depletion of the leucine pool. The results suggest a control of leucine uptake by feedback inhibition, in which leucine or other amino acids, e.g., isoleucine, inhibit leucine uptake. The results do not exclude control by a rapidly activated-inactivated system.