General properties and regulation of arginine transporting systems in Saccharomyces cerevisiae

The transport of arginine-¹⁴C by exponentially growing cellsof Saccharomyces cerevisiae (ATCC 9763) was studied in the presenceof various amino acids, ammonium and urea. Arginine transportwas inhibited when the cells were preincubated with these compoundsfor 1 hr. Little or no inhibition of transport occurred whenthe preincubation period was omitted. Kinetics studies revealedthat arginine was transported by two distinct systems havinghigh and low affinities for this amino acid. At given arginineconcentrations the high affinity system was capable of transportingarginine molecules at approximately seventy times the rate ofthe low affinity system. The general requirements for arginine transport revealed energyand temperature dependencies in addition to sensitivity to anumber of metabolic inhibitors. Transfer of cells to N-freemedium was accompanied by increased rates of transport. Thisincrease was shown for the uptake of ten different amino acids.For L-arginine, this increase was prevented by addition of cycloheximide. Analyses of amino acid pools, after various experimental treatments,failed to reveal any consistent correlation between transportrates and the concentrations of individual amino acids or ammonium. It is concluded that arginine transport of S. cerevisiae isregulated by inhibition and repression. In this respect theavailability of ammonium would appear to be of prime importancein the development of transport activity. (Received December 5, 1975; )     

Specific induction of catabolism and its relation to repression of biosynthesis in arginine metabolism of Saccharomyces cerevisiae.

Experimental results are presented in support of the model previously proposed for specific induction of the synthesis of enzymes for arginine catabolism in Saccharomyces cerevisiae (Wiame, 1971a,b), and its connection with end-product repression of arginine biosynthetic enzymes. The data support the occurrence of negative regulation of metabolism in a eukaryote.Operator regions, one for arginase and another for ornithine transaminase, are identified. The operator mutations are fully constitutive. A mutation compatible with the occurrence of a catabolic represser, CARGR, leads to partial pleiotropic constitutivity.The connection between the induction process and the repression of biosynthetic enzymes is due to a common receptor of metabolic signals, an ambivalent repressor ARGR endowed with the property of a usual repressor for anabolic enzymes and playing the role of inducer at the level of CARGR; this cascade process simulates a positive control. argR^? mutations, by producing defective ARGR, “turn on” anabolic enzyme synthesis and “turn off” the synthesis of catabolic enzymes (Fig. 2). The dual role of ARGR is confirmed by the isolation of a mutation argRII^d which, in contrast to the defective properties caused by usual argR^? mutations, causes a dominant hyperactivity toward induction of a catabolic enzyme, but retains recessive hypoactivity toward repression of an anabolic enzyme. Such an ambivalent repressor is a function necessary for mutual, balanced exclusion between opposite metabolisms.Many operator constitutive mutations for arginase, cargA⁺O^?, change the level of enzyme to a similar value, thus defining a genetic function. One of these mutations, cargA⁺O^h, in addition to having unusual genetic behaviour, leads to production of twice as much arginase as cargA⁺O^?. This suggests the existence of another genetic region near the structural gene for this enzyme and an additional regulatory function to be analyzed in a separate paper (Dubois &; Wiame, 1978).     

Coordinated regulation of ammonium assimilation and carbon catabolism by glyoxylate in Saccharomyces cerevisiae

The activities of citrate synthase (EC 4.1.3.7) and NADP+-dependent glutamate dehydrogenase (GDH) (EC 1.4.1.4) of Saccharomyces cerevisiae were inhibited in vitro by glyoxylate. In the presence of glyoxylate, pyruvate and glyoxylate pools increased, suggesting that glyoxylate was efficiently transported and catabolized. Pyruvate accumulation also indicates that citrate synthase was inhibited. A decrease in the glutamate pool was also observed under these conditions. This can be attributed to an increased transamination rate and to the inhibitory effect of glyoxylate on NADP+-dependent GDH. Furthermore, the increase in the ammonium pool in the presence of glyoxylate suggests that NADP+-dependent GDH was being inhibited in vivo, since the activity of glutamine synthetase did not decrease under these conditions. We propose that the inhibition of both citrate synthase and NADP+-dependent GDH could form part of a mechanism that regulates the internal 2-oxoglutarate concentration.     

Mks1p is a regulator of nitrogen catabolism upstream of Ure2p in Saccharomyces cerevisiae.

The supply of nitrogen regulates yeast genes affecting nitrogen catabolism, pseudohyphal growth, and meiotic sporulation. Ure2p of Saccharomyces cerevisiae is a negative regulator of nitrogen catabolism that inhibits Gln3p, a positive regulator of DAL5, and other genes of nitrogen assimilation. Dal5p, the allantoate permease, allows ureidosuccinate uptake (Usa(+)) when cells grow on a poor nitrogen source such as proline. We find that overproduction of Mks1p allows uptake of ureidosuccinate on ammonia and lack of Mks1p prevents uptake of ureidosuccinate or Dal5p expression on proline. Overexpression of Mks1p does not affect cellular levels of Ure2p. An mks1 ure2 double mutant can take up ureidosuccinate on either ammonia or proline. Moreover, overexpression of Ure2p suppresses the ability of Mks1p overexpression to allow ureidosuccinate uptake on ammonia. These results suggest that Mks1p is involved in nitrogen control upstream of Ure2p as follows: NH(3) dash, vertical Mks1p dash, vertical Ure2p dash, vertical Gln3p --> DAL5. Either overproduction of Mks1p or deletion of MKS1 interferes with pseudohyphal growth.     

Differential regulation and substrate preferences in two peptide transporters of Saccharomyces cerevisiae

Dal5p has been shown previously to act as an allantoate/ureidosuccinate permease and to play a role in the utilization of certain dipeptides as a nitrogen source in Saccharomyces cerevisiae. Here, we provide direct evidence that dipeptides are transported by Dal5p, although the affinity of Dal5p for allantoate and ureidosuccinate is higher than that for dipeptides. Allantoate, ureidosuccinate, and to a lesser extent allantoin competed with dipeptide transport by reducing the toxicity of the peptide Ala-Eth and decreasing the accumulation of [(14)C]Gly-Leu. In contrast to the well-studied di/tripeptide transporter Ptr2p, whose substrate specificity is very broad, Dal5p preferred to transport non-N-end rule dipeptides. S. cerevisiae W303 was sensitive to the toxic peptide Ala-Eth (non-N-end rule peptide) but not Leu-Eth (N-end rule peptide). Non-N-end rule dipeptides showed better competition with the uptake of [(14)C]Gly-Leu than N-end rule dipeptides. Similar to the regulation of PTR2, DAL5 expression was influenced by the addition of Leu and by the CUP9 gene. However, DAL5 expression was downregulated in the presence of leucine and the absence of CUP9, whereas PTR2 was upregulated. Toxic dipeptide and uptake assays indicated that either Ptr2p or Dal5p was predominantly used for dipeptide transport in the common laboratory strains S288c and W303, respectively. These studies highlight the complementary activities of two dipeptide transport systems under different regulatory controls in common laboratory yeast strains, suggesting that dipeptide transport pathways evolved to respond to different environmental conditions.     

Metabolic engineering of a phosphoketolase pathway for pentose catabolism in Saccharomyces cerevisiae

Low ethanol yields on xylose hamper economically viable ethanol production from hemicellulose-rich plant material with Saccharomyces cerevisiae. A major obstacle is the limited capacity of yeast for anaerobic reoxidation of NADH. Net reoxidation of NADH could potentially be achieved by channeling carbon fluxes through a recombinant phosphoketolase pathway. By heterologous expression of phosphotransacetylase and acetaldehyde dehydrogenase in combination with the native phosphoketolase, we installed a functional phosphoketolase pathway in the xylose-fermenting Saccharomyces cerevisiae strain TMB3001c. Consequently the ethanol yield was increased by 25% because less of the by-product xylitol was formed. The flux through the recombinant phosphoketolase pathway was about 30% of the optimum flux that would be required to completely eliminate xylitol and glycerol accumulation. Further overexpression of phosphoketolase, however, increased acetate accumulation and reduced the fermentation rate. By combining the phosphoketolase pathway with the ald6 mutation, which reduced acetate formation, a strain with an ethanol yield 20% higher and a xylose fermentation rate 40% higher than those of its parent was engineered.     

Protein glycation in Saccharomyces cerevisiae. Argpyrimidine formation and methylglyoxal catabolism

Methylglyoxal is the most important intracellular glycation agent, formed nonenzymatically from triose phosphates during glycolysis in eukaryotic cells. Methylglyoxal-derived advanced glycation end-products are involved in neurodegenerative disorders (Alzheimer's, Parkinson's and familial amyloidotic polyneurophathy) and in the clinical complications of diabetes. Research models for investigating protein glycation and its relationship to methylglyoxal metabolism are required to understand this process, its implications in cell biochemistry and their role in human diseases. We investigated methylglyoxal metabolism and protein glycation in Saccharomyces cerevisiae. Using a specific antibody against argpyrimidine, a marker of protein glycation by methylglyoxal, we found that yeast cells growing on d-glucose (100 mM) present several glycated proteins at the stationary phase of growth. Intracellular methylglyoxal concentration, determined by a specific HPLC based assay, is directly related to argpyrimidine formation. Moreover, exposing nongrowing yeast cells to a higher d-glucose concentration (250 mM) increases methylglyoxal formation rate and argpyrimidine modified proteins appear within 1 h. A kinetic model of methylglyoxal metabolism in yeast, comprising its nonenzymatic formation and enzymatic catabolism by the glutathione dependent glyoxalase pathway and aldose reductase, was used to probe the role of each system parameter on methylglyoxal steady-state concentration. Sensitivity analysis of methylglyoxal metabolism and studies with gene deletion mutant yeast strains showed that the glyoxalase pathway and aldose reductase are equally important for preventing protein glycation in Saccharomyces cerevisiae.     

Proline: an essential intermediate in arginine degradation in Saccharomyces cerevisiae.

Results of studies on proline-nonutilizing (Put-) mutants of the yeast Saccharomyces cerevisiae indicate that proline is an essential intermediate in the degradation of arginine. Put- mutants excreted proline when grown on arginine or ornithine as the sole nitrogen source. Yeast cells contained a single enzyme, delta 1-pyrroline-5-carboxylate (P5C) dehydrogenase, which is essential for the complete degradation of both proline and arginine. The sole inducer of this enzyme was found to be proline. P5C dehydrogenase converted P5C to glutamate, but only when the P5C was derived directly from proline. When the P5C was derived from ornithine, it was first converted to proline by the enzyme P5C reductase. Proline was then converted back to P5C and finally to glutamate by the Put enzymes proline oxidase and P5C dehydrogenase.     

alpha-Aminoadipate as a primary nitrogen source for Saccharomyces cerevisiae mutants.

In contrast to wild-type strains of the yeast Saccharomyces cerevisiae, lys2 and lys5 mutants are able to utilize alpha-aminoadipate as a primary source of nitrogen. Chattoo et al. (B. B. Chattoo, F. Sherman, D. A. Azubalis, T. A. Fjellstedt, D. Mehnert, and M. Ogur, Genetics 93:51-65, 1979) relied on this difference in the effective utilization of alpha-aminoadipate to develop a procedure for directly selecting lys2 and lys5 mutants. In this study we used a range of mutant strains and various media to determine why normal strains are unable to utilize alpha-aminoadipate as a nitrogen source. Our results demonstrate that the anabolism of high levels of alpha-aminoadipate through the biosynthetic pathway of lysine results in the accumulation of a toxic intermediate and, furthermore, that lys2 and lys5 mutants contain blocks leading to the formation of this intermediate.     

Organization and expression of a two-gene cluster in the arginine biosynthesis of Saccharomyces cerevisiae.

Summary Using the pMB9 recombinant plasmid pMY3, which contains a functional gene for the tRNA^Try mutant Su⁺7, the EcoRI fragment containing the tRNA^Try gene is mapped and oriented with respect to the HindIII site in the tetracycline region of pMB9. Complete HpaII and HaeIII maps of the EcoRI fragment are derived. The Su⁺7 tRNA gene is placed by hybridization to these fragments, and the tRNA gene is oriented by using the restriction sites for HinfI, TaqI, and HpaII in the tRNA gene itself. A tRNA^Asp gene is shown to lie adjacent to tRNA^Try, and is also placed and oriented in the map. The RI fragment itself originates in a locus adjacent to, and transcribed in the same direction as, the ribosomal RNA genes of 80d₃.The implications of the structure of the cloned DNA for its previously measured regulatory and tRNA gene activities are discussed. In particular, the effect on the regulation of RNA synthesis is attributable to an E. coli DNA sequence, but cannot be due to the presence of a normal tRNA promoter on the plasmid.Abbreviations MD megadaltons; expressions of the form HpaII:0.075 refer to a fragment generated by the indicated restriction nuclease, having the indicated molecular weight, in MD     

Isolation and characterization of Saccharomyces cerevisiae mutants defective in glycerol catabolism.

Mutants of the yeast Saccharomyces cerevisiae that are defective in the catabolism of glycerol were isolated, and two types of mutants were obtained. One type was deficient in glycerol kinase activity, whereas the other type was deficient in sn-glycerol 3-phosphate dehydrogenase activity. Genetic analysis indicated that each mutant strain owed its phenotype to a single nuclear mutation, and that the two mutations were complementary. The mutations were not linked to each other or to any of 10 loci tested. In addition, neither mutation was centromere linked. Possible mechanisms for the regulation of these enzymes were tested by growing the parental strain in the presence of various carbon sources.     

Glutamine synthesis is a regulatory signal controlling glucose catabolism in Saccharomyces cerevisiae.

The effect of glutamine biosynthesis and degradation on glucose catabolism in Saccharomyces cerevisiae was studied. A wild-type strain and mutants altered in glutamine biosynthesis and degradation were analyzed. Cells having low levels of glutamine synthetase activity showed high ATP/ADP ratios and a diminished rate of glucose metabolism. It is proposed that glutamine biosynthesis plays a role in the regulation of glucose catabolism.     

The CCAAT box-binding factor stimulates ammonium assimilation in Saccharomyces cerevisiae, defining a new cross-pathway regulation between nitrogen and carbon metabolisms.

In Saccharomyces cerevisiae, carbon and nitrogen metabolisms are connected via the incorporation of ammonia into glutamate; this reaction is catalyzed by the NADP-dependent glutamate dehydrogenase (NADP-GDH) encoded by the GDH1 gene. In this report, we show that the GDH1 gene requires the CCAAT box-binding activator (HAP complex) for optimal expression. This conclusion is based on several lines of evidence: (1) overexpression of GDH1 can correct the growth defect of hap2 and hap3 mutants on ammonium sulfate as a nitrogen source, (ii) Northern (RNA) blot analysis shows that the steady-state level of GDH1 mRNA is strongly lowered in a hap2 mutant, (iii) expression of a GDH1-lacZ fusion is drastically reduced in hap mutants, (iv) NADP-GDH activity is several times lower in the hap mutants compared with that in the isogenic wild-type strain, and finally, (v) site-directed mutagenesis of two consensual HAP binding sites in the GDH1 promoter strongly reduces expression of GDH1 and makes it HAP independent. Expression of GDH1 is also regulated by the carbon source, i.e., expression is higher on lactate than on ethanol, glycerol, or galactose, with the lowest expression being found on glucose. Finally, we show that a hap2 mutation does not affect expression of other genes involved in nitrogen metabolism (GDH2, GLN1, and GLN3 encoding, respectively, the NAD-GDH, glutamine synthetase, and a general activator of several nitrogen catabolic genes). The HAP complex is known to regulate expression of several genes involved in carbon metabolism; its role in the control of GDH1 gene expression, therefore, provides evidence for a cross-pathway regulation between carbon and nitrogen metabolisms.     

Bidirectional regulation between TORC1 and autophagy in Saccharomyces cerevisiae

It has been reported in various model organisms that autophagy and the target of rapamycin complex 1 (TORC1) signaling are strongly involved in eukaryotic cell aging and decreasing TORC1 activity extends longevity by an autophagy-dependent mechanism. Thus, to expand our knowledge of the regulation of eukaryotic cell aging, it is important to understand the relationship between TORC1 signaling and autophagy. Many researchers have shown that TORC1 represses autophagy under normal growth conditions, and TORC1 inactivation contributes to the upregulation of autophagy. However, it is poorly understood how autophagy is regulated or terminated when starvation is prolonged. Here, we report that bidirectional regulation between autophagy and TORC1 exists in the yeast Saccharomyces cerevisiae. We show that mutant cells with weak TORC1 activity maintain autophagy longer than wild-type cells, and TORC1 is partially reactivated under ongoing nitrogen starvation by an autophagy-dependent mechanism. In addition, we found that Atg13 is gradually rephosphorylated during prolonged nitrogen starvation, and the kinase activity of Atg1 is required for Atg13 rephosphorylation. Our data suggest that TORC1 can be substantially, if not fully, reactivated in an autophagy-dependent manner under ongoing starvation, and that partially reactivated TORC1 eventually plays a role in the attenuation of autophagy.     

Bidirectional regulation between TORC1 and autophagy in Saccharomyces cerevisiae

It has been reported in various model organisms that autophagy and the target of rapamycin complex 1 (TORC1) signaling are strongly involved in eukaryotic cell aging and decreasing TORC1 activity extends longevity by an autophagy-dependent mechanism. Thus, to expand our knowledge of the regulation of eukaryotic cell aging, it is important to understand the relationship between TORC1 signaling and autophagy. Many researchers have shown that TORC1 represses autophagy under normal growth conditions, and TORC1 inactivation contributes to the upregulation of autophagy. However, it is poorly understood how autophagy is regulated or terminated when starvation is prolonged. Here, we report that bidirectional regulation between autophagy and TORC1 exists in the yeast Saccharomyces cerevisiae. We show that mutant cells with weak TORC1 activity maintain autophagy longer than wild-type cells, and TORC1 is partially reactivated under ongoing nitrogen starvation by an autophagy-dependent mechanism. In addition, we found that Atg13 is gradually rephosphorylated during prolonged nitrogen starvation, and the kinase activity of Atg1 is required for Atg13 rephosphorylation. Our data suggest that TORC1 can be substantially, if not fully, reactivated in an autophagy-dependent manner under ongoing starvation, and that partially reactivated TORC1 eventually plays a role in the attenuation of autophagy.