Regulation of energy metabolism in yeast. Inheritance of a pleiotropic mutation causing defects in metabolism of energy reserves, ethanol utilization and formation of cytochrome a.a3

Mitochondrial modifications in a single nuclear mutant of Saccharomyces cerevisiae affected in cAMP-dependent protein phosphorylation

This paper reports studies of bioenergetic modifications in a TTR1 single-nuclear mutant, isolated as resistant to triethyltin, an inhibitor of mitochondrial ATPase, and effective in cAMP-dependent protein phosphorylation. This mutant appears to have lost the wild-type cell ability to respond to a decrease of oxygen concentration in the growth medium by a decrease of cytochrome concentration in the cell. ATP synthesis rate in mutant cells in both the prestationary and stationary phase of growth appeared increased in comparison to wild-type cells, as too was respiration rate. A comparative study of mitochondria extracted from wild-type and from TTR1 mutant cells showed an increase in respiration rate, an increase in ATP synthesis rate, and an increase in TPP+ uptake in mutant mitochondria. The specific ATPase activity, as well as its sensitivity to TET, appears to be similar for mitochondria extracted from both strains. It was proposed that the modification of mitochondrial biogenesis in the TTR1 mutant may be due to a response of the cell to an increase in ATP hydrolysis caused by the mutation. It is also possible that the modification in cAMP-dependent protein kinase regulation which appeared to occur in this mutant affects protein(s) involved in mitochondrial biogenesis.

Isolation and genetic study of triethyltin-resistant mutants of Saccharomyces cerevisiae

Three mutants of Saccharomyces cerevisiae resistant to triethyltin (an inhibitor of mitochondrial ATPase) on non-fermentative media, and non-resistant to this drug on fermentative media, were isolated and named TTR1, TTR2 and TTR3. Apart from triethyltin resistance, these mutants show the following common characteristics: (1) Increased intracellular cytochrome c concentration. (2) Increased respiration rate. (3) Decreased growth yield. (4) Increased growth sensitivity to several drugs inhibiting oxidative phosphorylation: namely, CCCP (permeabilizing inner mitochondrial membrane to protons), valinomycin (permeabilizing inner mitochondrial membrane to potassium) and oligomycin (inhibitor of mitochondrial ATPase). (5) Increased sensitivity to carbon source starvation. For each mutant, these characteristics appeared to be due to a single pleiotropic nuclear mutation. Mutation TTR1 causes additional phenotypic characteristics which do not appear in mutants TTR2 and TTR3: (1) Pinkish coloration of colonies which is more pronounced after a long growth period. (2) Inability of the cells to store glycogen. (3) Growth defect of the cells on a galactose-containing medium. (4) Inability of a diploid homozygote mutant strain to sporulate. All these phenotypic characteristics have already been described in yeast mutants deregulated in cAMP-dependent protein phosphorylation. Crossing of a strain bearing the TTR1 mutation with a strain mutated in the adenylate cyclase structural gene suggested that the TTR1 phenotype is due to a modification in regulation of cAPK by cAMP, making cell multiplication possible without intracellular cAMP.

Effects of hap mutations on heme and cytochrome formation in yeast

Simultaneous effects of mutations in the transcriptional regulatory genes, HAP1, HAP2 and HAP3, on all respiratory cytochromes of Saccharomyces cerevisiae were determined. Cytochrome behavior in hap mutants and in cyc4 and rhm1 mutants, altered in regulation of 5-aminolevulinate synthase, was compared. Although hap mutants were isolated as trans-acting, transcriptional regulators of the CYC1 (iso-1-cytochrome c) gene, each mutant exhibits partial deficiencies in all cytochrome types. In hap2 and hap3 strains all cytochromes were decreased proportionally to about 40-50% of wild type values. In contrast, hap1 caused a decrease in all cytochromes and an accumulation of a pigment, probably Zn porphyrin. Apparently apocytochrome and heme biosynthesis retain coordination in hap2 and hap3, but not in hap1, mutants. Unlike cyc4 and rhm1 mutants, hap mutants do not exhibit 5-aminolevulinate-dependent restoration of cytochromes. The hap1 mutant grew at near-normal rates on glycerol, whereas hap2 and hap3 mutants grew very slowly. The frequency of [rho-] was high (16-18%) in hap2 and hap3 strains. Results are consistent with generalized control of mitochondrial replication directed by the HAP1-HAP2 system and heme-directed control of formation of all apocytochromes mediated by HAP1. Neither system exerts all-or-nothing control.

Isolation and characterization of a phosphoprotein phosphatase-deficient mutant in yeast

The ppd1 mutant of yeast, Saccharomyces cerevisiae, was isolated as a suppressor of the cyr2 mutation which caused alteration of the catalytic subunit of cAMP-dependent protein kinase. Three peaks of phosphoprotein phosphatase activity (peak I, II and III) were identified by DEAE-Sephacel chromatography of crude extracts of the wild-type strain. The ppd1 mutant was deficient in peak III phosphoprotein phosphatase activity. The peak III enzyme efficiently utilized the phosphorylated forms of NAD-dependent glutamate dehydrogenase and trehalase as substrate. The ppd1 mutation did not suppress the cyr1, CYR3 or ras1 ras2 mutations. The ppd1 locus was located on chromosome II and had identical characteristics with glc1. The ppd1 mutation suppressed the G1 arrest caused by nutritional limitation, but maintained sensitivity to mating pheromone. In diploids homozygous for the ppd1 mutation, no premeiotic DNA replication and commitment to intragenic recombination occurred and no spores were formed, suggesting that the accumulation of phosphorylated proteins in the absence of one of the phosphoprotein phosphatases is required for mitosis but not for the initiation of meiosis.

RAS2 of Saccharomyces cerevisiae is required for gluconeogenic growth and proper response to nutrient limitation

Saccharomyces cerevisiae contains two genes with remarkable homology to members of the ras oncogene family. These two genes, RAS1 and RAS2, constitute an essential gene family since spores with disruptions of both genes fail to grow. We report here that strains containing RAS2 disruptions have three distinct phenotypes. First, they fail to grow efficiently on nonfermentable carbon sources. Second, they hyperaccumulate the storage carbohydrates glycogen and trehalose. Third, diploid cells homozygous for the RAS2 disruptions sporulate on rich media. Extragenic suppressors have been isolated that suppress the gluconeogenic defect. These suppressors fall into at least three complementation groups, mutations in two of which bypass the normal requirement of RAS for cell viability, allowing cells containing neither RAS gene to grow. The phenotype of the RAS2 mutant and extragenic suppressors implicate RAS with some function in the normal response to nutrient limitation.

In situ assay for 5-aminolevulinate dehydratase and application to the study of a catabolite repression-resistant Saccharomyces cerevisiae mutant

To facilitate the study of the effects of carbon catabolite repression and mutations on 5-aminolevulinate dehydratase (EC 4.2.1.24) from Saccharomyces cerevisiae, a sensitive in situ assay was developed, using cells permeabilized by five cycles of freezing and thawing. Enzymatic activity was measured by colorimetric determination of porphobilinogen with a modified Ehrlich reagent. For normal strains, porphobilinogen production was linear for 15 min, and the reaction rate was directly proportional to the permeabilized cell concentration up to 20 mg (dry weight) per ml. The reaction exhibited Michaelis-Menten-type kinetics, and an apparent Km of 2.6 mM was obtained for 5-aminolevulinic acid. This value is only slightly higher than the value of 1.8 mM obtained for the enzyme assayed in cell extracts. The in situ assay was used to assess catabolite repression-dependent changes in 5-aminolevulinate dehydratase during batch culture on glucose medium. In normal S. cerevisiae cells, the enzyme is strongly repressed as long as glucose is present in the medium. In contrast, a strain bearing the hex2-3 mutation exhibits derepressed levels of enzyme activity during growth on glucose. Synthesis of cytochromes by this strain is also resistant to catabolite repression. Similar studies employing a strain containing the glc1 mutation, which enhances porphyrin accumulation, did not reveal any significant phenotypic change in catabolite regulation of 5-aminolevulinate dehydratase.

Regulation of yeast trehalase by a monocyclic, cyclic AMP-dependent phosphorylation-dephosphorylation cascade system

Mutation at the GLC1 locus in Saccharomyces cerevisiae resulted in simultaneous deficiencies in glycogen and trehalose accumulation. Extracts of yeast cells containing the glc1 mutation exhibited an abnormally high trehalase activity. This elevated activity was associated with a defective cyclic AMP (cAMP)-dependent monocyclic cascade which, in normal cells, regulates trehalase activity by means of protein phosphorylation and dephosphorylation. Trehalase in extracts of normal cells was largely in a cryptic form which could be activated in vitro by ATP . Mg in the presence of cAMP. Normal extracts also exhibited a correlated cAMP-dependent protein kinase which catalyzed incorporation of label from [gamma-32P]ATP into protamine. In contrast, cAMP had little or no additional activating effect on trehalase or on protamine phosphorylation in extracts of glc1 cells. Similar, unregulated activation of cryptic trehalase was also found in glycogen-deficient strains bearing a second, independently isolated mutant allele, glc1-2. Since trehalase activity was not directly affected by cAMP, the results indicate that the glc1 mutation results in an abnormally active protein kinase which has lost its normal dependence on cAMP. Trehalase in extracts of either normal or mutant cells underwent conversion to a cryptic form in an Mg2+-dependent, fluoride-sensitive reaction. Rates of this reversible reduction of activity were similar in extracts of mutant and normal cells. This same, unregulated protein kinase would act on glycogen synthase, maintaining it in the phosphorylated low-activity D-form. The glc1 mutants provide a novel model system for investigating the in vivo metabolic functions of a specific, cAMP-dependent protein kinase.