Effects of supersuppressor genes on enzymes controlling lysine biosynthesis in Saccharomyces

Physical and biochemical characterization of the cloned LYS5 gene required for alpha-aminoadipate reductase activity in the lysine biosynthetic pathway of Saccharomyces cerevisiae

The LYS5 and LYS2 genes of Saccharomyces cerevisiae are required for the synthesis of alpha-aminoadipate reductase in the lysine pathway. The LYS5 gene, originally cloned as a DNA insert of the plasmid pSC5, has been subcloned on a 3.2 kb SphI-Sau3AI DNA fragment of the recombinant plasmid pSR7. An internal 2.1 kb HpaI-HpaI DNA fragment of the subclone, upon Southern hybridization, exhibits homology with HpaI-restricted wild-type S. cerevisiae genomic DNA. The lys5+ transformants exhibited alpha-aminoadipate reductase activity similar to that of wild-type cells. S1 nuclease analysis localizes the transcription initiation site relative to the detailed restriction map, and reveals the direction of transcription, as well as the transcript size of the LYS5 gene which can be no greater than 1.65 kb. From this it is estimated that the encoded polypeptide is appreciably smaller than the 4 kb LYS2 gene product. These results provide a physical and biochemical characterization of the cloned LYS5 gene. Based on these observations, it is concluded that the LYS5 gene encodes a relatively small polypeptide of the large heteropolymeric alpha-aminoadipate reductase.

Properties of revertants of lys2 and lys5 mutants as well as alpha-aminoadipate-semialdehyde dehydrogenase from Saccharomyces cerevisiae

alpha-Aminoadipate-semialdehyde dehydrogenase catalyzes the conversion of alpha-aminoadipate to alpha-aminoadipate-semialdehyde in the biosynthetic pathway of lysine in yeasts and molds. Mutants belonging to lys2 and lys5 loci of Saccharomyces cerevisiae lacked the alpha-aminoadipate-semialdehyde dehydrogenase activity. Complementation in vitro was demonstrated by combining the extracts from different lys2 and lys5 mutants. Some of the revertants of lys2 and lys5 mutants exhibited lower specific activity and higher thermolability of alpha-aminoadipate-semialdehyde dehydrogenase than the enzyme from wild-type cells. The enzyme was partially purified from wild-type cells and the molecular weight of the enzyme was estimated on a Sephacryl S-300 column at 180,000. Results from the revertant analysis and in vitro complementation indicated LYS2 and LYS5 as structural genes, each encoding a subunit of this large enzyme.

Cloning and biochemical characterization of LYS5 gene of Saccharomyces cerevisiae

In Saccharomyces cerevisiae, the functions of two unlinked genes (LYS2 and LYS5) are required for the synthesis of the lysine biosynthetic enzyme, alpha-aminoadipate reductase. The LYS5 gene of S. cerevisiae was cloned by functional complementation of a lys5 mutant, X4004-3A, using a YEp24 plasmid library. The cloned LYS5 gene was contained within a 7.5 kb DNA insert of the recombinant plasmid pSC5. Cloning of LYS5 gene was confirmed by second cycle transformation of a lys5 mutant with the pSC5 plasmid, growth response studies, and plasmid loss experiments with Lys5+ transformants. Analysis of restriction digests of the pSC5 plasmid revealed 3 EcoRI, 5 PvuII, 1 PstI, 1 BglII and 2 HpaI sites in the 7.5 kb insert. A 3.9 kb internal pSC5 fragment hybridized only to the plasmid pSC5, but no homology was observed with LYS2 DNA or the YEp24 vector. The pSC5 transformed Lys5+ cells and the wild-type strain exhibited same level of alpha-aminoadipate reductase activity, whereas lys5 mutant and plasmid-cured transformed strain exhibited none. Lys2+ transformants consistently had five times greater alpha-aminoadipate reductase activity when compared with the wild-type and the Lys5+ transformant. The alpha-aminoadipate reductase activity was repressed in lysine-grown wild-type and Lys5+ transformed cells but not in Lys2+ transformed cells. A Lys2+ and Lys5+ double transformant exhibited higher alpha-aminoadipate reductase activity than lys2+ or lys5+ transformant.

Purification and properties of saccharopine dehydrogenase (glutamate forming) in the Saccharomyces cerevisiae lysine biosynthetic pathway

Saccharopine dehydrogenase (glutamate forming) of the biosynthetic pathway of lysine in Saccharomyces cerevisiae was purified 1,122-fold by using acid precipitation, ammonium sulfate precipitation, DEAE-Sepharose, gel filtration, and Reactive Red-120 agarose chromatography. The enzyme exhibited a native molecular size of 69,000 daltons by gel filtration and consisted of a single 50,000-dalton polypeptide based upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The enzyme was readily denatured by exposures to temperatures exceeding 46 degrees C. The pH optimum for the reverse reaction was 9.5. The apparent Kms for L-saccharopine and NAD+ were 2.32 and 0.054 mM, respectively. The enzyme was inhibited by mercuric chloride but not by carbonyl or metal complexing agents.

alpha-Aminoadipate pathway for the biosynthesis of lysine in lower eukaryotes

Bacteria and green plants use the diaminopimelate pathway for the biosynthesis of the essential amino acid, lysine; however, yeast and other higher fungi use the alpha-aminoadipate (AA) pathway. The AA pathway has been investigated in detail biochemically, genetically, and in terms of regulatory mechanisms in the baker's yeast Saccharomyces cerevisiae. The genetic analysis of lysine auxotrophs of S. cerevisiae revealed that there are more than 12 lysine genes for 8 enzyme-catalyzed steps. Lysine genes are not linked to each other and seven of the genes are mapped on six different linkage groups (chromosomes). The gene-enzyme relationships have been determined for ten of the lysine loci which include two unlinked gene functions required for each of AA reductase (LYS2 and LYS5) and Saccharopine reductase (LYS9 and LYS14). Five of the lysine enzymes are localized in mitochondria and three in cytosol. The lysine pathway of S. cerevisiae is regulated by feedback inhibition and end product repression. Two, and possibly three, of the enzymes exhibit general control of amino acid biosynthesis and at least five of the enzymes coded for, by unlinked genes, are simultaneously depressed in a regulatory (repressor) gene-mutant.

Two unlinked lysine genes (LYS9 and LYS14) are required for the synthesis of saccharopine reductase in Saccharomyces cerevisiae

Three lysine auxotrophs, strains AU363, 7305d, and 8201-7A, were investigated genetically and biochemically to determine their gene loci, biochemical lesions, and roles in the lysine biosynthesis of Saccharomyces cerevisiae. These mutants were leaky and blocked after the alpha-aminoadipate step. Complementation studies placed these three mutations into a single, new complementation group, lys14. Tetrad analysis from appropriate crosses provided evidence that the lys14 locus represented a single nuclear gene and that lys14 mutants were genetically distinct from the other mutants (lys1, lys2, lys5, and lys9) blocked after the alpha-aminoadipate step. The lys14 strains, like lys9 mutants, accumulated alpha-aminoadipate-semialdehyde and lacked significant amounts of saccharopine reductase activity. On the bases of these results, it was concluded, therefore, that LYS9 and LYS14, two distinct genes, were required for the biosynthesis of saccharopine reductase in wild-type S. cerevisiae.

Amino acid transport and metabolism in nitrogen-starved cells of Saccharomyces cerevisiae

Nitrogen-starved yeast derepress a general amino acid permease which transports basic and hydrophobic amino acids. Although both groups of amino acids are metabolized, the derivatives of the basic amino acids are retained by the cells, whereas those of the hydrophobic amino acids are released as acidic and neutral deaminated derivatives. The release of the deaminated derivatives of the hydrophobic amino acids only occurs in the presence of glucose, which presumably produces amino acceptors. The accumulation of intracellular amino acids results in trans-inhibition of the uptake of exogenous amino acids whether the intracellular amino acid is a basic amino acid or the product of intracellular transamination from a hydrophobic amino acid. Variation of permease and transaminase activity was measured during growth under repressed (ammonia-grown) and derepressed (proline-grown) conditions. Maximum levels for both activities occurs at the mid-exponential phase.

Lysine biosynthesis in Saccharomyces. Conversion of -aminoadipate into -aminoadipic -semialdehyde

The reduction of alpha-aminoadipate to alpha-aminoadipic delta-semialdehyde by a cell-free extract of Saccharomyces is shown to be a three-step process. First the amino acid reacts with ATP to form an adenylyl derivative. Then the adenylyl derivative of alpha-aminoadipate is reduced in the presence of NADPH. In the third step the reduced adenylyl derivative of the amino acid is cleaved to form alpha-aminoadipic delta-semialdehyde. The presence of Mg(2+) is necessary for the first and second steps. The third step does not need any cofactors. The product of the first step was isolated by chromatography after incubating the cell-free extract of Saccharomyces with alpha-aminoadipate, ATP and Mg(2+). The isolated product was identified as an adenylyl derivative of alpha-aminoadipate and could be converted into alpha-aminoadipic delta-semialdehyde under the stated experimental conditions. The product of the second step was too unstable to be identified.