Regulation of the ilv 1 multifunctional gene in Saccharomyces cerevisiae

Substitution of histone H3 arginine 72 to alanine leads to deregulation of isoleucine biosynthesis in budding yeast Saccharomyces cerevisiae

Histone residues play an essential role in the regulation of various biological processes. In the present study, we have utilized the H3/H4 histone mutant library to probe functional aspects of histone residues in amino acid biosynthesis. We found that histone residue H3R72 plays a crucial role in the regulation of isoleucine biosynthesis. Substitution of arginine residue (H3R72) of histone H3 to alanine (H3R72A) renders yeast cells unable to grow in the minimal media. Histone mutant H3R72A requires the external supplementation of either isoleucine, serine, or threonine for the growth in minimal media. We also observed that H3R72 residue and leucine amino acid in synthetic complete media might play a crucial role in determining the intake of isoleucine and threonine in yeast. Further, gene deletion analysis of ILV1 and CHA1 in H3R72A mutant confirmed that isoleucine is the sole requirement for growth in minimal medium. Altogether, we have identified that histone H3R72 residue may be crucial for yeast growth in the minimal medium by regulating isoleucine biosynthesis through the Ilv1 enzyme in budding yeast Saccharomyces cerevisiae.

Autophagy sustains glutamate and aspartate synthesis in Saccharomyces cerevisiae during nitrogen starvation

Autophagy catabolizes cellular constituents to promote survival during nutrient deprivation. Yet, a metabolic comprehension of this recycling operation, despite its crucial importance, remains incomplete. Here, we uncover a specific metabolic function of autophagy that exquisitely adjusts cellular metabolism according to nitrogen availability in the budding yeast Saccharomyces cerevisiae. Autophagy enables metabolic plasticity to promote glutamate and aspartate synthesis, which empowers nitrogen-starved cells to replenish their nitrogen currency and sustain macromolecule synthesis. Our findings provide critical insights into the metabolic basis by which autophagy recycles cellular components and may also have important implications in understanding the role of autophagy in diseases such as cancer.

Effect of threonine, cystathionine, and the branched-chain amino acids on the metabolism of Zygosaccharomyces rouxii*

Zygosaccharomyces rouxii is an important yeast in the formation of flavor in soy sauce. In this study, we investigated the separate effects of exogenous threonine, cystathionine, and the branched-chain amino acids on the metabolism of Z. rouxii. The addition of these amino acids had significant effects on both Z. rouxii growth and glycerol and higher alcohol production. It also seemed that Z. rouxii displayed the Crabtree effect, which was independent of the added amino acids. Furthermore, we investigated the regulation of the metabolism of alpha-ketobutyrate, which is a key-intermediate in Z. rouxii amino acid metabolism. Threonine and cystathionine were introduced separately to stimulate the formation rate of alpha-ketobutyrate and the branched-chain amino acids to inhibit its conversion rate. Enzyme activities showed that these amino acids had a significant effect on the formation and conversion rate of alpha-ketobutyrate but that the alpha-ketobutyrate pool size in Z. rouxii was in balance all the time. The latter was confirmed by the absence of alpha-ketobutyrate accumulation.

Regulation of isoleucine-valine biosynthesis in Saccharomyces cerevisiae

The threonine deaminase gene (ILV1) of Saccharomyces cerevisiae has been designated "multifunctional" since Bollon (1974) indicated its involvement both in the catalysis of the first step in isoleucine biosynthesis and in the regulation of the isoleucine-valine pathway. Its role in regulation is characterized by a decrease in the activity of the five isoleucine-valine enzymes when cells are grown in the presence of the three branched-chain amino acids, isoleucine, valine and leucine (multivalent repression). We have demonstrated that the regulation of AHA reductoisomerase (encoded by ILV5) and branched-chain amino acid transaminase is unaffected by the deletion of ILV1, subsequently revealing that the two enzymes can be regulated in the absence of threonine deaminase. Both threonine deaminase activity and ILV1 mRNA levels increase in mutants (gcd2 and gcd3) having constitutively depressed levels of enzymes under the general control of amino acid biosynthesis, as well as in response to starvation for tryptophan and branched-chain amino acid imbalance. Thus, the ILV1 gene is under general amino acid control, as is the case for both the ILV5 and the transaminase gene. Multivalent repression of reductoisomerase and transaminase can be observed in mutants defective in general control (gcn and gcd), whereas this is not the case for threonine deaminase. Our analysis suggests that repression effected by general control is not complete in minimal medium. Amino acid dependent regulation of threonine deaminase is only through general control, while the branched-chain amino acid repression of AHA reducto isomerase and the transaminase is caused both by general control and an amino acid-specific regulation.

Occurrence of a catabolic L-serine (L-threonine) deaminase in Saccharomyces cerevisiae

Saccharomyces cerevisiae mutants lacking the anabolic L-threonine deaminase, the ilv1- mutants, have been found to exhibit a normal ability to grow, without auxotrophy towards isoleucine, on L-threonine of L-serine as only nitrogen nutrient. Starting from a strain carrying a ilv1- mutation, a new mutation affecting the ability to utilize L-threonine as nitrogen source was selected. This mutation, which also impairs the ability to utilize L-serine, has been denominated cha-, for 'catabolism of hydroxyamino acids' and was found to result in the lack of a catabolic L-serine (L-threonine) deaminase. This enzyme which, unlike the anabolic threonine deaminase, is more active towards serine than towards threonine, differs from the latter enzyme by a number of biochemical and regulatory properties. Whereas the anabolic enzyme is an allosteric enzyme sensitive to feedback inhibition by isoleucine, the catabolic enzyme exhibits Michaelian kinetics: no control of its activity has been detected. Its synthesis is induced by L-serine and L-threonine. These two enzymes, which thus can be easily differentiated by means of their regulations, display a limited ability to compensate for one another's absence and appear to play clearly distinct roles under normal physiological conditions.

Analysis of yeast ilv 1 CIS control and domain mutants

We have identified a cis control region specific for the ilv 1 gene of Saccharomyces cerevisiae. Mutants designated ilv 1-OPc map in this control region located between arg 6 and ilv 1 and result in increased basal levels and constitutive synthesis of the ilv 1 gene products. Furthermore, ilv 1 mutants have been isolated in three different structural domains indicating that the ilv 1 gene may contain a functional intervening sequence specific for one of the two gene products.

Characterization of yeast ribosomal DNA fragments generated by EcoR1 restriction endonuclease

The action of Escherichia coli restriction endonuclease R1 (EcoR1) on DNA isolated from Saccharomyces cerevisiae (strain MAR-33) generates three predominent homogenously sized DNA fragments (species of 1.8, 2.2 and 2.5 kilo nucleotide base pairs (KB). Many DNA species of molecular weight greater than 2 million daltons can be recognized upon incomplete EcoR1 digestion of yeast DNA. Four additional DNA species ranging from 0.3--0.9 KB can be identified as the second major class of EcoR1-yeast DNA products. Hybridization with radioactive ribosomal RNA (rRNA) and competition with nonradioactive rRNA show that of the three predominent EcoR1-yeast DNA species, the 2.5 KB species hybridizes only with the 25S rRNA while the lighter 1.8 KB species hybridizes with the 18S rRNA. The intermediate DNA species of 2.2 KB hybridizes to a small extent with the 25S rRNA and could be a result of the presence of the 2.5 KB DNA species. The mass proportions and hybridization values of these 3 DNA species account for about 60% of the total ribosomal DNA (rDNA). The 5 Eco-R1-yeast DNA species of less than 0.9 KB (4 major and 1 minor species) hybridize to varying degrees with the 2 rRNA and can be grouped in two classes. In one class there are 3 DNA species that hybridize exclusively with the 18S rRNA. In the second class there are 2 DNA species that besides hybridizing predominently with the 25S rRNA also hybridize with the 18S rRNA. The 7 EcoR1-yeast DNA species (excluding the 2.2 KB DNA species) that hybridize with the two rRNA account for nearly a 5 million dalton DNA segment, which is very close to the anticipated gene size of rRNA precursor molecule. If the 2.2 KB DNA species is a part of the rDNA that is not transcribed or 5 sRNA then the cistron encoding the rRNA in S. cerevisiae has at least 8 EcoR1 recognition sites resulting in 8 DNA fragments upon digestion with the EcoR1. Consideration is given to the relationship of the rRNA species generated by EcoR1 digestion and the chromosomes containing ribosomal cistrons.