Transcriptional regulation of Saccharomyces cerevisiae CYS3 encoding cystathionine gamma-lyase

Identification of Core Regulatory Genes and Metabolic Pathways for the n-Propanol Synthesis in Saccharomyces cerevisiae

The n-propanol produced by Saccharomyces cerevisiae has a remarkable effect on the taste and flavor of Chinese Baijiu. The n-propanol metabolism-related genes were deleted to evaluate the role in the synthesis of n-propanol to ascertain the key genes and pathways for the production of n-propanol by S. cerevisiae. The results showed that CYS3, GLY1, ALD6, PDC1, ADH5, and YML082W were the key genes affecting the n-propanol metabolism in yeast. The n-propanol concentrations of α5ΔGLY1, α5ΔCYS3, and α5ΔALD6 increased by 121.75, 22.75, and 17.78%, respectively, compared with α5. The n-propanol content of α5ΔPDC1, α5ΔADH5, and α5ΔYML082W decreased by 24.98, 8.35, and 8.44%, respectively, compared with α5. The contents of intermediate metabolites were measured, and results showed that the mutual transformation of glycine and threonine in the threonine pathway and the formation of propanal from 2-ketobutyrate were the core pathways for the formation of n-propanol. Additionally, YML082W played important role in the synthesis of n-propanol by directly producing 2-ketobutyric acid through l-homoserine. This study provided valuable insights into the n-propanol synthesis in S. cerevisiae and the theoretical basis for future optimization of yeast strains in Baijiu making.

mTORC1-Sch9 regulates hydrogen sulfide production through the transsulfuration pathway

Endogenous hydrogen sulfide mediates anti-aging benefits of dietary restriction (DR). However, it is unclear how H₂S production is regulated by pathways related to DR. Due to the importance of mTORC1 pathway in DR, we investigated the effects of Sch9, a yeast homolog of mammalian S6K1 and a major substrate of mTORC1 on H₂S production in yeast Saccharomyces cerevisiae. We found that inhibition of the mTORC1-Sch9 pathway by SCH9 deletion, rapamycin or myriocin treatment resulted in a dramatic decrease in H₂S production. Although deficiency of SCH9 did not alter the intracellular level of methionine, the intracellular level of cysteine increased in Δsch9 cells. The expression of CYS3 and CYS4, two transsulfuration pathway genes encoding cystathionine gamma-lyase (CGL) and cystathionine beta-synthase (CBS), were also decreased under mTORC1-Sch9 inhibition. Overexpression of CYS3 or CYS4 in Δsch9 cells or WT cells treated with rapamycin rescued the deficiency of H₂S production. Finally, we also observed a reduction in H₂S production and lowering of both mRNA and protein levels of CGL and CBS in cultured human cells treated with rapamycin to reduce mTORC1 pathway activity. Thus, our findings reveal a probably conserved mechanism in which H₂S production by the transsulfuration pathway is regulated by mTORC1-Sch9 signaling.

Trans-sulfuration Pathway Seleno-amino Acids Are Mediators of Selenomethionine Toxicity in Saccharomyces cerevisiae

Toxicity of selenomethionine, an organic derivative of selenium widely used as supplement in human diets, was studied in the model organism Saccharomyces cerevisiae. Several DNA repair-deficient strains hypersensitive to selenide displayed wild-type growth rate properties in the presence of selenomethionine indicating that selenide and selenomethionine exert their toxicity via distinct mechanisms. Cytotoxicity of selenomethionine decreased when the extracellular concentration of methionine or S-adenosylmethionine was increased. This protection resulted from competition between the S- and Se-compounds along the downstream metabolic pathways inside the cell. By comparing the sensitivity to selenomethionine of mutants impaired in the sulfur amino acid pathway, we excluded a toxic effect of Se-adenosylmethionine, Se-adenosylhomocysteine, or of any compound in the methionine salvage pathway. Instead, we found that selenomethionine toxicity is mediated by the trans-sulfuration pathway amino acids selenohomocysteine and/or selenocysteine. Involvement of superoxide radicals in selenomethionine toxicity in vivo is suggested by the hypersensitivity of a Δsod1 mutant strain, increased resistance afforded by the superoxide scavenger manganese, and inactivation of aconitase. In parallel, we showed that, in vitro, the complete oxidation of the selenol function of selenocysteine or selenohomocysteine by dioxygen is achieved within a few minutes at neutral pH and produces superoxide radicals. These results establish a link between superoxide production and trans-sulfuration pathway seleno-amino acids and emphasize the importance of the selenol function in the mechanism of organic selenium toxicity.

Yeast genes involved in sulfur and nitrogen metabolism affect the production of volatile thiols from Sauvignon Blanc musts

Two volatile thiols, 3-mercaptohexan-1-ol (3MH), and 3-mercaptohexyl-acetate (3MHA), reminiscent of grapefruit and passion fruit respectively, are critical varietal aroma compounds in Sauvignon Blanc (SB) wines. These aromatic thiols are not present in the grape juice but are synthesized and released by the yeast during alcoholic fermentation. Single deletion mutants of 67 candidate genes in a laboratory strain of Saccharomyces cerevisiae were screened using gas chromatography mass spectrometry for their thiol production after fermentation of SB grape juice. None of the deletions abolished production of the two volatile thiols. However, deletion of 17 genes caused increases or decreases in production by as much as twofold. These 17 genes, mostly related to sulfur and nitrogen metabolism in yeast, may act by altering the regulation of the pathway(s) of thiol production or altering substrate supply. Deleting subsets of these genes in a wine yeast strain gave similar results to the laboratory strain for sulfur pathway genes but showed strain differences for genes involved in nitrogen metabolism. The addition of two nitrogen sources, urea and di-ammonium phosphate, as well as two sulfur compounds, cysteine and S-ethyl-L-cysteine, increased 3MH and 3MHA concentrations in the final wines. Collectively these results suggest that sulfur and nitrogen metabolism are important in regulating the synthesis of 3MH and 3MHA during yeast fermentation of grape juice.

Nature and distribution of large sequence polymorphisms in Saccharomyces cerevisiae

To obtain a better understanding of the genome-wide distribution and the nature of large sequence polymorphisms (LSPs) in Saccharomyces cerevisiae, we hybridized genomic DNA of 88 haploid or homozygous diploid S. cerevisiae strains of diverse geographic origins and source substrates onto high-density tiling arrays. On the basis of loss of hybridization, we identified 384 LSPs larger than 500 bp that were located in 188 non-overlapping regions of the genome. Validation by polymerase chain reaction-amplification and/or DNA sequencing revealed that 39 LSPs were due to deletions, whereas 74 LSPs involved sequences diverged far enough from the S288c reference genome sequence as to prevent hybridization to the microarray features. The LSP locations were biased toward the subtelomeric regions of chromosomes, where high genetic variation in genes involved in transport or fermentation is thought to facilitate rapid adaptation of S. cerevisiae to new environments. The diverged LSP sequences appear to have different allelic ancestries and were in many cases identified as Saccharomyces paradoxus introgressions.

Aspergillus nidulans genes encoding reverse transsulfuration enzymes belong to homocysteine regulon

Homocysteine is an intermediate in methionine synthesis in Aspergillus nidulans, but it can also be converted to cysteine by the reverse transsulfuration pathway involving cystathionine beta-synthase (CBS) and cystathionine gamma-lyase (CGL). Because homocysteine is toxic to the cell at high concentrations, this pathway also functions as a means of removal of its excess. We found that the transcription of the mecA and mecB genes encoding CBS and CGL was upregulated by excess of homocysteine as well as by shortage of cysteine. Homocysteine induced transcription of both genes when added to the growth medium or overproduced in a regulatory mutant. The derepressing effect of cysteine shortage was observed in some mutants and in the wild-type strain during sulfur starvation. An increase in the level of mecA or mecB transcript roughly parallel with the elevation of the respective enzyme activity. On the basis of the mode of mecA and mecB regulation by homocysteine, these genes may be classified in a group of genes upregulated directly or indirectly by this amino acid. We call this group of genes the "homocysteine regulon".