Physical localization of yeast CYS3, a gene whose product resembles the rat gamma-cystathionase and Escherichia coli cystathionine gamma-synthase enzymes

Plant homocysteine, a methionine precursor and plant's hallmark of metabolic disorders

Homocysteine (Hcy) is a sulfur-containing non-proteinogenic amino acid, which arises from redox-sensitive methionine metabolism. In plants, Hcy synthesis involves both cystathionine β-lyase and S-adenosylhomocysteine hydrolase activities. Thus, Hcy itself is crucial for de novo methionine synthesis and S-adenosylmethionine recycling, influencing the formation of ethylene, polyamines, and nicotianamine. Research on mammalian cells has shown biotoxicity of this amino acid, as Hcy accumulation triggers oxidative stress and the associated lipid peroxidation process. In addition, the presence of highly reactive groups induces Hcy and Hcy derivatives to modify proteins by changing their structure and function. Currently, Hcy is recognized as a critical, independent hallmark of many degenerative metabolic diseases. Research results indicate that an enhanced Hcy level is also toxic to yeast and bacteria cells. In contrast, in the case of plants the metabolic status of Hcy remains poorly examined and understood. However, the presence of the toxic Hcy metabolites and Hcy over-accumulation during the development of an infectious disease seem to suggest harmful effects of this amino acid also in plant cells. The review highlights potential implications of Hcy metabolism in plant physiological disorders caused by environmental stresses. Moreover, recent research advances emphasize that recognizing the Hcy mode of action in various plant systems facilitates verification of the potential status of Hcy metabolites as bioindicators of metabolism disorders and thus may constitute an element of broadly understood biomonitoring.

Deactivation of the autotrophic sulfate assimilation pathway substantially reduces high-level β-lactam antibiotic biosynthesis and arthrospore formation in a production strain from Acremonium chrysogenum

The filamentous ascomycete Acremonium chrysogenum is the only industrial producer of the β-lactam antibiotic cephalosporin C. Synthesis of all β-lactam antibiotics starts with the three amino acids l-α-aminoadipic acid, l-cysteine and l-valine condensing to form the δ-(l-α-aminoadipyl)-l-cysteinyl-d-valine tripeptide. The availability of building blocks is essential in every biosynthetic process and is therefore one of the most important parameters required for optimal biosynthetic production. Synthesis of l-cysteine is feasible by various biosynthetic pathways in all euascomycetes, and sequencing of the Acr. chrysogenum genome has shown that a full set of sulfur-metabolizing genes is present. In principle, two pathways are effective: an autotrophic one, where the sulfur atom is taken from assimilated sulfide to synthesize either l-cysteine or l-homocysteine, and a reverse transsulfuration pathway, where l-methionine is the sulfur donor. Previous research with production strains has focused on reverse transsulfuration, and concluded that both l-methionine and reverse transsulfuration are essential for high-level cephalosporin C synthesis. Here, we conducted molecular genetic analysis with A3/2, another production strain, to investigate the autotrophic pathway. Strains lacking either cysteine synthase or homocysteine synthase, enzymes of the autotrophic pathway, are still autotrophic for sulfur. However, deletion of both genes results in sulfur amino acid auxotrophic mutants exhibiting delayed biomass production and drastically reduced cephalosporin C synthesis. Furthermore, both single- and double-deletion strains are more sensitive to oxidative stress and form fewer arthrospores. Our findings provide evidence that autotrophic sulfur assimilation is essential for growth and cephalosporin C biosynthesis in production strain A3/2 from Acr. chrysogenum.

Formation of hydrogen sulfide from cysteine in Saccharomyces cerevisiae BY4742: genome wide screen reveals a central role of the vacuole

Discoveries on the toxic effects of cysteine accumulation and, particularly, recent findings on the many physiological roles of one of the products of cysteine catabolism, hydrogen sulfide (H₂S), are highlighting the importance of this amino acid and sulfur metabolism in a range of cellular activities. It is also highlighting how little we know about this critical part of cellular metabolism. In the work described here, a genome-wide screen using a deletion collection of Saccharomyces cerevisiae revealed a surprising set of genes associated with this process. In addition, the yeast vacuole, not previously associated with cysteine catabolism, emerged as an important compartment for cysteine degradation. Most prominent among the vacuole-related mutants were those involved in vacuole acidification; we identified each of the eight subunits of a vacuole acidification sub-complex (V₁ of the yeast V-ATPase) as essential for cysteine degradation. Other functions identified included translation, RNA processing, folate-derived one-carbon metabolism, and mitochondrial iron-sulfur homeostasis. This work identified for the first time cellular factors affecting the fundamental process of cysteine catabolism. Results obtained significantly contribute to the understanding of this process and may provide insight into the underlying cause of cysteine accumulation and H₂S generation in eukaryotes.

Transcriptional regulation of Saccharomyces cerevisiae CYS3 encoding cystathionine gamma-lyase

In studying the regulation of GSH11, the structural gene of the high-affinity glutathione transporter (GSH-P1) in Saccharomyces cerevisiae, a cis-acting cysteine responsive element, CCGCCACAC (CCG motif), was detected. Like GSH-P1, the cystathionine gamma-lyase encoded by CYS3 is induced by sulfur starvation and repressed by addition of cysteine to the growth medium. We detected a CCG motif (-311 to -303) and a CGC motif (CGCCACAC; -193 to -186), which is one base shorter than the CCG motif, in the 5'-upstream region of CYS3. One copy of the centromere determining element 1, CDE1 (TCACGTGA; -217 to -210), being responsible for regulation of the sulfate assimilation pathway genes, was also detected. We tested the roles of these three elements in the regulation of CYS3. Using a lacZ-reporter assay system, we found that the CCG/CGC motif is required for activation of CYS3, as well as for its repression by cysteine. In contrast, the CDE1 motif was responsible for only activation of CYS3. We also found that two transcription factors, Met4 and VDE, are responsible for activation of CYS3 through the CCG/CGC and CDE1 motifs. These observations suggest a dual regulation of CYS3 by factors that interact with the CDE1 motif and the CCG/CGC motifs.

Telomeric silencing of an open reading frame in Saccharomyces cerevisiae

The endmost chromosome I ORF is silenced by a natural telomere position effect. YAR073W/IMD1 was found to be transcribed at much higher levels in sir3 mutants and when its adjacent telomere was removed from it. These results suggest that telomeres play a role in silencing actual genes.

CYS3, a hotspot of meiotic recombination in Saccharomyces cerevisiae. Effects of heterozygosity and mismatch repair functions on gene conversion and recombination intermediates

We have examined meiotic recombination at the CYS3 locus. Genetic analysis indicates that CYS3 is a hotspot of meiotic gene conversion, with a putative 5'-3' polarity gradient of conversion frequencies. This gradient is relieved in the presence of msh2 and pms1 mutations, indicating an involvement of mismatch repair functions in meiotic recombination. To investigate the role of mismatch repair proteins in meiotic recombination, we performed a physical analysis of meiotic DNA in wild-type and msh2 pms1 strains in the presence or absence of allelic differences at CYS3. Neither the mutations in CYS3 nor the absence of mismatch repair functions affects the frequency and distribution of nearby recombination-initiating DNA double-strand breaks (DSBs). Processing of DSBs is also similar in msh2 pms1 and wild-type strains. We conclude that mismatch repair functions do not control the distribution of meiotic gene conversion events at the initiating steps. In the MSH2 PMS1 background, strains heteroallelic for frameshift mutations in CYS3 exhibit a frequency of gene conversion greater than that observed for either marker alone. Physical analysis revealed no modification in the formation of DSBs, suggesting that this marker effect results from subsequent processing events that are not yet understood.

Metabolism of sulfur amino acids in Saccharomyces cerevisiae

Sulfur amino acid biosynthesis in Saccharomyces cerevisiae involves a large number of enzymes required for the de novo biosynthesis of methionine and cysteine and the recycling of organic sulfur metabolites. This review summarizes the details of these processes and analyzes the molecular data which have been acquired in this metabolic area. Sulfur biochemistry appears not to be unique through terrestrial life, and S. cerevisiae is one of the species of sulfate-assimilatory organisms possessing a larger set of enzymes for sulfur metabolism. The review also deals with several enzyme deficiencies that lead to a nutritional requirement for organic sulfur, although they do not correspond to defects within the biosynthetic pathway. In S. cerevisiae, the sulfur amino acid biosynthetic pathway is tightly controlled: in response to an increase in the amount of intracellular S-adenosylmethionine (AdoMet), transcription of the coregulated genes is turned off. The second part of the review is devoted to the molecular mechanisms underlying this regulation. The coordinated response to AdoMet requires two cis-acting promoter elements. One centers on the sequence TCACGTG, which also constitutes a component of all S. cerevisiae centromeres. Situated upstream of the sulfur genes, this element is the binding site of a transcription activation complex consisting of a basic helix-loop-helix factor, Cbf1p, and two basic leucine zipper factors, Met4p and Met28p. Molecular studies have unraveled the specific functions for each subunit of the Cbf1p-Met4p-Met28p complex as well as the modalities of its assembly on the DNA. The Cbf1p-Met4p-Met28p complex contains only one transcription activation module, the Met4p subunit. Detailed mutational analysis of Met4p has elucidated its functional organization. In addition to its activation and bZIP domains, Met4p contains two regulatory domains, called the inhibitory region and the auxiliary domain. When the level of intracellular AdoMet increases, the transcription activation function of Met4 is prevented by Met30p, which binds to the Met4 inhibitory region. In addition to the Cbf1p-Met4p-Met28p complex, transcriptional regulation involves two zinc finger-containing proteins, Met31p and Met32p. The AdoMet-mediated control of the sulfur amino acid pathway illustrates the molecular strategies used by eucaryotic cells to couple gene expression to metabolic changes.

LTE1 of Saccharomyces cerevisiae is a 1435 codon open reading frame that has sequence similarities to guanine nucleotide releasing factors

The DNA sequence of the LTE1 gene on the left arm of chromosome I of Saccharomyces cerevisiae has been determined. The LTE1 open reading frame comprises 4305 bp that can be translated into 1435 amino acid residues. The position of this open reading frame corresponds well to that of a 4.7 kb transcript that has been mapped to this position. The derived amino acid sequence has significant similarities to the amino acid sequence of the guanine nucleotide releasing factor isolated from a rat brain library. The carboxy-terminus of the LTE1 protein also shows similarities to other guanine nucleotide exchange factors of the S. cerevisiae CDC25 family.

Molecular cloning of chromosome I DNA from Saccharomyces cerevisiae: analysis of the genes in the FUN38-MAK16-SPO7 region

Transcribed regions on a 42-kb segment of chromosome I from Saccharomyces cerevisiae were mapped. Polyadenylated transcripts corresponding to eight previously characterized genes (MAK16, LTE1, CCR4, FUN30, FUN31, TPD3, DEP1, and CYS3) and eight new genes were identified. All transcripts were present at one to four copies per cell except for one which was significantly less abundant. This region has been sequenced, and the sizes, locations, and orientations of the transcripts were in nearly perfect agreement with the open reading frames. Disruptions in eight genes identified solely on the basis of a transcribed region, FUN38, FUN25, FUN26, FUN28, FUN30, FUN31, FUN33, and FUN34, indicated that all were nonessential for growth on rich medium at 30 degrees C. Disruption of FUN30, a gene closely related to RAD16 and RAD54, surprisingly resulted in increased resistance to UV irradiation. No additional phenotypes, other than slow growth, were observed for all other mutants. The distribution of essential genes on chromosome I is discussed.