Role of homocysteine synthetase in an alternate route for methionine biosynthesis in Saccharomyces cerevisiae

Enzymatic Synthesis of l-Methionine Analogues and Application in a Methyltransferase Catalysed Alkylation Cascade

Chemical modification of small molecules is a key step for the development of pharmaceuticals. S-adenosyl-l-methionine (SAM) analogues are used by methyltransferases (MTs) to transfer alkyl, allyl and benzyl moieties chemo-, stereo- and regioselectively onto nucleophilic substrates, enabling an enzymatic way for specific derivatisation of a wide range of molecules. l-Methionine analogues are required for the synthesis of SAM analogues. Most of these are not commercially available. In nature, O-acetyl-l-homoserine sulfhydrolases (OAHS) catalyse the synthesis of l-methionine from O-acetyl-l-homoserine or l-homocysteine, and methyl mercaptan. Here, we investigated the substrate scope of ScOAHS from Saccharomyces cerevisiae for the production of l-methionine analogues from l-homocysteine and organic thiols. The promiscuous enzyme was used to synthesise nine different l-methionine analogues with modifications on the thioether residue up to a conversion of 75 %. ScOAHS was combined with an established MT dependent three-enzyme alkylation cascade, allowing transfer of in total seven moieties onto two MT substrates. For ethylation, conversion was nearly doubled with the new four-enzyme cascade, indicating a beneficial effect of the in situ production of l-methionine analogues with ScOAHS.

Biotransformation of organic selenium compounds in budding yeast,Saccharomyces cerevisiae

Organic selenium metabolites of plants and animals such as selenoamino acids and selenosugars are metabolized to selenomethionine in yeast.

Candida albicans Hom6 is a homoserine dehydrogenase involved in protein synthesis and cell adhesion

BACKGROUND/PURPOSE

Candida albicans is a common fungal pathogen in humans. In healthy individuals, C. albicans represents a harmless commensal organism, but infections can be life threatening in immunocompromised patients. The complete genome sequence of C. albicans is extremely useful for identifying genes that may be potential drug targets and important for pathogenic virulence. However, there are still many uncharacterized genes in the Candida genome database. In this study, we investigated C. albicans Hom6, the functions of which remain undetermined experimentally.

METHODS

HOM6-deleted and HOM6-reintegrated mutant strains were constructed. The mutant strains were compared with wild-type in their growth in various media and enzyme activity. Effects of HOM6 deletion on translation were further investigated by cell susceptibility to hygromycin B or cycloheximide, as well as by polysome profiling, and cell adhesion to polystyrene was also determined.

RESULTS

C. albicans Hom6 exhibits homoserine dehydrogenase activity and is involved in the biosynthesis of methionine and threonine. HOM6 deletion caused translational arrest in cells grown under amino acid starvation conditions. Additionally, Hom6 protein was found in both cytosolic and cell-wall fractions of cultured cells. Furthermore, HOM6 deletion reduced C. albicans cell adhesion to polystyrene, which is a common plastic used in many medical devices.

CONCLUSION

Given that there is no Hom6 homologue in mammalian cells, our results provided an important foundation for future development of new antifungal drugs.

Cloning and analysis of the gene for cystathionine gamma-synthase from Arabidopsis thaliana

A cDNA clone, CGS1, encoding cystathionine gamma-synthase (CGS) from Arabidopsis thaliana was selected by complementation of CGS mutant strain of Escherichia coli (metB). Cells expressing CGS1 can grow on medium lacking Met and contain CGS enzyme activity. Genomic DNA blot analysis of A. thaliana revealed that there is a single gene homologous with CGS1. A genomic fragment carrying CGS1 was cloned and sequenced. Through combined analysis of the cDNA and genomic clone it was determined that the CGS1 coding sequence is 1692 bp, encodes a 563 amino acid, 60 kDa protein, and is interrupted by ten introns. A transcriptional initiation site was detected 260 bp 5' of the initiator codon. The predicted amino acid sequence of CGS1 contains a consensus pyridoxal phosphate-binding site and is similar to MetB of E. coli, with which it is 35 percent identical. The CGS1 product has a sequence at the amino terminus that resembles a transit peptide for localization to plastids. At least 160 amino acids from the amino terminus of the CGS1 enzyme are not essential for enzymatic activity.

Cysteine biosynthesis in Saccharomyces cerevisiae occurs through the transsulfuration pathway which has been built up by enzyme recruitment

The transsulfuration pathways allow the interconversion of homocysteine and cysteine with the intermediary formation of cystathionine. The various organisms studied up to now incorporate reduced sulfur into a three- or a four-carbon chain and use differently the transsulfuration pathways to synthesize sulfur amino acids. In enteric bacteria, the synthesis of cysteine is the first step of organic sulfur metabolism and homocysteine is derived from cysteine. Fungi are capable of incorporating reduced sulfur into a four-carbon chain, and they possess two operating transsulfuration pathways. By contrast, synthesis of cysteine from homocysteine is the only existing transsulfuration pathway in mammals. In Saccharomyces cerevisiae, genetic, phenotypic, and enzymatic study of mutants has allowed us to demonstrate that homocysteine is the first sulfur amino acid to be synthesized and cysteine is derived only from homocysteine (H. Cherest and Y. Surdin-Kerjan, Genetics 130:51-58, 1992). We report here the cloning of genes STR4 and STR1, encoding cystathionine beta-synthase and cystathionine gamma-lyase, respectively. The only phenotypic consequence of the inactivation of STR1 or STR4 is cysteine auxotrophy. The sequencing of gene STR4 has allowed us to compare all of the known sequences of transsulfuration enzymes and enzymes catalyzing the incorporation of reduced sulfur in carbon chains. These comparisons reveal a partition into two families based on sequence motifs. This partition mainly correlates with similarities in the catalytic mechanisms of these enzymes.

Unstable S-Adenosylmethionine synthetase in an ethionine-resistant strain of Neurospora crassa

A pleitropic ethionine-resistant mutant of Neurospora contains an S-adenosylmethionine synthetase that is labile to heat and dialysis but exhibits normal kinetics for methionine and ethionine.

Methionine adenosyltransferase and ethionine resistance in Saccharomyces cerevisiae

The methionine adenosyltransferase is repressed in Saccharomyces cerevisiae during growth in the presence of excess methionine. The relationship of this repression to the level of intracellular S-adenosylmethionine is discussed. In conjunction with these studies, an ethionine-resistant mutant has been investigated which has a low level of methionine adenosyltransferase under all conditions tested. The mechanism of ethionine resistance in the latter strain apparently depends on its inability to form large quantities of intracellular S-adenosylethionine. With respect to the methionine adenosyltransferase, there is no apparent interaction between ethionine-resistant and ethionine-sensitive alleles when both are present in the heterozygous diploid.