Molecular genetics of met 17 and met 25 mutants of Saccharomyces cerevisiae: intragenic complementation between mutations of a single structural gene

We cloned the MET 17 gene of Saccharomyces cerevisiae by functional complementation after transformation of a yeast met 17 mutant. Restriction mapping and nucleotide sequencing of the MET 17 clones revealed that these were from the same genomic region as clones isolated previously and shown to contain the MET 25 gene encoding the enzyme O-acetylhomoserine, O-acetylserine sulphydrylase (OAH-OAS sulphydrylase). Transformation studies with MET 25 clones showed that the MET 17 and MET 25 functions were both endoced in a single transcription unit. We conclude that met 17 and met 25 are both mutations in the structural gene for the OAH-OAS sulphydrylase subunit and that each affects a different functional domain of the enzyme allowing subunit complementation in the met 17 X met 25 diploid. Enzyme assays indicated that the diploid, although not requiring methionine, had a low OAH-OAS sulphydrylase activity (10% of wild type). This is consistent with MET 17 and MET 25 being the same gene. We found that both met 17 and met 25 mutants were devoid of 3' phospho-adenosine 5' phospho-sulphite (PAPS) reductase activity and that this activity was fully restored in the met 17 X met 25 diploid. The possible interactions between OAH-OAS sulphydrylase and PAPS reductase are discussed.

Nucleotide sequence of the Saccharomyces cerevisiae MET25 gene

To elucidate further the molecular basis of the specific regulatory mechanism modulating the expression of the genes implicated in methionine metabolism, we have cloned and characterized two genes, MET3 and MET25, and shown that the regulation of their expression is transcriptional. The sequence of the cloned yeast MET25 gene which encodes the O-acetyl homoserine - O-acetyl serine (OAH-OAS) sulfhydrylase is reported here along with its 5' and 3' flanking regions. The amino acid composition predicted from the DNA sequence is in good agreement with that determined by hydrolysis of the purified enzyme. In the 5' flanking region the signal for general amino acid control was not found, corroborating our previous finding that the synthesis of OAH-OAS sulfhydrylase is not submitted to general control. The transcription start points have been determined. The 5' and 3' flanking regions of the MET25 gene suggest initiation and termination signals similar to those associated with other yeast genes.

O-Acetylhomoserine sulfhydrylase of the fission yeast Schizosaccharomyces pombe: partial purification, characterization, and its probable role in homocysteine biosynthesis

A crude extract of Schizosaccharomyces pombe cells catalyzed sulfhydrylation of both O-acetyl-L-serine and O-acetyl-L-homoserine with H2S, but did not synthesize cystathionine from O-acetyl-L-homoserine and L-cysteine. The O-acetylhomoserine sulfhydrylase [EC 4.2.99.10] was very unstable; however, it could be stabilized by the addition of 25% (w/w) sucrose or glycerol. The optimal pH for activity was 8.0 and that for stability was 7.0. The enzyme was purified approximately 300-fold from an ammonium sulfate-precipitated fraction. L-Methionine was the most effective inhibitor among the amino acids examined. It inhibited the enzyme competitively with respect to OAH with a Ki value of 2.6 mM. Sulfhydrylase activity was inhibited to various extents by some carbonyl reagents, but sulfhydryl reagents such as p-chloromercuribenzoic acid, 5,5'-dithio-bis(2-nitrobenzoic acid), and monoiodoacetic acid had no inhibitory effect. The enzyme also reacted with O-succinylhomoserine and L-homoserine to synthesize homocysteine directly, but could not utilize cysteine as a co-substrate in place of H2S. In the sulfhydrylation reactions, Km values for the substrates ranged from 10.4-12.5 mM. The enzyme was resolved to the apoenzyme by incubation with phenylhydrazine and reactivated by the addition of pyridoxal 5'-phosphate, whose Km value was 0.083 microM. The molecular weight of the enzyme was estimated to be approximately 186,000 by gel filtration and 170,000 by ultracentrifugation in sucrose density gradients. The isolectric point of the protein was pH 4.1. The characteristics of this enzyme are compared with those of physiologically functional sulfhydrylases reported for other organisms, and the possibility of the enzyme functioning as a homocysteine synthase is discussed.

Methionine biosynthesis in Brevibacterium flavum: properties and essential role of O-acetylhomoserine sulfhydrylase

Out of 27 strains of methionine auxotrophs of Brevibacterium flavum, 14 strains did not grow on homoserine but grew on O-acetylhomoserine, and all were found to lack homoserine O-acetyltransferase [EC 2.3.1.31] alone. Another 3 strains did not grow on O-acetylhomoserine but grew on homocysteine, and the two strains tested were found to lack O-acetylhomoserine sulfhydrylase (AHS) alone, without any changes in the activities of cystathionine gamma-synthase [EC4.2.99.9] and beta-cystathionase [EC 4.4.1.8]. Prototrophic revertants of the AHS-lacking mutants showed concomitant reversion of AHS activity. None of the methionine auxotrophs grew on cystathionine. From these results it was concluded that the methionine biosynthetic pathway of this bacterium involves formation of O-acetylhomoserine from homoserine by the action of homoserine O-acetyltransferase, and direct formation of homocysteine from O-acetylhomoserine by the AHS reaction. AHS synthesis was strongly repressed by methionine. AHS was purified to 70% purity. The purified preparation was activated by pyridoxal phosphate after treatment with hydroxylamine. The enzyme showed a molecular weight of 360,000, an optimum pH of 8.7 for activity, and specifically reacted with O-acetyl-L-homoserine and showed with O-acetyl-L-serine one hundredth as much activity as that with O-acetylo -homoserine, but did not show activity with O-succinyl-L-homoserine, homoserine, or serine. The Km values for O-acetylhomoserine and H2S were 2.0 mM and 0.08 mM, respectively. The enzyme was inhibited 50, 23. and 29% by 10 mM L-methionine, l-homoserine, and O-acetyl-L-serine, respectively, but it was not inhibited by cystathionine or S-adenosyl-L-methionine.

Partial purification and comparison of some properties of L-serine sulfhydro-lyase of Saccharomyces cerevisiae

In order to ascertain the role of L-serine sulfhydro-lyase (L-serine hydro-lyase (adding homocysteine) EC 4.2.1.22) which also catalyzes sulfhydrylation of O-acetyl-L-serine (Yamagata, S. (1981) J. Bacteriol. 147, 688-690), the enzyme was partially purified from a wild-type strain and three cysteine auxotrophs of Saccharomyces cerevisiae, and the molecular and enzymatic properties of these preparations were compared. The results showed no significant difference in properties investigated, indicating that cysteine synthesis is exclusively performed in this organism through sulfhydrylation of O-acetyl-L-serine, catalyzed not by serine sulfhydro-lyase but by O-acetylserine . O-acetylhomoserine sulfhydro-lyase (Yamagata, S., Takeshima, K. and Naiki, N. (1974) J. Biochem. 75, 1221-1229). Insensitivity of the former enzyme to L-methionine also supported this conclusion.

O-Acetylserine and O-acetylhomoserine sulfhydrylase of yeast. Subunit structure

The molecular weight of O-acetylserine (OAS)-O-acetylhomoserine (OAH) sulfhydrylase purified from yeast was estimated to be about 200,000 by Sephadex G-200 gel chromatography in various buffers. The S20, w value of this protein was determined to be about 9.0 by sucrose density gradient centrifugation. The calculated molecular weight based on this value was similar to that estimated by gel chromatography. Treatment with 1% sodium dodesylsulfate (SDS) or 6 M urea dissociated the enzyme into 4 subunits; these had a molecular weight estimated to be 51,000 by SDS-poly-acrylamide gel electrophoresis and to be 57,000 by Sephadex G-100 gel chromatography in the presence of 6 M urea and 0.5% beta-mercaptoethanol. The 4 subunits appeared to be identical, based on the symmetric subunit elution pattern from a Sephadex column, a single peptide band on SDS-polyacrylamide gel, and the detection of histidine as the sole N-terminal amino acid in the native enzyme. Since dissociation into the subunits occurred without the use of reducing agents, the association of the subunits seems to require no disulfide linkage. One mole of the subunit contained one mole of sulfhydryl group which appeared to be buried inside the molecule. Partial restoration of the catalytic activity was observed when the urea-denatured enzyme was dialyzed to remove urea, especially in the presence of reducing agents such as dithiothreitol. The urea-denatured enzyme showed a tendency in the absence of reducing agents to form a subunit dimer linked by a disulfide bond between the cystine residues exposed by denaturation. The amino acid composition of the enzyme was determined; it contained one half-cystine residue per subunit, and the content of acidic residues was much higher than that of basic residues. Based on these findings, the subunit structure of the enzyme is discussed.