The two methionine adenosyl transferases in Saccharomyces cerevisiae: evidence for the existence of dimeric enzymes

Engineered Pichia pastoris for enhanced production of S-adenosylmethionine

A genetically engineered strain of Pichia pastoris expressing S-adenosylmethionine synthetase gene from Saccharomyces cerevisiae under the control of AOX 1 promoter was developed. Induction of recombinant strain with 1% methanol resulted in the expression of SAM2 protein of ~ 42 kDa, whereas control GS115 showed no such band. Further, the recombinant strain showed 17-fold higher enzyme activity over control. Shake flask cultivation of engineered P. pastoris in BMGY medium supplemented with 1% L-methionine yielded 28 g/L wet cell weight and 0.6 g/L S-adenosylmethionine, whereas control (transformants with vector alone) with similar wet cell weight under identical conditions accumulated 0.018 g/L. The clone cultured in the bioreactor containing enriched methionine medium showed increased WCW (117 g/L) as compared to shake flask cultures and yielded 2.4 g/L S-adenosylmethionine. In spite of expression of SAM 2 gene up to 90 h, S-adenosylmethionine accumulation tended to plateau after 72 h, presumably because of the limited ATP available in the cells at stationery phase. The recombinant P pastoris seems promising as potential source for industrial production of S-adenosylmethionine.

A dominant negative effect of eth-1r, a mutant allele of the Neurospora crassa S-adenosylmethionine synthetase-encoding gene conferring resistance to the methionine toxic analogue ethionine

eth-1r, a thermosensitive allele of the Neurospora crassa S-adenosylmethionine (AdoMet) synthetase gene that confers ethionine resistance, has been cloned and sequenced. Replacement of an aspartic amino acid residue (D48-->N48), perfectly conserved in prokaryotic, fungal and higher eukaryotic AdoMet synthetases, was found responsible for both thermosensitivity and ethionine resistance conferred by eth-1r. Gene fusion constructs, designed to overexpress eth-1r in vivo, render transformant cells resistant to ethionine. Dominance of ethionine resistance was further demonstrated in eth-1+/eth-1r partial diploids carrying identical gene doses of both alleles. Heterozygous eth-1+/eth-1r cells have, at the same time, both the thermotolerance conferred by eth-1+ and the ethionine-resistant phenotype conferred by eth-1r. AdoMet levels and AdoMet synthetase activities were dramatically decreased in heterozygous eth-1+/ eth-1r cells. We propose that this negative effect exerted by eth-1r results from the in vivo formation of heteromeric eth-1+/eth-1r AdoMet synthetase molecules.

Methionine adenosyltransferase: structure and function

Methionine adenosyltransferase (MAT), a key enzyme in metabolism, catalyzes the synthesis of one of the most important and pivotal biological molecules, S-adenosyl-methionine. In every organism studied thus far, MAT exists in multiple forms; most are encoded by related, but distinct genes. Molecular and immunological studies revealed the presence of considerable conservation in the structure of MAT from different species; however, the various MAT isozymes differ in their physical and kinetic properties in ways that allow them to be regulated differently. Recent studies suggest that human MAT is composed of nonidentical subunits that can assume multiple states of aggregation, each with different kinetic characteristics. The tissue distribution of MAT isozymes and the ability of cells within the same tissue to switch between the different forms of MAT suggest that this mode of regulation is important for cellular function and differentiation. Therefore, understanding the regulation and structure-function relationship of this fascinating enzyme should help us clarify its role in biology and may provide us with tools to effectively manipulate its activity in clinical situations such as cancer, autoimmunity and organ transplantation.

Antigenic conservation of primary structural regions of S-adenosylmethionine synthetase

Although the physical and kinetic properties of S-adenosylmethionine (AdoMet) synthetases from different sources are quite different, it appears that these enzymes have structurally or antigenically conserved regions as demonstrated by studies with AdoMet synthetase specific antibodies. Polyclonal anti-human lymphocyte AdoMet synthetase crossreacted with enzyme from rat liver (beta isozyme), Escherichia coli and yeast. In addition, polyclonal anti-E. coli enzyme and antibodies to synthetic peptides copying several regions of the yeast enzyme reacted with the human gamma and rat beta isozymes. Antibodies to yeast SAM1 encoded protein residues 6-21, 87-113 and 87-124 inhibited the activity of human lymphocyte AdoMet synthetase, while antibodies to residues 272-287 had no effect on the enzyme activity. Our results suggest that these conserved regions may be important in enzyme activity.

Transcriptional regulation of the MET3 gene of Saccharomyces cerevisiae

The MET3 gene, coding for ATP sulfurylase (ATPS), an enzyme implicated in methionine biosynthesis in Saccharomyces cerevisiae, was cloned by functional complementation, after transformation, of a yeast met3 mutant strain. The cloned MET3 gene was used as a probe to measure the specific MET3 messenger RNA in a wild-type strain grown under conditions which promote or fail to promote repression of ATPS synthesis. It was found that the level of MET3 messenger RNA is reduced ten-fold when the strain is grown under conditions where ATPS synthesis is repressed, suggesting that the MET3 expression is regulated transcriptionally. The direction of transcription and the size of the transcript have been determined.

Targeted selection of recombinant clones through gene dosage effects

The availability of multicopy plasmid vectors for the yeast Saccharomyces cerevisiae allows the selective amplification of individual segments of the genome. Increased dosage of particular genes results in overproduction of specific gene products and thereby confers resistance to certain metabolic inhibitors. Advantage was taken of this fact to isolate recombinant clones that increase the activities of the enzymes UDP-N-acetylglucosamine-1-P transferase and 3-hydroxy-3-methylglutaryl-CoA reductase.