Downregulation of p21/WAF1 expression by thymidylate synthase

We developed a cell system where expression of thymidylate synthase (TS), an enzyme essential for DNA synthesis, can be modulated by a Zn(2+)-inducible promoter in MCF-7 cells. We found that overexpression of TS resulted in downregulation of p21 protein and mRNA levels. Statistical analysis demonstrated a significant downregulation of p21, but not a statistically significant decrease in p53 protein levels following TS induction. Since p21 is known to be transcriptionally activated by p53, these results suggest that TS downregulation of p21 may be occurring through a p53-independent mechanism in this in vitro cell system. In addition, cell cycle analysis demonstrated that downregulation of p21 by TS resulted in a decreased G(1)/S ratio in MCF-7 cells.

Saccharomyces cerevisiae expresses two genes encoding isozymes of methylenetetrahydrofolate reductase

The identification, expression, and assay of two Saccharomyces cerevisiae genes encoding methylenetetrahydrofolate reductases (MTHFR) is described. MTHFR catalyzes the reduction of 5, 10-methylenetetrahydrofolate to 5-methyltetrahydrofolate, used to methylate homocysteine in methionine synthesis. The MET12 gene is located on chromosome XVI and encodes a protein of 657 amino acids. The MET13 gene is located on chromosome VII and encodes a protein of 599 amino acids. The deduced amino acid sequences of these two genes are 34% identical to each other and 32-37% identical to the human MTHFR. A phenotype for the single disruption of MET12 was not observed, however, single disruption of MET13 resulted in methionine auxotrophy. Double disruption of both MET12 and MET13 also resulted in methionine auxotrophy. Growth of the methionine auxotrophs was supported by both methionine and S-adenosylmethionine. Transcripts of both MET12 and MET13 were detected in total RNA from wild type cells grown in the presence or absence of methionine. The methionine requirement of the met12 met13 double disruptant was complemented by plasmid-borne MET13, but not MET12 even when a multicopy plasmid was used. Furthermore, overexpression of the human MTHFR in the met12 met13 double disruptant complemented the methionine auxotrophy of this strain. In contrast, overexpression of the Escherichia coli metF gene did not complement the methionine requirement of met12 met13 cells. Assays for MTHFR in crude extracts and expression of the yeast proteins in Escherichia coli verified that both MET12 and MET13 encode functional MTHFR isozymes.

Role of mitochondrial and cytoplasmic serine hydroxymethyltransferase isozymes in de novo purine synthesis in Saccharomyces cerevisiae

One-carbon units are essential to a variety of anabolic processes which yield necessary cellular components including purines, pyrimidines, amino acids, and lipids. Serine hydroxymethyltransferase (SHMT) is the major provider of one-carbon units in the cell. The other product of this reaction is glycine. Both of these metabolites are required in de novo purine biosynthesis. In Saccharomyces cerevisiae, mitochondrial and cytoplasmic SHMT isozymes are encoded by distinct nuclear genes (SHM1 and SHM2). Molecular genetic analyses have begun to define the roles of these two isozymes in folate-mediated one-carbon metabolism [McNeil, J. B., et al. (1996) Genetics 142, 371-381]. In our study, the SHM1 and SHM2 genes were disrupted singly and in combination to investigate the contributions of the two SHMT isozymes to the production of glycine and one-carbon units required in purine biosynthesis. Cell subfractionation experiments indicated that while only 5% of total activity was localized in the mitochondria, the specific activity in that compartment was much higher than in the cytoplasm. Growth and 13C NMR experiments indicate that the two isozymes function in different directions, depending on the nutritional conditions of the cell. When yeast was grown on serine as the primary one-carbon source, the cytoplasmic isozyme was the main provider of glycine and one-carbon groups for purine synthesis. When grown on glycine, the mitochondrial SHMT was the predominant isozyme catalyzing the synthesis of serine from glycine and one-carbon units. However, when both serine and glycine were present, the mitochondrial SHMT made a significant contribution of one-carbon units, but not glycine, for purine synthesis. Finally, NMR data are presented that suggest the existence of at least two sites of de novo purine biosynthesis in growing yeast cells, each being fed by distinct pools of precursors.