Rate and extent of interconversion of tetrahydrofolate cofactors to dihydrofolate after cessation of dihydrofolate reductase activity in stationary versus log phase L1210 leukemia cells.

Previous studies from this laboratory established that the rapid but partial interconversion of tetrahydrofolate cofactors to dihydrofolate after exposure of L1210 leukemia cells to antifolates cannot be due to direct feedback inhibition of thymidylate synthase by dihydrofolate or any other endogenous folylpolyglutamates when dihydrofolate reductase activity is abolished by antifolates. Rather, the data suggested this preservation of tetrahydrofolate cofactor pools is likely due to a fraction of cellular folates unavailable for oxidation to dihydrofolate. This paper explores the role of cell cycle phase in L1210 leukemia cells in logarithmic versus stationary phase growth as a factor in the rate and extent of tetrahydrofolate cofactor interconversion to dihydrofolate after exposure of cells to the dihydrofolate reductase inhibitor trimetrexate. The S phase fraction was reduced by inoculating L1210 leukemia cells at high density to achieve a stationary state. Flow cytometric analysis of DNA content indicated that log phase cultures were 53.0% S phase; this decreased to 42.1% at 24 h and 24.1% at 48 h in stationary phase cultures. 5-Bromo-2'-deoxyuridine incorporation into DNA decreased 80 and 96%, while [3H]dUrd incorporation into DNA declined 70 and 95% for stationary cultures at 24 and 48 h, respectively, as compared with the log phase rates. Log phase cells interconverted 28.0% of the total pool of radiolabeled folates to dihydrofolate with a half-time of approximately 30 s. Stationary cells at 24 h interconverted 20.4% of the total folate pool with a t1/2 of approximately 3 min, and at 48 h, net interconversion to dihydrofolate decreased further to 12.1% with a t1/2 of approximately 6 min. The decrease in the extent of tetrahydrofolate cofactor interconversion to dihydrofolate in stationary phase cells was directly proportional to the decrease in the S phase fraction determined by total DNA content. This suggests that tetrahydrofolate cofactor depletion occurs only in S phase cells. The much larger drop in [3H]dUrd and 5-bromo-2'-deoxyuridine incorporation into DNA in comparison with the decline in the S phase fraction measured by DNA content along with the reduced rate of tetrahydrofolate cofactor interconversion to dihydrofolate indicates that the rate of DNA synthesis is decreased in S phase cells in stationary cultures. Network thermodynamic simulations suggest that a reduction in the number of S phase cells and their thymidylate synthase catalytic activity would account for the observed decrease in the rate and extent of interconversion of tetrahydrofolate cofactors to dihydrofolate after trimetrexate in stationary phase cultures.(ABSTRACT TRUNCATED AT 400 WORDS)     

Thymidylate synthase activity from chlamydomonas cells and cultured tissues of Nicotiana, pinus, and daucus

Prior preparation of N(5),N(10)-methylenetetrahydrofolate from dl-tetrahydrofolate and [(14)C]formaldehyde resulted in an improved assay for thymidylate synthase. Although preparations from tobacco seedlings and cotton root tips (0.25 centimeter) were inconsistent with respect to enzyme activity, extracts from actively growing cell cultures of Chlamydomonas, Nicotiana hybrid callus, Pinus callus, and Daucus proembryonic cells contained significant levels of thymidylate synthase (12.3 to 23.8 nanomoles of thymidylate formed per milligram of protein per hour).     

Effect of growth phase on phospholipid biosynthesis in Saccharomyces cerevisiae.

The effect of growth phase on the membrane-associated phospholipid biosynthetic enzymes CDP-diacylglycerol synthase, phosphatidylserine synthase, phosphatidylinositol synthase, and the phospholipid N-methyltransferases in wild-type Saccharomyces cerevisiae was examined. Maximum activities were found in the exponential phase of cells grown in complete synthetic medium. As cells entered the stationary phase of growth, the activities of the CDP-diacylglycerol synthase, phosphatidylserine synthase, and the phospholipid N-methyltransferases decreased 2.5- to 5-fold. The subunit levels of phosphatidylserine synthase and the cytoplasmic-associated enzyme inositol-1-phosphate synthase were not significantly affected by the growth phase. When grown in medium supplemented with inositol-choline, cells in the exponential phase of growth had reduced CDP-diacylglycerol synthase, phosphatidylserine synthase, and phospholipid N-methyltransferase activities, with repressed subunit levels of phosphatidylserine synthase and inositol-1-phosphate synthase compared with cells grown without inositol-choline. Enzyme activity levels remained reduced in the stationary phase of growth of cells supplemented with inositol-choline. The phosphatidylserine synthase and inositol-1-phosphate synthase subunit levels, however, were depressed. Phosphatidylinositol synthase (activity and subunit) was not affected by growth in medium supplemented with or without inositol-choline or the growth phase of the culture. The phospholipid composition of cells in the exponential and stationary phase of growth was also examined. The phosphatidylinositol to phosphatidylserine ratio doubled in stationary-phase cells. The phosphatidylcholine to phosphatidylethanolamine ratio was not significantly affected by the growth phase of cells.     

Two glycogen synthase isoforms in Saccharomyces cerevisiae are coded by distinct genes that are differentially controlled

In previous work, we identified a Saccharomyces cerevisiae glycogen synthase gene, GSY1, which codes for an 85-kDa polypeptide present in purified yeast glycogen synthase (Farkas, I., Hardy, T.A., DePaoli-Roach, A.A., and Roach, P.J. (1990) J. Biol. Chem. 265, 20879-20886). We have now cloned another gene, GSY2, which encodes a second S. cerevisiae glycogen synthase. The GSY2 sequence predicts a protein of 704 residues, molecular weight 79,963, with 80% identity to the protein encoded by GSY1. Amino acid sequences obtained from a second polypeptide of 77 kDa present in yeast glycogen synthase preparations matched those predicted by GSY2. GSY1 resides on chromosome VI, and GSY2 is located on chromosome XII. Disruption of the GSY1 gene produced a strain retaining about 85% of wild type glycogen synthase activity at stationary phase, while disruption of the GSY2 gene yielded a strain with only about 10% of wild type enzyme activity. The level of glycogen synthase activity in yeast cells disrupted for GSY1 increased in stationary phase, whereas the activity remained at a constant low level in cells disrupted for GSY2. Disruption of both genes resulted in a viable haploid that totally lacked glycogen synthase activity and was defective in glycogen deposition. In conclusion, yeast expresses two forms of glycogen synthase with activity levels that behave differently in the growth cycle. The GSY2 gene product appears to be the predominant glycogen synthase with activity linked to nutrient depletion.     

Construction and expression of mouse thymidylate synthase minigenes

Mouse thymidylate synthase minigenes that lack introns were constructed by ligating restriction fragments containing 4.5, 1.0, or 0.25 kilobase pairs (kb) of 5'-flanking DNA of the normal thymidylate synthase gene and as little as 0.25 kb of 3'-flanking DNA to full-length thymidylate synthase cDNA. All three minigenes were expressed at approximately the same levels following transfection into hamster V79 cells that were deficient in thymidylate synthase. S1 nuclease protection assays revealed that the multiple 5' and 3' termini of thymidylate synthase mRNA in cells transfected with these minigenes were at the same positions as those of the normal mRNA in mouse cells. Deletion analysis of the promoter region revealed that minigenes extending to position -150 nucleotides (relative to the AUG codon) were expressed at approximately the same level as those extending to -1 kb. However, minigenes extending to -53 nucleotides were inactive. To determine if the minigenes were capable of being regulated in a cell cycle-dependent manner, thymidylate synthase gene expression was measured in hamster cells that were stably transfected with the largest minigene and synchronized by serum-stimulation. Thymidylate synthase enzyme level and mRNA content increased 3-5-fold as cells progressed from G1 through S phase.     

Regulation of thymidylate synthase gene expression in mouse fibroblasts synchronized by mitotic selection

Previous studies have shown that thymidylate synthase gene expression is regulated over a wide range in response to growth stimulation in cultured mouse fibroblasts. In the present study we show that the gene is also regulated during the cell cycle in continuously growing cells. Our analyses were conducted with a fluorodeoxyuridine-resistant mouse 3T6 cell line that overproduces thymidylate synthase and its mRNA by a factor of 50 due to gene amplification. Cells were synchronized by mitotic selection. RNA blot analyses showed that the amount of thymidylate synthase mRNA increased 5- to 10-fold as cells progressed from G1 through the middle of S phase. S1 nuclease protection assays showed that the pattern of 5' termini of thymidylate synthase mRNA was the same in G1 and S phase. Despite the large increase in thymidylate synthase mRNA content, the level of the enzyme increased only by a factor of 2 as cells progressed from G1 to mid S phase. This apparent discrepancy can be explained by the fact that the enzyme is highly stable.     

Thymidine 5′-Monophosphate-Requiring Mutants of Saccharomyces cerevisiae Are Deficient in Thymidylate Synthetase

Thymidylate synthetase activity was measured in crude extracts of the yeast Saccharomyces cerevisiae by a sensitive radiochemical assay. Spontaneous non-conditional mutants auxotrophic for thymidine 5'-monophosphate (tmp1) lacked detectable thymidylate synthetase activity in cell-free extracts. In contrast, the parent strains (tup1, -2, or -4), which were permeable to thymidine 5'-monophosphate, contained levels of activity similar to those found in wild-type cells. Specific activity of thymidylate synthetase in crude extracts of normal cells or of cells carrying tup mutations was essentially unaffected by the ploidy or mating type of the cells, by the medium used for growth, by the respiratory capacity of the cells, by concentrations of exogenous thymidine 5'-monophosphate as high as 50 mug/ml, or by subsequent removal of thymidine 5'-monophosphate from the medium. Extracts of a strain bearing the temperature-sensitive cell division cycle mutation cdc21 lacked detectable thymidylate synthetase activity under all conditions tested. Its parent and another mutant (cdc8), which arrests with the same terminal phenotype under restrictive conditions, had normal levels of the enzyme. Cells of a temperature-sensitive thymidine 5'-monophosphate auxotroph arrested with a morphology identical to the cdc21 strain at the nonpermissive temperature and contained demonstrably thermolabile thymidylate synthetase activity. Tetrad analysis and the properties of revertants showed that the thymidylate synthetase defects were a consequence of the same mutation causing, in the auxotrophs, a requirement for thymidine 5'-monophosphate and, in the conditional mutants, temperature sensitivity. Complementation tests indicated that tmp1 and cdc21 are the same locus. These results identify tmp1 as the structural gene for yeast thymidylate synthetase.     

Ornithine decarboxylase activity and cell cycle regulation in Saccharomyces cerevisiae.

In the yeast Saccharomyces cerevisiae, the specific activity of the enzyme ornithine decarboxylase (ODC) was correlated with overall growth status. The activity of ODC was highest in actively growing cells, whereas the specific activity was lower in slow-growing cultures limited for nitrogen or inhibited by low concentrations of cycloheximide. Specific activities of ODC were also low in cultures arrested in the stationary phase (in the G1 portion of the cell cycle) by starvation for required nutrients. Although correlated with overall growth, ODC activity was not required for growth or cell cycle regulation. Cells continued to grow in the presence of the polyamine spermidine or spermine, which markedly reduced ODC specific activities. Thus, high levels of ODC activity were not necessary for growth, nor were decreased ODC specific activities sufficient to cause cells to arrest in G1. Conversely, one agent (o-phenanthroline) which causes growing cells to arrest in G1 did so with no effect on ODC specific activity. Therefore, ODC specific activity changes are not necessary for cell cycle regulation but simply reflect the normal growth status of cells.     

Stimulation by heme of steryl ester synthase and aerobic sterol exclusion in the yeast Saccharomyces cerevisiae.

Saccharomyces cerevisiae sterol and heme auxotrophs were used to elucidate a role for hemes in sterol esterification. Steryl ester synthase (SES) activity was stimulated on average fourfold in cells supplemented with 50 micrograms/ml delta-aminolevulinic acid (ALA). This stimulation was not dependent on ALA per se, but on the ability of this precursor to effect heme competency. The addition of ALA stimulated SES activity of yeast on either fermentative or respiratory carbon sources. The elevation of SES activity was independent of intracellular free sterol, unsaturated fatty acid, or methionine levels. SES activity increases as the cells enter stationary phase, and this increase is enhanced by heme competency. SES was directly inhibited by the hypocholesterolemic drug lovastatin (mevinolin). The inhibition of SES activity by lovastatin was enhanced in heme-competent cells.     

The role of cellular folates in the enhancement of activity of the thymidylate synthase inhibitor 10-propargyl-5,8-dideazafolate against hepatoma cells in vitro by inhibitors of dihydrofolate reductase

Exposure of growing cultures of hepatoma cells in vitro to the lipid-soluble dihydrofolate reductase inhibitors metoprine (36 nM) or trimetrexate (2 nM) at subtoxic concentrations causes little change in cell growth rate, colony forming ability, cell cycle distribution, and de novo purine and thymidylate biosynthesis. The reductase inhibitors augment the cytotoxic activity of the thymidylate synthase inhibitor, 10-propargyl-5,8-dideazafolate by nearly 10-fold under optimal conditions. Treatment of the hepatoma cells with the reductase inhibitors for 72 h during growth caused approximately a 75% reduction in total cellular folates and 5,10-methylenetetrahydrofolate (primarily as polyglutamates) the substrate for thymidylate synthase. The reductase inhibitors also cause a doubling in the accumulation of 10-propargyl-5,8-dideazafolate polyglutamates. The combined antifolate treatment (metoprine or trimetrexate plus 10-propargyl-5,8-dideazafolate) expands the dUMP pool by 30-fold, which is more than the sum of either of the antifolates alone. Consequently, it is postulated that the enhanced activity of 10-propargyl-5,8-dideazafolate in combination with low concentrations of dihydrofolate reductase inhibitors is due to an increase in the ratio of inhibitor to substrate for thymidylate synthase of nearly 10-fold and an extensive enhancement of the dUMP pool. These conditions predispose the target enzyme and the cells to more effective metabolic blockade by 10-propargyl-5,8-dideazafolate which is presumably caused by the formation of an inhibited 10-propargyl-5,8-dideazafolate[polyglutamate]-thymidylate synthase-dUMP ternary complex.     

In situ autoradiographic detection of folylpolyglutamate synthetase activity

The enzyme folylpolyglutamate synthetase (FPGS) catalyzes the conversion of folate (pteroylmonoglutamate) to the polyglutamate forms (pteroylpolyglutamates) that are required for folate retention by mammalian cells. A rapid in situ autoradiographic assay for FPGS was developed which is based on the folate cofactor requirement of thymidylate synthase. Chinese hamster AUX B1 mutant cells lack FPGS activity and are unable to accumulate folate. As a result, the conversion of [6-3H]deoxyuridine to thymidine via the thymidylate synthase reaction is impaired in AUX B1 cells and no detectable label is incorporated into DNA. In contrast, FPGS in wild-type Chinese hamster CHO cells causes folate retention and enables the incorporation of [6-3H]deoxyuridine into DNA. Incorporation may be detected by autoradiography of monolayer cultures or of colonies replica plated onto polyester discs. Introduction of Escherichia coli FPGS into AUX B1 cells restores the activity of the thymidylate synthase pathway and demonstrates that the E. coli FPGS enzyme can provide pteroylpolyglutamates which function in mammalian cells.     

Two chitin synthases in Saccharomyces cerevisiae

Disruption of the yeast CHS1 gene, which encodes trypsin-activable chitin synthase I, yielded strains that apparently lacked chitin synthase activity in vitro, yet contained normal levels of chitin (Bulawa, C. E., Slater, M., Cabib, E., Au-Young, J., Sburlati, A., Adair, W. L., and Robbins, P. W. (1986) Cell 46, 213-225). It is shown here that disrupted (chs1 :: URA3) strains have a particulate chitin synthetic activity, chitin synthase II, and that wild type strains, in addition to chitin synthase I, have this second activity. Chitin synthase II is measured in wild type strains without preincubation with trypsin, the condition under which highest chitin synthase II activities are obtained in extracts from the chs1 :: URA3 strain. Chitin synthase II, like chitin synthase I, uses UDP-GlcNAc as substrate and synthesizes alkali-insoluble chitin (with a chain length of about 170 residues). The enzymes are equally sensitive to the competitive inhibitor Polyoxin D. The two chitin synthases are distinct in their pH and temperature optima, and in their responses to trypsin, digitonin, N-acetyl-D-glucosamine, and Co2+. In contrast to the report by Sburlati and Cabib (Sburlati, A., and Cabib, E. (1986) Fed. Proc. 45, 1909), chitin synthase II activity in vitro is usually lowered on treatment with trypsin, indicating that chitin synthase II is not activated by proteolysis. Chitin synthase II shows highest specific activities in extracts from logarithmically growing cultures, whereas chitin synthase I, whether from growing or stationary phase cultures, is only measurable after trypsin treatment, and levels of the zymogen do not change. Chitin synthase I is not required for alpha-mating pheromone-induced chitin synthesis in MATa cells, yet levels of chitin synthase I zymogen double in alpha factor-treated cultures. Specific chitin synthase II activities do not change in pheromone-treated cultures. It is proposed that of yeast's two chitin synthases, chitin synthase II is responsible for chitin synthesis in vivo, whereas nonessential chitin synthase I, detectable in vitro only after trypsin treatment, may not normally be active in vivo.     

Changes in Membrane Fatty Acid Composition and Δ12-Desaturase Activity during Growth of Acanthamoeba castellanii in Batch Culture

ABSTRACT. Growth of Acanthamoeba castellanii in batch culture at 30° C was associated with marked changes in cellular fatty acid composition. The largest change occurred in the linoleate to oleate ratio, which was maximal in early- to mid-exponential phase cultures but decreased approximately 10-fold as cells approached stationary phase. The higher degree of lipid unsaturation in young cultures was accentuated by a greater proportion of 20-carbon polyunsaturated fatty acids than in stationary phase cultures. The unsaturation index (average number of double bonds per fatty acid) was maximal in mid-exponential phase cultures after 24 hours growth. Incorporation of [1-¹⁴C]acetate into polyunsaturated fatty acids in short-term (2 hour) experiments was high in 12 and 24 hour old cultures, where linoleate and eicosadienoate accounted for up to 26% of total labelled fatty acids. Incorporation of [1-¹⁴CJacetate into these fatty acids was negligible in stationary phase cultures. These results were correlated with changes in the specific activity of the Δ12-desaturase. Δ12-Desaturase activity was greatest in microsomal membranes isolated from early- to mid-exponential phase cells, but declined by approximately 50% as cultures progressed towards stationary phase. Membrane fractionation studies revealed that although some differences in fatty acid composition between plasma-membrane, mitochondrial (enriched), and microsomal membrane fractions were evident, the large changes in lipid unsaturation in whole cells of A. castellanii could not be accounted for by differential development of particular subcellular membranes.     

The yeast GLC7 gene required for glycogen accumulation encodes a type 1 protein phosphatase.

The glc7 mutant of the yeast Saccharomyces cerevisiae does not accumulate glycogen due to a defect in glycogen synthase activation (Peng, Z., Trumbly, R. J., and Reimann, E.M. (1990) J. Biol. Chem. 265, 13871-13877) whereas wild-type strains accumulate glycogen as the cell cultures approach stationary phase. We isolated the GLC7 gene by complementation of the defect in glycogen accumulation and found that the GLC7 gene is the same as the DIS2S1 gene (Ohkura, H., Kinoshita, N., Miyatani, S., Toda, T., and Yanagida, M. (1989) Cell 57, 997-1007). The protein product predicted by the GLC7 DNA sequence has a sequence that is 81% identical with rabbit protein phosphatase 1 catalytic subunit. Protein phosphatase 1 activity was greatly diminished in extracts from glc7 mutant cells. Two forms of protein phosphatase 1 were identified after chromatography of extracts on DEAE-cellulose. Both forms were diminished in the glc7 mutant and were partly restored by transformation with a plasmid carrying the GLC7 gene. Southern blots indicate the presence of a single copy of GLC7 in S. cerevisiae, and gene disruption experiments showed that the GLC7 gene is essential for cell viability. The GLC7 mRNA was identified as a 1.4-kilobase RNA that increases 4-fold at the end of exponential growth in wild-type cells, suggesting that activation of glycogen synthase is mediated by increased expression of protein phosphatase 1 as cells reach stationary phase.     

Analysis of growth phase regulated KatA and CatE and their physiological roles in determining hydrogen peroxide resistance in Agrobacterium tumefaciens

Agrobacterium tumefaciens possesses two catalases, a bifunctional catalase-peroxidase, KatA and a homologue of a growth phase regulated monofunctional catalase, CatE. In stationary phase cultures and in cultures entering stationary phase, total catalase activity increased 2-fold while peroxidase activity declined. katA and catE were found to be independently regulated in a growth phase dependent manner. KatA levels were highest during exponential phase and declined as cells entered stationary phase, while CatE was detectable at early exponential phase and increased during stationary phase. Only small increases in H2O2 resistance levels were detected as cells entering stationary phase. The katA mutant was more sensitive to H2O2 than the parental strain during both exponential and stationary phase. Inactivation of catE alone did not significantly change the level of H2O2 resistance. However, the katA catE double mutant was more sensitive to H2O2 during both exponential and stationary phase than either of the single catalase mutants. The data indicated that KatA plays the primary role and CatE acts synergistically in protecting A. tumefaciens from H2O2 toxicity during all phases of growth. Catalase-peroxidase activity (KatA) was required for full H2O2 resistance. The expression patterns of the two catalases in A. tumefaciens reflect their physiological roles in the protection against H2O2 toxicity, which are different from other bacteria.