Temperature control of growth and productivity in mutant Chinese hamster ovary cells synthesizing a recombinant protein

The use of a temperature switch to control the growth and productivity of temperature-sensitive (ts) mutants was investigated to extend the productive life span of recombinant Chinese hamster ovary (CHO) cells in batch culture. Bromodeoxyuridine was used at 39 degrees C to select mutagenized CHO-K1 cells, which resulted in the isolation of 31 temperature-sensitive mutants that were growth inhibited at 39 degrees C. Two of these mutants were successfully transfected with the gene for tissue inhibitor of metalloproteinases (TIMP) using glutamine synthetase amplification, and a permanent recombinant cell line established (5G1-B1) that maintains the ts phenotype.Continuous exposure to the nonpermissive temperature (npt) of 39 degrees C led to a rapid decline in cell viability. However, a temperature regime using alternating incubations at 34 degrees C and 39 degrees C arrested the 5G1-B1 cells while retaining a high cell viability for up to 170 h in culture. The specific production rate of the growth-arrested cells was 3-4 times that of control cultures maintained at a constant 34 degrees C over the crucial 72-130-h period of culture, which resulted in a 35% increase in the maximum product yield. Glucose uptake and lactate production both decreased in arrested cells. Flow cytometric analysis indicated that 5G1-B1 cells arrested in the G(1) or G(0) phase of the cell cycle, and no major structural damage was caused to these cells by the alternating temperature regime.These results demonstrate that growth-arrested ts CHO cells have increased productivity compared to growing cultures and maintain viability for longer periods. The system offers the prospect of enhancing the productivity of recombinant mammalian cells grown in simple batch fermentors. (c) 1993 John Wiley & Sons, Inc.     

A Chinese hamster cell cycle mutant arrested at G2 phase has a temperature-sensitive ubiquitin-activating enzyme, E1

The effect of restrictive temperature on ubiquitin conjugation activity has been studied in cells of ts20, a temperature-sensitive cell cycle mutant of the Chinese hamster cell line E36. Ts20 is arrested in early G2 phase at nonpermissive temperature. Immunoblotting with antibodies to ubiquitin conjugates shows that conjugates disappear rapidly at restrictive temperatures in ts20 mutant but not in wild type E36 cells. The incorporation of 125I-ubiquitin into permeabilized ts20 cells is temperature-sensitive. Addition of extracts of another G2 phase mutant, FM3A ts85, with a temperature-sensitive ubiquitin activation enzyme (E1), to permeabilized ts20 cells at restrictive temperatures fails to complement their ubiquitin ligation activity. This indicates that the lesions in the two mutants are similar. Purified E1 from reticulocytes restores the conjugation activity of heat-inactivated permeabilized ts20 cells. Ubiquitin conjugation activity of cell-free extracts of ts20 cells was temperature-sensitive and could be restored by adding purified reticulocyte E1. Purified reticulocyte E2 or E3, on the other hand, did not restore the ubiquitin conjugation activity of heat-treated ts20 extracts. These results are consistent with the conclusion that ts20 has temperature-sensitive ubiquitin-activating enzyme (E1). The fact that two E1 mutants (ts20 and ts85) derived from different cell lines are arrested at the S/G2 boundary at restrictive temperatures strongly indicates that ubiquitin ligation is necessary for passage through this part of the cell cycle. The temperature thresholds of heat shock protein synthesis of ts20 and wild type E36 cells were identical. The implications of these findings with respect to a suggested role of ubiquitin in coupling between protein denaturation and the heat shock response are discussed.     

Isolation of temperature-sensitive cell cycle mutants from mouse FM3A cells. Characterization of mutants with special reference to DNA replication

A large number of mutants that are temperature sensitive (ts) for growth have been isolated from mouse mammary carcinoma FM3A cells by an improved selection method consisting of cell synchronization and short exposures to restrictive temperature. The improved method increased the efficiency of isolating DNA ts mutants, which showed a rapid decrease in DNA-synthesizing ability after temperature shift-up. Sixteen mutants isolated by this and other methods were selected for this study. Flow microfluorometric analysis of these mutants cultured at a nonpermissive temperature (39 degrees C) for 16 h indicated that five clones were arrested in the G1 to S phase of the cell cycle, six clones were in the S to G2 phase, and two clones were arrested in the G2 phase. The remaining three clones exhibited 8C DNA content after incubation at 39 degrees C for 28 h, indicating defects in mitosis or cytokinesis. These mutants were classified into 11 complementation groups. All the mutants except for those arrested in the G2 phase and those exhibiting defects in mitosis or cytokinesis showed a rapid decrease in DNA synthesis after temperature shift-up without a decrease in RNA and protein synthesis. The polyomavirus DNA cell-free replication system, which consists of polyomavirus large tumor antigen and mouse cell extracts, was used for further characterization of these DNA ts mutants. Among these ts mutants, only the tsFT20 strain, which contains heat-labile DNA polymerase alpha, was unable to support the polyomavirus DNA replication. Analysis by DNA fiber autoradiography revealed that DNA chain elongation rates of these DNA ts mutants were not changed and that the initiation of DNA replication at the origin of replicons was impaired in the mutant cells.     

Analysis of a Chinese hamster temperature-sensitive cell cycle mutant arrested in early S phase

E36 ts24 is a temperature-sensitive cell cycle mutant which has been derived from the Chinese hamster lung cell line E36. This mutant is arrested in phase S when incubated at the restrictive temperature (40.3 degrees C) for growth. At this temperature, proliferation of the mutant cells ceases after 10 h. About 2 h earlier, DNA synthesis is arrested. These kinetic studies indicate that the execution point of the mutant cells is in early S phase well beyond the G1/S boundary. The pattern of replication bands in E36 ts24 cell grown for 9 h at 40.3 degrees C strengthen the kinetic studies and map the execution point to early S phase. The exact point of arrest of the mutant cells in phase S was mapped in early S phase near the execution point. At the point of arrest the cells continue to synthesize DNA at at a high rate but practically all of the newly synthesized DNA is degraded. This high rate of DNA degradation is limited to nascent DNA at the point of arrest. In the presence of 5-bromodeoxyuridine (5-BudR), the last E36 ts24 cells which reach mitosis at the restrictive temperature for growth show asymmetric replication bands which illustrate DNA degradation and resynthesis occurring in these cells at 40.3 degrees C.     

Complementation between two temperature-sensitive mammalian cell mutants, each defective in the G1 phase of the cell cycle

Two mammalian temperature-sensitive (ts) G1 cell cycle mutants of different species origin (Syrian hamster and mouse) have been tested for complementation using somatic cell hybrid analysis. All hamster-mouse hybrid clones tested were found to exhibit normal growth properties at the restrictive temperature, while neither mutant alone was capable of normal growth at this temperature. The two mutant lines therefore complement for growth in a somatic cell hybrid and most likely represent ts lesions in different cellular functions specific to the G1 phase of the cell cycle.     

Control of cell division in Saccharomyces cerevisiae mutants defective in adenylate cyclase and cAMP-dependent protein kinase

Examination of the proportion of unbudded cells, terminal nuclear phenotype and DNA content of nuclei indicated that cyr1 mutants of yeast defective in adenylate cyclase activity were arrested at the G1 phase of the cell cycle. The step of G1 arrest due to the cyr1 mutation preceded the step sensitive to the mating pheromone. The temperature-sensitive cyr1 cells did not continue growth, nor retain the capacity to conjugate at a restrictive temperature. The phenotypes of the cyr1 mutant mimicked those of nutritionally limited cells. The G1 arrest caused by the cyr1 mutation was overcome by the presence of a suppressor mutation, bcy1, that resulted in deficiency of a regulatory subunit of cAMP-dependent protein kinase and production of high level of cAMP-independent protein kinase. The bcy1 mutation suppressed G1 arrest caused by nutritional limitation, and continued bud emergence for multiple cycles without further nuclear division. The data suggest that cAMP works as a positive effector at the start of a yeast cell cycle via activation of cAMP-dependent protein kinase.     

HYS2, an essential gene required for DNA replication in Saccharomyces cerevisiae.

To investigate cell cycle regulation at the S or G2 phase in Saccharomyces cerevisiae, we have isolated mutants displaying supersensitivity to hydroxyurea (HU), a chemical that inhibits DNA replication. Such mutants, which we have named hydroxyurea sensitive (hys), defined four linkage groups and we characterized the hys2 mutation in this study. The hys2-1 mutant displays temperature sensitive growth and a constellation of phenotypes indicating defective DNA metabolism. At the restrictive temperature, hys2-1 cells arrest as large budded cells with a single nucleus at the neck of the bud and a short spindle. The hys2-1 mutant exhibits increased rates of chromosome loss and recombination. Additionally, hys2-1 appears to accumulate incompletely replicated DNA that can be detected by a pulse field electrophoresis assay. Finally, deletion of RAD9 in a hys2-1 strain decreases the percentage of arrested cells, suggesting that an intact RAD9-checkpoint is required for the cell cycle arrest in hys2-1 cells. HYS2 encodes a 55 kDa protein that is essential for viability at all temperatures. Taken together, these data suggest that Hys2 plays a role in DNA replication.     

Yeast cell death caused by mutation of the OST2 gene encoding the epsilon-subunit of Saccharomyces cerevisiae oligosaccharyltransferase

An essential epsilon-subunit of oligosaccharyltransferase Ost2 is a yeast homolog of mammalian highly conserved DAD1 (defender against apoptotic death). In hamster cells, the Gly38Arg mutation in DAD1 causes apoptosis at restrictive temperatures due to a defect in N-linked glycosylation. To analyze the function of Ost2 in yeast cell death, we constructed Saccharomyces cerevisiae strains expressing Gly58Arg (corresponding to the Gly38Arg mutation in hamster DAD1), Gly86Arg, and Glu113Val mutant Ost2. At elevated temperatures, ost2 mutants arrested growth by decreasing cell viability. Phosphatidylserine exposure, a phenotypic marker of apoptosis in mammalian cells, was found in ost2 mutant cells at 37 degrees C, although DNA fragmentation was not clearly detected. A high concentration of sorbitol compensates for the temperature sensitivity of the ost2 mutant. These results suggest that apoptosis-like cell death in ost2 mutants is caused by the secondary effect of overall reduced protein N-linked glycosylation.     

A novel protein kinase gene ssp1+ is required for alteration of growth polarity and actin localization in fission yeast.

Temperature-sensitive suppressor mutants were isolated from two fission yeast mutants defective in cell shape control: ppe1, encoding a type 2A-like protein phosphatase, and sts5, one of 11 staurosporine-supersensitive mutants. Complementation tests showed that suppression was due to two chromosomal loci, ssp1 and ssp2. Cells of the ssp1 mutant grown at the restrictive temperature arrested uniformly with an elongated cell body and a 2C content of DNA. Interestingly, these mutant cells grew only in a monopolar manner. At a specific point in the G2 phase of the cell cycle, wild-type cells exhibit a drastic alteration in growth polarity, from mono- to bipolar. This change coincides with the distribution of cortical actin from one end of the cell to both ends. In the ssp1 mutant cells, cortical actin was localized only at one end, suggesting that the mutant fails to change growth polarity. Nucleotide sequence determination showed that ssp1+ encodes a novel protein kinase. Ectopic overexpression of ssp1+ resulted in an altered cell morphology and cortical actin was randomly dispersed within the cells. Immunocytological analysis revealed that the protein was primarily localized in the cytoplasm and that half of the protein existed in an insoluble fraction. These results show that the dynamics of actin-based growth polarity during the cell cycle are regulated, at least in part, by a novel set of protein kinases and phosphatases.     

Cell Cycle Arrest of Cdc Mutants and Specificity of the Rad9 Checkpoint

In eucaryotes a cell cycle control called a checkpoint ensures that mitosis occurs only after chromosomes are completely replicated and any damage is repaired. The function of this checkpoint in budding yeast requires the RAD9 gene. Here we examine the role of the RAD9 gene in the arrest of the 12 cell division cycle (cdc) mutants, temperature-sensitive lethal mutants that arrest in specific phases of the cell cycle at a restrictive temperature. We found that in four cdc mutants the cdc rad9 cells failed to arrest after a shift to the restrictive temperature, rather they continued cell division and died rapidly, whereas the cdc RAD cells arrested and remained viable. The cell cycle and genetic phenotypes of the 12 cdc RAD mutants indicate the function of the RAD9 checkpoint is phase-specific and signal-specific. First, the four cdc RAD mutants that required RAD9 each arrested in the late S/G(2) phase after a shift to the restrictive temperature when DNA replication was complete or nearly complete, and second, each leaves DNA lesions when the CDC gene product is limiting for cell division. Three of the four CDC genes are known to encode DNA replication enzymes. We found that the RAD17 gene is also essential for the function of the RAD9 checkpoint because it is required for phase-specific arrest of the same four cdc mutants. We also show that both X- or UV-irradiated cells require the RAD9 and RAD17 genes for delay in the G(2) phase. Together, these results indicate that the RAD9 checkpoint is apparently activated only by DNA lesions and arrests cell division only in the late S/G(2) phase.     

Cell cycle analysis of p53-induced cell death in murine erythroleukemia cells.

A temperature-sensitive mutant of murine p53 (p53Val-135) was transfected by electroporation into murine erythroleukemia cells (DP16-1) lacking endogenous expression of p53. While the transfected cells grew normally in the presence of mutant p53 (37.5 degrees C), wild-type p53 (32.5 degrees C) was associated with a rapid loss of cell viability. Genomic DNA extracted at 32.5 degrees C was seen to be fragmented into a characteristic ladder consistent with cell death due to apoptosis. Following synchronization by density arrest, transfected cells released into G1 at 32.5 degrees C were found to lose viability more rapidly than did randomly growing cultures. Following release into G1, cells became irreversibly committed to cell death after 4 h at 32.5 degrees C. Commitment to cell death correlated with the first appearance of fragmented DNA. Synchronized cells allowed to pass out of G1 prior to being placed at 32.5 degrees C continued to cycle until subsequently arrested in G1; loss of viability occurred following G1 arrest. In contrast to cells in G1, cells cultured at 32.5 degrees C for prolonged periods during S phase and G2/M, and then returned to 37.5 degrees C, did not become committed to cell death. G1 arrest at 37.5 degrees C, utilizing either mimosine or isoleucine deprivation, does not lead to rapid cell death. Upon transfer to 32.5 degrees C, these G1 synchronized cell populations quickly lost viability. Cells that were kept density arrested at 32.5 degrees C (G0) lost viability at a much slower rate than did cells released into G1. Taken together, these results indicate that wild-type p53 induces cell death in murine erythroleukemia cells and that this effect occurs predominantly in the G1 phase of actively cycling cells.     

The Glc7 type 1 protein phosphatase of Saccharomyces cerevisiae is required for cell cycle progression in G2/M.

We isolated a mutant carrying a conditional mutation in the GLC7 gene, encoding the catalytic subunit of a type 1 protein phosphatase, by selection of suppressors that restored the growth defect of cdc24 mutants at high temperature and simultaneously conferred cold-sensitive growth. This cold sensitivity for growth is caused by a single mutation (glc7Y-170) at position 170 of the Glc7 protein, resulting in replacement of cysteine with tyrosine. Genetic analysis suggested that the glc7Y-170 allele is associated with a recessive negative phenotype, reducing the activity of Glc7 in the cell. The glc7Y-170 mutant missegregated chromosome III at the permissive temperature, arrested growth as large-budded cells at the restrictive temperature, exhibited a significant increase in the number of nuclei at or in the neck, and had a short spindle. Furthermore, the glc7Y-170 mutant exhibited a high level of CDC28-dependent protein kinase activity when incubated at the restrictive temperature. These findings suggest that the glc7Y-170 mutation is defective in the G2/M phase of the cell cycle. Thus, type 1 protein phosphatase in Saccharomyces cerevisiae is essential for the G2/M transition.     

Mad1p, a phosphoprotein component of the spindle assembly checkpoint in budding yeast

The spindle assembly checkpoint prevents cells from initiating anaphase until the spindle has been fully assembled. We previously isolated mitotic arrest deficient (mad) mutants that inactivate this checkpoint and thus increase the sensitivity of cells to benomyl, a drug that interferes with mitotic spindle assembly by depolymerizing microtubules. We have cloned the MAD1 gene and show that when it is disrupted yeast cells have the same phenotype as the previously isolated mad1 mutants: they fail to delay the metaphase to anaphase transition in response to microtubule depolymerization. MAD1 is predicted to encode a 90-kD coiled-coil protein. Anti-Mad1p antibodies give a novel punctate nuclear staining pattern and cell fractionation reveals that the bulk of Mad1p is soluble. Mad1p becomes hyperphosphorylated when wild-type cells are arrested in mitosis by benomyl treatment, or by placing a cold sensitive tubulin mutant at the restrictive temperature. This modification does not occur in G1- arrested cells treated with benomyl or in cells arrested in mitosis by defects in the mitotic cyclin proteolysis machinery, suggesting that Mad1p hyperphosphorylation is a step in the activation of the spindle assembly checkpoint. Analysis of Mad1p phosphorylation in other spindle assembly checkpoint mutants reveals that this response to microtubule- disrupting agents is defective in some (mad2, bub1, and bub3) but not all (mad3, bub2) mutant strains. We discuss the possible functions of Mad1p at this cell cycle checkpoint.     

Abortive transformation of temperature-sensitive mutants of rat 3Y1 cells by simian virus 40: effect of cellular arrest states on entry into S phase and cellular proliferation

Four temperature-sensitive (ts) mutants of rat 3Y1 fibroblasts, representing independent complementation groups, cease to proliferate predominantly with a 2n DNA content, at the restrictive temperature (39.8 degrees C) (temperature arrest) or at the permissive temperature (33.8 degrees C) at a confluent cell density (density arrest) (Ohno et al., 1984). We studied the temperature- or the density-arrested cells of these mutants infected with simian virus 40 (SV40) or its mutants affecting large T or small t antigen with respect to kinetics at 39.8 degrees C of entry into S phase and cellular proliferation. Three mutants, 3Y1tsD123, 3Y1tsF121 and 3Y1tsG125, expressed T antigen and entered S phase at 39.8 degrees C from both the arrested states after infection with either wild-type, tsA mutants, or a .54/.59 deletion mutant of SV40, whereas in the density-arrested 3Y1tsH203, expression of T antigen and entry into S phase were inefficient and ts. Following the WT-SV40 induced entry into S phase, the temperature-arrested 3Y1tsD123 detached from the substratum with no detectable increase in cell number, whereas the density-arrested ones completed a round of the cell cycle and then detached. 3Y1tsF121 and 3Y1tsG125 in the both arrested states proliferated through more than one generation. 3Y1tsF121 and 3Y1tsG125 in the density-arrested state infected with tsA mutants once proliferated and then ceased to increase in number as the percentage of T-antigen positive population decreased. These results suggest that wild-type and tsA-mutated large T antigens are able to overcome the cellular ts blocks of entry into S phase in the 3 ts mutants of 3Y1 cells in both the arrested states, and that small t antigen is not required to overcome the blocks. It is also suggested that cellular behaviors subsequent to S phase (viability, mitosis, and proliferation in the following generations) depend on cellular arrest states, on traits of cellular ts defects, and on the duration of large T antigen expression.     

An Enrichment for Temperature-Sensitive Mutants in TETRAHYMENA THERMOPHILA

The parameters for the killing of Tetrahymena by 5-bromodeoxyuridine(BUdR) and near-ultraviolet light have been determined. Significant preferential killing by UV of cells that have incorporated BUdR was obtained when the cells were irradiated in a nonnutrient buffer. UV alone was found to be toxic to cells irradiated in growth medium. Mutants defective in division at a restrictive temperature were isolated from mutagenized cultures that had been treated with BUdR and UV and from mutagenized cultures that had no such treatment. Results indicate that the number of temperature sensitive (ts) growth mutants can be increase five to six times using the BUdR/UV treatment. Data are presented that indicate differences in the frequency of occurrence of various types of ts mutants, with and without enrichment. A mutant that immediately stopped macromolecular synthesis and cell division upon being placed at the restrictive temperature was more resistant to BUdR/UV treatment than wild type by 1000-fold. Using the above techniques, BUdR-resistant mutants altered in the phosphorylation of thymidine have been isolated.     

Serum-mediated regulation of amino acid transport in cultured chick embryo fibroblasts

Three different temperature sensitive mutants derived from the Syrian hamster cell line BHK 21 were found to have greatly reduced DNA synthesis at the non-permissive temperature. These mutants are distinct by complementation analysis and behave at the non-permissive temperature as cell cycle traverse defective mutants. Microfluorometric analysis of mutant populations arrested at the non-permissive temperature shows an accumulation of cells with G1 DNA content. Mutants ts 13 and ts HJ4 synchronized in G1 by serum or isoleucine deprivation and shifted to the non-permissive temperature at the time of release do not enter the S phase, while in the case of mutant ts 11 preincubation at the non-permissive temperature before release is required to completely prevent its entry into S. Ts 13 and ts 11 are able to traverse the S phase at the non-permissive temperature when synchronized at the boundary G1/S; in this case, preincubation of ts 11 at the non-permissive temperature before release does not affect the ability of these cells to perform DNA synthesis. On the other hand, ts HJ4 appears to traverse S only partially when tested under similar conditions. Temperature shift experiments of mutant populations at different times after isoleucine synchronization suggest that ts 13 and ts 11 are blocked at the non-permissive temperature in early G1, whereas ts HJ4 is probably affected near the initiation of DNA synthesis, or in some early S function.     

Regulation of the proliferating cell nuclear antigen cyclin and thymidine kinase mRNA levels by growth factors

The enzymes of the DNA synthesizing machinery constitute a group of gene products that are generally expressed co-ordinately at the G1/S boundary of the cell cycle. We have investigated how growth factors regulate the steady-state mRNA levels of two of these genes, the PCNA (proliferating cell nuclear antigen)/cyclin and the thymidine kinase genes. To detect the PCNA/cyclin mRNA, we isolated a cDNA clone from a human library. Two different cell lines were used for these studies: BALB/c3T3 cells, which are exquisitely sensitive to growth factors, and ts13 cells, a temperature-sensitive (ts) mutant of the cell cycle, which arrests in G1 at the restrictive temperature. The steady-state levels of the RNAs for these two genes under different growth conditions were also compared with the levels of histone H3 RNA which are good indicators of the fraction of cells in S phase. Both PCNA/cyclin and thymidine kinase genes share two fundamental characteristics, i.e. they are not inducible in a G1-specific ts mutant of the cell cycle at the restrictive temperature and their expression is inhibited by cycloheximide, indicating that unlike early growth-regulated genes, they require the previous expression of other growth-regulated genes. However, the two genes also show differences, the most notable being that PCNA/cyclin is inducible by epidermal growth factor alone, while thymidine kinase is not.     

MIF2 is required for mitotic spindle integrity during anaphase spindle elongation in Saccharomyces cerevisiae

The function of the essential MIF2 gene in the Saccharomyces cerevisiae cell cycle was examined by overepressing or creating a deficit of MIF2 gene product. When MIF2 was overexpressed, chromosomes missegregated during mitosis and cells accumulated in the G2 and M phases of the cell cycle. Temperature sensitive mutants isolated by in vitro mutagenesis delayed cell cycle progression when grown at the restrictive temperature, accumulated as large budded cells that had completed DNA replication but not chromosome segregation, and lost viability as they passed through mitosis. Mutant cells also showed increased levels of mitotic chromosome loss, supersensitivity to the microtubule destabilizing drug MBC, and morphologically aberrant spindles. mif2 mutant spindles arrested development immediately before anaphase spindle elongation, and then frequently broke apart into two disconnected short half spindles with misoriented spindle pole bodies. These findings indicate that MIF2 is required for structural integrity of the spindle during anaphase spindle elongation. The deduced Mif2 protein sequence shared no extensive homologies with previously identified proteins but did contain a short region of homology to a motif involved in binding AT rich DNA by the Drosophila D1 and mammalian HMGI chromosomal proteins.     

Yeast Cell Death Caused by Mutation of the OST2 Gene Encoding the ε-Subunit of Saccharomyces cerevisiae Oligosaccharyltransferase

An essential ε-subunit of oligosaccharyltransferase Ost2 is a yeast homolog of mammalian highly conserved DAD1 (defender against apoptotic death). In hamster cells, the Gly38Arg mutation in DAD1 causes apoptosis at restrictive temperatures due to a defect in N-linked glycosylation. To analyze the function of Ost2 in yeast cell death, we constructed Saccharomyces cerevisiae strains expressing Gly58Arg (corresponding to the Gly38Arg mutation in hamster DAD1), Gly86Arg, and Glu113Val mutant Ost2. At elevated temperatures, ost2 mutants arrested growth by decreasing cell viability. Phosphatidylserine exposure, a phenotypic marker of apoptosis in mammalian cells, was found in ost2 mutant cells at 37 °C, although DNA fragmentation was not clearly detected. A high concentration of sorbitol compensates for the temperature sensitivity of the ost2 mutant. These results suggest that apoptosis-like cell death in ost2 mutants is caused by the secondary effect of overall reduced protein N-linked glycosylation.     

Effect of cell cycle position on thermotolerance in Saccharomyces cerevisiae.

We showed that the heat killing curve for exponentially growing Saccharomyces cerevisiae was biphasic. This suggests two populations of cells with different thermal killing characteristics. When exponentially growing cells separated into cell cycle-specific fractions via centrifugal elutriation were heat shocked, the fractions enriched in small unbudded cells showed greater resistance to heat killing than did other cell cycle fractions. Cells arrested as unbudded cells fell into two groups on the basis of thermotolerance. Sulfur-starved cells and the temperature-sensitive mutants cdc25, cdc33, and cdc35 arrested as unbudded cells were in a thermotolerant state. Alpha-factor-treated cells arrested in a thermosensitive state, as did the temperature-sensitive mutant cdc36 when grown at the restrictive temperature. cdc7, which arrested at the G1-S boundary, arrested in a thermosensitive state. Our results suggest that there is a subpopulation of unbudded cells in exponentially growing cultures that is in G0 and not in G1 and that some but not all methods which cause arrest as unbudded cells lead to arrest in G0 as opposed to G1. It has been shown previously that yeast cells acquire thermotolerance to a subsequent challenge at an otherwise lethal temperature during a preincubation at 36 degrees C. We showed that this acquisition of thermotolerance was corrected temporally with a transient increase in the percentage of unbudded cells during the preincubation at 36 degrees C. The results suggest a relationship between the heat shock phenomenon and the cell cycle in S. cerevisiae and relate thermotolerance to transient as well as to more prolonged residence in the G0 state.