A temperature-sensitive mutation affecting S-phase progression can lead to accumulation of cells with a G2 DNA content

Cultures of ts BN75, a temperature-sensitive mutant of BHK 21 cells, show a gradual biphasic drop in [3H]thymidine incorporation together with an accumulation of cells having a G2 DNA content when incubated at 39.5 degrees. However, when higher (41 degrees - 42 degrees) nonpermissive temperatures were used, the major block was in S-phase DNA synthesis. The cultures of ts BN75 shifted to 42 degrees at the start of the S phase, cell-cycle progress was arrested in the middle of S, while under these conditions wild-type BHK cells underwent at least one cycle of DNA synthesis. When ts BN75 cells growth-arrested at high temperature with a G2 DNA content were shifted to the permissive temperature (33.5 degrees C), the restart of DNA synthesis preceded the appearance of mitotic cells. These data suggest that the ts defect of ts BN75 cells might affect primarily the S phase of the cycle rather than the G2 phase.     

A temperature-sensitive cell cycle mutant of the BHK cell line

A temperature-sensitive growth mutant derived from the BHK 21 cell Line, ts AF8, was found to have greatly reduced DNA synthesis at the nonpermissive temperature. This reduction is mainly due to a decrease in the frequency of cells synthesizing DNA. Upon shift up, ts AF8 becomes blocked in the G1 phase of the cell cycle. The cells acquire elevated cAMP levels and a unimodal distribution of DNA content, equivalent to that of G1 cells at the permissive temperature, Ts AF8 cells blocked at the G1/S boundary with hydroxyurea will enter S when shifted to the nonpermissive temperature. On the other hand, ts AF8 cells arrested m G1 by serum deprivation and shifted to the nonpermissive temperature at the moment of serum addition do not enter S, while those synchronized by isoleucine deprivation and shifted at the time of isoleucine addition will enter S. These data suggest that the cycle arrest point of the ts AF8 mutation is located in G1 between the blocks induced by serum starvation and isoleucine deprivation. The reduction in DNA synthesis caused by the ts AF8 mutation is not reversed by infection or transformation with Polyoma virus. Mitochondrial DNA continues to be synthesized at wild-type levels at the nonpermissive temperature.     

Characterization of ts13 cells a temperature-sensitive mutant of the G1 phase of the cell cycle

ts 13 cells are a temperature-sensitive (ts) mutant of BHK cells that are known to arrest in G1 when shifted to the nonpermissive temperature. We have determined the entry into S of ts13 cells in five different growth conditions, namely: 1) quiescent, sparse cultures stimulated to proliferate by serum. 2) Quiescent, dense cultures stimulated by serum. 3) Quiescent, sparse cultures stimulated by trypsinization and replating. 4) Quiescent, dense cultures stimulated by trypsinization and replating. 5) Mitotic cells collected by mitotic detachment. For each different growth condition we have also determined the execution point of the mutant function, i.e. the time at which a shift-up to the nonpermissive temperature no longer prevents the entry of cells into S. The median time of entry into S and the execution point varied in different growth conditions, but the distance between the median execution point and the median time of entry into S was remarkably constant, i.e. 3.2 hr. In addition we have fused ts 13 cells cells with chick erythrocytes and studied the ability of ts13 cells in heterokaryon formation to induce DNA synthesis in chick nuclei. Although ts13 cells can induce DNA synthesis in chick nuclei at the permissive temperature, they fail to do so when fused and stimulated at the nonpermissive temperature of 39.5 degrees C.     

Replication of a mammalian genome: the role of de novo protein biosynthesis during S phase

ts14 is a temperature-sensitive Chinese hamster lung cell mutant that ceases protein biosynthesis within a short time of transfer to nonpermissive temperature (Haralson and Roufa, 1975; Roufa and Haralson, 1975; Roufa and Reed, 1975). This mutant contains a revertible, presumably a point mutation that renders its 60S ribosomal subunit thermolabile (Haralson and Roufa, 1975). In this report, we describe the relationship between the conditional ability of ts14 to synthesize protein during S phase and the replication of its DNA.After transfer to nonpermissive temperature (39°C), where ts14 synthesizes protein at a rate approximately 20 fold less than wild-type cells, synchronous cultures of the mutant performed all the processes required for replication of their DNA. During prolonged incubations at nonpermissive temperature, S phase ts14 completed approximately one round of DNA replication semi-conservatively as judged by density-transfer experiments. Pulse-labeling experiments performed on S phase cells revealed that ts14 synthesized the intermediates of discontinuous DNA replication at nonpermissive and permissive temperatures at similar rates. In these tests, the mutant was not substantially different from wild-type at both culture temperatures. At the nonpermissive temperature, however, ts14 synthesized significantly less nuclear protein (that is, histone) than did wild-type cells, and the mutant's chromatin appeared deficient in histone by virtue of its increased sensitivity to nuclease.     

Effects of temperature changes on thymidine kinase in heat- and cold-sensitive cell-cycle mutants and ‘wild-type’ murine P-815 cells

Two heat-sensitive (arrested in G1 at 39.5°C) and two cold-sensitive (arrested in G1 at 33°C) clonal cell-cycle mutants that had been isolated from the same clone (K 21), of the murine mastocytoma P-815 cell line, were tested for thymidine kinase (EC 2.7.1.21) activity. After shift of mutant cells to the nonpermissive temperature, thymidine kinase activity decreased, and minimal levels (i.e., less than 3% of those observed for ‘wild-type’ K 21 cells at the respective temperature) were attained within 16 h in heat-sensitive and after 3–4 days in cold-sensitive mutants, which is in good agreement with kinetics of accumulation of heat-sensitive and cold-sensitive cells in G1 phase. After return of arrested mutant cells to the permissive temperature, thymidine kinase of heat-sensitive cells increased rapidly and in parallel with entry of cells into the S phase. In cultures of cold-sensitive cells, however, initiation of DNA synthesis preceded the increase of thymidine kinase activity by approx. one cell-cycle time. Thymidine kinase activities in revertants of the heat-sensitive and cold-sensitive mutants were similar to those of ‘wild-type’ cells. In ‘wild-type’ K 21 cells incubated at 39.5°C, thymidine kinase activity was approx. 30% of that at 33°C. This difference is attributable, at least in part, to a higher rate of inactivation of the enzyme at 39.5°C, as determined in cultures incubated with cycloheximide. The rapid increase of thymidine kinase activity that occurred after shift of K 21 cells and of arrested heat-sensitive mutant cells from 39.5°C to 33°C was inhibited by actinomycin D and cycloheximide.     

Failure of reactivation of chick erythrocytes after fusion with temperature-sensitive mutants of mammalian cells arrested in G1

Two temperature-sensitive (ts) mutants of mammalian cell lines (AF8 and cs4D3) that arrest in G1 at the nonpermissive temperature were fused with chick erythrocytes and the induction of DNA synthesis was studied in the resulting heterokaryons. While both AF8 and cs4D3 could induce DNA synthesis in chick nuclei at the permissive temperature, they both failed to do so when arrested in G1 at the nonpermissive temperature. When S phase AF8 cells were fused with chick erythrocytes, chick nuclei were reactivated even if the heterokaryons were incubated at the temperature nonpermissive for AF8. A third ts mutant, ts111, that is blocked in cytokinesis but continues to synthesize DNA, reactivated chick nuclei at both permissive and nonpermissive temperature. It is concluded that chick erythrocyte reactivation depends on the presence of S phase-specific factors.     

A temperature-sensitive mutant of cultured mouse cells defective in chromosome condensation

A temperature-sensitive mutant, designated ts85, was isolated from a mouse mammary carcinoma cell line, FM3A. The ts85 cells grew at 33 °C (permissive temperature) with a doubling time of 18 h, which was almost the same as with wild-type cells, whereas the cell number scarcely increased at all at 39 °C (non-permissive temperature). When the ts85 cells were shifted from 33 to 39 °C, their DNA synthesis fell to below 1% of the initial value in 14 h. RNA or protein synthesis, however, was maintained at the initial levels for at least 14 h at 39 °C. Cytofluorometric analysis of asynchronous cultures and studies with synchronous cultures suggested that the bulk of the cells cultured at 39 °C for 12–18 h were arrested in late S and G2 phases. Electron microscopic observations revealed that chromatin was abnormally condensed into fragmented and compact forms, particularly around nucleoli, in about 80% of cells of an asynchronous culture incubated at 39 °C for 16 h. Cells in mitosis were not detected in such cultures and nuclear membrane and nucleoli were still intact. Such abnormal chromosome condensation was not observed in the ts85 cells at 33 °C or in wild-type cells at either temperature. Since these findings suggest that a ts gene product of ts85 cells is necessary for chromosome condensation, ts85 cells may represent a useful tool for establishing the mechanisms of chromosome condensation. The interrelationship between abnormal chromosome condensation and reduction in DNA synthesis of the ts85 cells is discussed.     

Murine temperature-sensitive DNA polymerase α mutant displays a diminished capacity to stimulate DNA synthesis in senescent human fibroblast nuclei in heterokaryons at the nonpermissive condition

We have investigated the capacity of a murine cell line with a temperature-sensitive (ts) mutation in the DNA polymerase α (Pola) locus and a series of ts non-Pola mutant cell lines from separate complementation groups to stimulate DNA synthesis, in senescent fibroblast nuclei in heterokaryons. In the Pola mutant × senescent heterodikaryons, both human and murine nuclei display significantly diminished levels of DNA synthesis at the restrictive temperature (39.5°C) as determined by [³H]thymidine labeling in autoradiographs. In contrast, all of the non-Pola mutants, as well as the parental (wild-type) murine cells, induced similar levels of DNA synthesis in both parental nuclei at the nonpermissive and permissive temperatures. Similarly, young human fibroblasts are also able to initiate DNA synthesis in heterokaryons with the ts Pola mutant at the two temperatures. In order to determine if complementation of the non-Pola mutants requires induction of serum responsive factors in the senescent cells, fusion studies of similar design were conducted with young and old human fibroblasts incubated in low serum (0.2%) for 48 hr prior to and after cell fusion. Again, a diminished level of DNA synthesis was observed at 39.5°C in the Pola mutant x senescent cell heterokaryons. In these low-serum studies, both parental nuclei in the Pola x young cell heterokaryons and the human nuclei in heterokaryons with one of the non-Pola mutants (FT107) also displayed diminished levels of DNA synthetic activity. All of the other mutants are able to support similar levels of synthetic activity at both temperatures in the presence of reduced serum. The nature of the mutation in three of the non-Pola lines has not been determined but, like the Pola mutant cells, are inhibited in the G₁ phase of the cell cycle when incubated at the nonpermissive temperature (39.5°C). The fourth non-Pola mutant line is known to have at least one ts mutation in the cdc2 gene and is inhibited in the G₂ phase when exposed to 39.5°C. These results suggest that there may be a functional deficiency of pol α in senescent human fibroblasts, and this replication factor may be one of the rate-limiting factors involved in loss of the capacity to initiate DNA synthesis in senescent cells. © 1994 Wiley-Liss, Inc.     

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.     

The ts41 mutation in Chinese hamster cells leads to successive S phases in the absence of intervening G2, M, and G1.

The ts41 mutation of Chinese hamster cells was first isolated and characterized by Hirschberg and Marcus (1982) who showed that at nonpermissive temperature, cells accumulate up to 16C equivalents of DNA. Here we show that the mutation is recessive and at nonpermissive temperature, cells replicate their genome normally, but instead of going on into G2, M, and G1, they pass directly into a second S phase. Entry into a second S phase does not require serum nor is it inhibited by G2 checkpoints or mitotic inhibitors. Temperature-shift experiments suggest that the ts41 gene product participates in two functions in the cell cycle: entry into mitosis and inhibition of entry into S phase. The ts41 mutation seems to define a class of cell cycle mutant that couples the sequential events of DNA replication and mitosis.     

Disappearance of a basic chromosomal protein from cells of a mouse temperature-sensitive mutant defective in histone phosphorylation

The amount of a basic nuclear protein which migrates a little slower than histone H1 in urea-acetic acid-polyacrylamide gel electrophoresis and a little faster than H1 in sodium dodecylsulfate-polyacrylamide gel electrophoresis, decreases when cells of a temperature-sensitive mutant, ts85, derived from a mouse carcinoma cell line, are incubated at the nonpermissive temperature (39°C). This protein appears again, when cells cultured at 39°C are shifted down to the permissive temperature, indifferent to the added cycloheximide. Neither in wild type nor in a revertant of ts85, the protein disappeared at 39°C. Since the ts85 cells were found to be defective in chromosome condensation and in the phosphorylation of histone H1 at 39°C (1,2), this basic protein may relate to the both events.     

Changes in RNA polymerase II in a cell cycle-specific temperature-sensitive mutant of hamster cells

tsAF8 cells are a temperature-sensitive mutant of BHK cells that arrest at the nonpermissive temperature in the G₁ phase of the cell cycle. The activity of solubilized RNA polymerase II and its ability to bind [³H]-γ-amanitin decrease in tsAF8 cells at 40.6°, with a half-life of ～ 10 hr. No appreciable changes occur in these two parameters in tsAF8 cells at 34° or in BHK cells at either 34° or 40.6°. Protein synthesis is not appreciably affected for at least 24 hr after tsAF8 cells are shifted to 40.6°. These results indicate that in tsAF8 cells at the nonpermissive temperature, there is a defect in either the synthesis, the assembly, or the stability of RNA polymerase II, and that the loss of RNA polymerase II molecules is not due to widespread cellular damage.     

Cytoplasmic regulation of two G1-specific temperature-sensitive functions

tsAF8 and ts13 cells are temperature-sensitive (ts) mutants of BHK cells that specifically arrest, at nonpermissive temperature, in the G1 phase of the cell cycle. These two mutants can complement each other. Both cell lines can be made quiescent by serum deprivation (G0). When subsequently stimulated by serum, they can enter S phase at 34 degrees C but not at 39.5 degrees-40.6 degrees C. We have used these mutants to determine whether the nucleus is needed during the G0 leads to S transition for the expression of the G1 ts functions. For this purpose, we fused cytoplasts of G0-tsAF8 with whole ts13 cells in G0, and cytoplasts of G0-ts13 with whole tsAF8 cells in G0. Serum stimulation at the nonpermissive temperature induced DNA synthesis in both types of such fusion products. No DNA synthesis was induced by serum stimulation at the nonpermissive temperature in fusion products constructed between either G0-tsAF8 cytoplasts and whole G0-tsAF8 cells or G0-ts13 cytoplasts and whole G0-ts13 cells. These results demonstrate that the information for these two ts functions, which are required for entry of serum-stimulated cells into the S phase, are already present in the cytoplasm of G0 cells--that is, before serum stimulation commits them to the transition from the nonproliferating to the proliferating state.     

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.     

Alteration of the largest subunit of RNA polymerase II and its effect on chromosome stability in Schizosaccharomyces pombe

A mutation in the RNA polymerase II largest subunit (RpII LS) that is related to abnormal induction of sister chromatid exchange has previously been described the CHO-K1 cell mutant tsTM4. To elucidate the molecular basis of this effect we introduced the mutation into the homologous site in the Schizosaccharomyces pombe rpb1 gene, which encodes RpII LS. Since the tsTM4 mutant exhibited a decrease in the rate of DNA synthesis in cells arrested in S phase at the nonpermissive temperature, we focussed on the study of growth, the cell cycle, and chromosome stability at various temperatures. First, we examined the effects of the mutation on haploid yeast cells. The mutant showed slower growth than the wild type, but cell growth was not arrested at the nonpermissive temperature. When growing cells were shifted to the nonpermissive temperature, an accumulation of cells in G1 and/or G0 was observed. Tetrad analysis suggested that these phenotypes were associated with the mutation. In diploid cells, chromosome instability was detected by loss of intragenic complementation between two alleles of the ade6 gene. An abnormal fraction of cells containing an intermediate DNA content was also observed by FACS analysis. The accumulation of this fraction may reflect the fact that a large number of cells are in S phase or have an abnormal DNA content as a result of chromosome instability. These observations demonstrate that the S. pomberpb1 mutant exhibits a phenotype very similar to that of the CHO-K1 cell mutant tsTM4.     

Identification of adenovirus 2 early genes required for induction of cellular DNA synthesis in resting hamster cells.

tsAF8 cells are temperature-sensitive (ts) mutants of BHK-21 cells that arrest at the nonpermissive temperature in the G1 phase of the cell cycle. When made quiescent by serum restriction, they can be stimulated to enter the S phase by 10% serum at 34 degrees C, but not at 40.6 degrees C. Infection by adenovirus type 2 or type 5 stimulates cellular DNA synthesis in tsAF8 cells at both 34 and 40.6 degrees C. Infection of these cells with deletion Ad5dl312, Ad5dl313, Ad2 delta p305, and Ad2+D1) and temperature-sensitive (H5ts125, H5ts36) mutants of adenovirus indicates that the expression of both early regions 1A and 2 is needed to induce quiescent tsAF8 cells to enter the S phase at the permissive temperature. This finding has been confirmed by microinjection of selected adenovirus DNA fragments into the nucleus of tsAF8 cells. In addition, we have shown that additional viral functions encoded by early regions 1B and 5 are required for the induction of cellular DNA synthesis at the nonpermissive temperature.     

Reactivation of G0 nuclei by S-phase cells

tsAF8 cells are a temperature sensitive (ts) mutant of BHK that arrest in G₁ at the nonpermissive temperature or after serum deprivation. G₀ tsAF8 were fused by polyethylene glycol with other G₀ tsAF8 cells, and the fusion products were incubated at the nonpermissive temperature. The homokaryons were incapable of entering S phase under these conditions. However, when S phase tsAF8 cells were fused with G₀ tsAF8 cells, both nuclei in the homokaryons entered S phase, even when the fusion products were incubated at the nonpermissive temperature. In addition S phase tsAF8 cells, if fused with chick erythrocytes, can reactivate the chick nucleus, even if the heterokaryons are placed at nonpermissive temperature. Therefore S phase information of tsAF8 can induce DNA synthesis, after fusion, in mammalian G₀ AF8 cells and chick erythrocytes even when the homokaryons or heterokaryons are incubated at the nonpermissive temperature.     

Alteration of the largest subunit of RNA polymerase II and its effect on chromosome stability in Schizosaccharomyces pombe

A mutation in the RNA polymerase II largest subunit (RpII LS) that is related to abnormal induction of sister chromatid exchange has previously been described the CHO-K1 cell mutant tsTM4. To elucidate the molecular basis of this effect we introduced the mutation into the homologous site in the Schizosaccharomyces pombe rpb1 gene, which encodes RpII LS. Since the tsTM4 mutant exhibited a decrease in the rate of DNA synthesis in cells arrested in S phase at the nonpermissive temperature, we focussed on the study of growth, the cell cycle, and chromosome stability at various temperatures. First, we examined the effects of the mutation on haploid yeast cells. The mutant showed slower growth than the wild type, but cell growth was not arrested at the nonpermissive temperature. When growing cells were shifted to the nonpermissive temperature, an accumulation of cells in G1 and/or G0 was observed. Tetrad analysis suggested that these phenotypes were associated with the mutation. In diploid cells, chromosome instability was detected by loss of intragenic complementation between two alleles of the ade6 gene. An abnormal fraction of cells containing an intermediate DNA content was also observed by FACS analysis. The accumulation of this fraction may reflect the fact that a large number of cells are in S phase or have an abnormal DNA content as a result of chromosome instability. These observations demonstrate that the S. pomberpb1 mutant exhibits a phenotype very similar to that of the CHO-K1 cell mutant tsTM4. Received: 1 October 1997 / Accepted: 29 December 1997     

Coordination between chromosome replication and cell division in Escherichia coli.

Cell division properties of Escherichia coli B/r containing either a dnaC or a dnaI mutation were examined. Incubation at nonpermissive temperature resulted in the eventual production of cells of approximately normal size, or slightly smaller, which lacked chromosomal DNA. The cell division patterns in cultures which were grown at permissive temperature and then shifted to nonpermissive temperature were consistent with: first, division and equipartition of chromosomes by cells which were in the C and D periods at the time of the shift; second, an apparent delay in cell division; and third, commencement of the formation of chromosomeless cells. In glucose-grown cultures of the dnaI mutant, production of chromosomeless cells continued for at least 120 min, whereas in the dnaC mutant chromosomeless cells were formed during a single interval between 110 and 130 min after the temperature shift. The results are discussed in light of the hypothesis that replication of a specific chromosomal region is not an obligatory requirement for the initiation and completion of the processes leading to division in a cell which contains at least one functioning chromosome.