Genetic and biochemical characterization of mutants of CHO cells resistant to the protein synthesis inhibitor trichodermin

Genetic analysis of a vital mammalian housekeeping locus using CHO cells that express a transfected mutant allele

We describe a novel approach for the isolation of null mutations in a vital Chinese hamster ovary (CHO) cell housekeeping gene. Our experimental strategy required introduction of an expressible DNA clone encoding a recessive emetine-resistance allele of ribosomal protein S14 into wild-type CHO cells. Transgene heterozygote (TGH) cell lines, which harbor multiple emetine-resistance S14 transgenes, survive mutations that inactivate the CHO RPS14 locus by virtue of the transgenes' biological function. Null mutations in RPS14 yield TGH clones that display the transgene's drug-resistance phenotype. A large collection of emetine-resistant clones was isolated from one TGH cell line and shown to consist of three types of S14 mutations: (1) nonsense null mutations in the RPS14 protein coding sequence; (2) missense null mutations that affect S14 amino acid residues that have been conserved stringently during eukaryotic evolution; and (3) a recurrent missense mutation that results in a new, functional RPS14 emetine-resistance allele.

Effects of T-2 toxin on induction of petite mutants and mitochondrial function in Saccharomyces cerevisiae

The influence of the trichothecene mycotoxin T-2 on the mitochondria of Saccharomyces cerevisiae was studied. T-2 is a cytotoxic molecule inhibiting growth and macromolecular synthesis in S. cerevisiae. At low concentrations, T-2 toxin arrested yeast growth on glycerol medium and at higher concentrations, it arrested growth on glucose medium. The toxin was not capable itself of inducing petite mutations. Its inhibitory effect on the growth of petite strains, of both chromosomally isogenic and non-isogenic strains was less than that of grande strains. One exception to this was equally low susceptibility of psi+ SUP4-3 strain in both rho+ and rho- state. T-2 toxin was also capable of retarding the petite inducing activity of the mutagen, ethidium bromide. T-2 toxin inhibited the polymerization of P-ribo-sylaminoimidazole in an ade2 strain of S. cerevisiae. These results show that T-2 toxin is capable of interfering with the activity of the mitochondria in addition to its well studied effects on cytoplasmic protein synthesis.

Synthesis and incorporation of human ribosomal protein S14 into functional ribosomes in human-Chinese hamster cell hybrids containing human chromosome 5: human RPS14 gene is the structural gene for ribosomal protein S14

In Chinese hamster ovary cells, mutations in the RPS14 gene (which was previously designated emtB) render cells resistant to normally cytotoxic concentrations of the protein synthesis inhibitor, emetine. Several lines of evidence indicate the RPS14 gene in Chinese hamster is the structural gene for ribosomal protein S14, including the finding that mutants with alterations in this gene produce an electrophoretically altered form of this protein. A human gene which complements the defect in CHO RPS14 mutants and renders them sensitive to emetine has previously been assigned to the long arm of chromosome 5. The analysis of ribosomal proteins extracted from CHO Emtr X human cell hybrids, which contain human chromosome 5 and are emetine sensitive, demonstrated the presence of both the normal human and altered hamster forms of ribosomal protein S14. Human chromosome 5, the emetine-sensitive phenotype, and the human form of ribosomal protein S14 segregate concordantly from hybrids, confirming that the human gene in question is the structural gene for this protein. In addition, the results indicate that in interspecific cell hybrids, the human form of S14 is either incorporated into functional ribosomes more efficiently than the altered hamster protein or the human gene is overexpressed relative to the corresponding hamster gene.

Morphological changes in CHO and VERO cells treated with T-2 mycotoxin. Correlation with inhibition of protein synthesis

Exposure of Chinese hamster ovary and African green monkey kidney cells to T-2 mycotoxin resulted in several morphological changes which were related to inhibition of protein synthesis, the basic in vitro mechanism of action of the toxin. These changes, which occurred in both cell types, included disassociation of polysomes and mitochondrial cristae alterations. In addition, CHO cells displayed membrane bleb formations similar to those found in CHO cells after exposure to established inhibitors of protein synthesis, puromycin and anisomycin. Blebs could be either a result of protein synthesis inhibition or a non-specific early pathological response. Bleb formations were not observed in VERO cells under any experimental condition.

Autoradiographic detection of diphtheria toxin resistant mutants in human diploid fibroblasts

An autoradiographic procedure for the detection of diphtheria toxin (DT) resistant (DipR) mutants in human diploid fibroblast (HDF) cells has been developed. The assay is based on the observation that when HDFs from confluent cultures are seeded in medium containing 0.01 flocculating units/ml or higher concentration of DT, protein synthesis in sensitive cells is severely inhibited by 4-6 hr. If at this or later time, a radiolabeled protein precursor (eg, 3H-leucine) is added to the culture, it is almost exclusively incorporated into the resistant cells, which are then readily identified by autoradiography. The DipR cells can also be identified by labeling in the presence of 3H-thymidine, although a higher background is observed in these experiments. Reconstruction experiments using DipS and DipR HDFs show that the frequency of heavily labeled cells that are detected by autoradiography show an excellent correlation with the number of DipR cells added and to the number of DipR cells as detected by conventional colony forming assay. These studies provide strong evidence that the labeled cells identified by autoradiography are bona fide DipR mutants. The detection of DipR cells by autoradiography is apparently not affected by the presence of the sensitive cells in the mixtures. The spontaneous frequency of DipR cells in HDFs has been found to be in the range of 1-5 X 10(-6), and this increases in a dose dependent manner upon treatment with the mutagen ethyl methanesulfonate. These results indicate that the autoradiographic assay could be used for quantitative mutagenesis. Since the autoradiographic assay does not depend on cell division, it may prove useful in estimating the incidence of pre-existing mutations in cell populations that either do not divide or have very limited growth potential (eg, lymphocytes, muscle cells, neurons, senescent fibroblasts, etc).

A specific mutation abolishing Na+/H+ antiport activity in hamster fibroblasts precludes growth at neutral and acidic pH

A H+-suicide technique based on the reversibility of Na+/H+ antiport was developed for the selection of mutants deficient in this membrane-bound activity. The strategy was to use the Na+/H+ antiporter as a H+-vector killing device. Chinese hamster lung fibroblasts (CCL39) were loaded with LiCl and incubated in Na+-, Li+-free choline Cl saline solution (pH 5.5). Under these conditions, intracellular pH dropped in 5 min from 7.1 to 4.8, leading to a rapid loss of cell viability (less than 0.1% survival after 30 min). Cytoplasmic acidification and cell death were prevented by treatment with 5-N,N-dimethylamiloride, a potent inhibitor of Na+/H+ antiport. Of the H+-suicide resistant clones that survived two cycles of selection, 90% were found deficient in Na+/H+ antiport activity. One class of mutants (PS10, PS12) fully resistant to the H+-suicide test, does not acidify the cell interior in response to an outward-directed Li+ gradient and has no detectable amiloride-sensitive Na+ influx measured either in Li+- or H+-loaded cells. Growth of these fibroblast clones lacking Na+/H+ antiport was found to be pH conditional in HCO3(-)-free medium. Whereas wild-type cells can grow over a wide range of external pHs (6.6-8.2), PS mutants cannot grow at neutral and acidic pHs (pH less than 7.2); their optimal growth occurs at alkaline pH values (pH 8-8.3). These findings strongly suggest that the Na+/H+ antiport activity through regulation of intracellular pH plays a crucial role in growth control.

Genetic and biochemical distinction among Chinese hamster cell emtA, emtB, and emtC mutants

Genetic and biochemical experiments have enabled us to more clearly distinguish three genetic loci, emtA, emtB, and emtC, all of which can be altered to give rise to resistance to the protein synthesis inhibitor, emetine, in cultured Chinese hamster cells. Genetic experiments have demonstrated that, unlike the emtB locus, neither the emtA locus nor the emtC locus is linked to chromosome 2 in Chinese hamster cells, clearly distinguishing the latter two genes from emtB. emtA mutants can also be distinguished, biochemically, from emtB and emtC mutants based upon different degrees of cross-resistance to another inhibitor of protein synthesis, cryptopleurine. Two-dimensional gel electrophoretic analysis of ribosomal proteins failed to detect any electrophoretic alterations in ribosomal proteins from emtA or emtC mutants that could be correlated with emetine resistance. However, a distinct electrophoretic alteration in ribosomal protein S14 was observed in an emtB mutant. In addition, the parental Chinese hamster peritoneal cell line of an emtC mutant, and the emtC mutant itself, are apparently heterozygous for an electrophoretic alteration in ribosomal protein L9.

Construction and characterization of Chinese hamster cell EmtA EmtB double mutants

Starting with hybrid cell lines between a Chinese hamster cell EmtA mutant and a Chinese hamster cell EmtB mutant, we have constructed cell lines that are homozygous for mutant alleles at both the emtA locus and the emtB locus, by using a two-step segregation protocol. The EmtA EmtB double mutants are approximately 10-fold more resistant to emetine inhibition than either of the parental mutants. Having both the EmtA mutation and the EmtB mutation expressed in the same cell also results in a level of resistance to cryptopleurine that is significantly higher than a simple additive effect of the two mutations alone. Analysis of ribosomal proteins by two-dimensional polyacrylamide gel electrophoresis demonstrated that a parental hybrid and a first-step segregant, which has lost the wild-type emtA allele, synthesize both a normal and an altered form of ribosomal protein S14, whereas an EmtA EmtB double mutant synthesizes only the altered form of this ribosomal protein. This result confirms that the emtB locus is the structural gene for ribosomal protein S14. Our results also suggest that the products of the emtA and emtB loci interact directly, indicating that the emtA locus, like the emtB locus, encodes a component of the ribosome.

Construction and characterization of Chinese hamster cell EmtA EmtB double mutants

Starting with hybrid cell lines between a Chinese hamster cell EmtA mutant and a Chinese hamster cell EmtB mutant, we have constructed cell lines that are homozygous for mutant alleles at both the emtA locus and the emtB locus, by using a two-step segregation protocol. The EmtA EmtB double mutants are approximately 10-fold more resistant to emetine inhibition than either of the parental mutants. Having both the EmtA mutation and the EmtB mutation expressed in the same cell also results in a level of resistance to cryptopleurine that is significantly higher than a simple additive effect of the two mutations alone. Analysis of ribosomal proteins by two-dimensional polyacrylamide gel electrophoresis demonstrated that a parental hybrid and a first-step segregant, which has lost the wild-type emtA allele, synthesize both a normal and an altered form of ribosomal protein S14, whereas an EmtA EmtB double mutant synthesizes only the altered form of this ribosomal protein. This result confirms that the emtB locus is the structural gene for ribosomal protein S14. Our results also suggest that the products of the emtA and emtB loci interact directly, indicating that the emtA locus, like the emtB locus, encodes a component of the ribosome.

Genetic and biochemical distinction among Chinese hamster cell emtA, emtB, and emtC mutants

Genetic and biochemical experiments have enabled us to more clearly distinguish three genetic loci, emtA, emtB, and emtC, all of which can be altered to give rise to resistance to the protein synthesis inhibitor, emetine, in cultured Chinese hamster cells. Genetic experiments have demonstrated that, unlike the emtB locus, neither the emtA locus nor the emtC locus is linked to chromosome 2 in Chinese hamster cells, clearly distinguishing the latter two genes from emtB. emtA mutants can also be distinguished, biochemically, from emtB and emtC mutants based upon different degrees of cross-resistance to another inhibitor of protein synthesis, cryptopleurine. Two-dimensional gel electrophoretic analysis of ribosomal proteins failed to detect any electrophoretic alterations in ribosomal proteins from emtA or emtC mutants that could be correlated with emetine resistance. However, a distinct electrophoretic alteration in ribosomal protein S14 was observed in an emtB mutant. In addition, the parental Chinese hamster peritoneal cell line of an emtC mutant, and the emtC mutant itself, are apparently heterozygous for an electrophoretic alteration in ribosomal protein L9.

Isolation and characterization of interspecific heat-resistant hybrids between a temperature-sensitive chinese hamster cell asparaginyl-tRNA synthetase mutant and normal human leukocytes: assignment of human asnS gene to chromosome 18

We isolated interspecific somatic cell hybrids between human peripheral leukocytes and a temperature-sensitive CHO cell line with a thermolabile asparaginyl-tRNA synthetase. The hybrids were selected at 39 degrees C so as to require the expression of the human gene complementing the deficient CHO enzyme. In vitro heat-inactivation profiles of cell-free extracts from temperature-resistant hybrid cells indicate the presence of two forms of asparaginyl-tRNA synthetase. One form is very resistant to thermal inactivation, like the normal human enzyme, while the other form is very thermolabile, like the altered enzyme from the CHO parent. Hybrids and temperature-sensitive segregants derived from them were analyzed for the expression of known human chromosomal marker enzymes. The strong correlation between the expression of the human form of asparaginyl-tRNA synthetase and the presence of human chromosome 18 in hybrids suggests that the human gene, asnS, which corrects the heat-sensitive phenotype of the CHO asparaginyl-tRNA synthetase mutant, is located on chromosome 18.

High-frequency mutation at the adenine phosphoribosyltransferase locus in Chinese hamster ovary cells due to deletion of the gene

Evidence for a two-step model to explain the high-frequency expression of the recessive phenotype at the autosomal adenine phosphoribosyltransferase (APRT; EC 2.4.2.7) locus in Chinese hamster ovary (CHO) cells was given by Simon et al. [Simon, A. E., Taylor, M. W., Bradley, W. E. C. & Thompson, L. (1982) Mol. Cell. Biol. 2, 1126-1133]. This model proposed a high-frequency event, leading to allelic inactivation or a loss of gene function, and a low-frequency event, causing a structural alteration of the APRT protein. Either event could occur first, resulting in two classes of heterozygotes. We have analyzed the low-frequency event that gave rise to the class 2 aprt heterozygote D416 and the high-frequency event that led to APRT- cells derived from D416. Genomic Southern blots of Msp I- or Hpa II-digested DNA from wild-type CHO, aprt heterozygote D416, and two APRT- cell lines derived from D416 indicate a loss of a specific Msp I/Hpa II recognition sequence at one aprt locus in the heterozygote that correlates with the production of the electrophoretically altered APRT protein found in D416. The APRT- mutants are homozygous for the loss of this Msp I/Hpa II site. By using an additional CHO gene as an internal control, it was determined that the APRT- mutants contain only a single copy of the altered aprt gene. Thus, the high-frequency event that produces APRT- mutants derived from D416 is not an inactivation event but rather a deletion of the wild-type aprt gene.

Isolation and characterization of 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole-resistant mutants of the Chinese hamster ovary cell line

Mutants resistant to the RNA synthesis inhibitor 5,6-dichloro-1-beta-D-ribofurano-sylbenzimidazole (DRB) have been isolated in the Chinese hamster ovary cell line CHO-K1. Three independently isolated mutants, DRB6 DRB10, and DRB13, were 3-, 5-, and 3.5-fold, respectively, more resistant to DRB than the parental cell line WTCHO. The DRB-resistant mutations were expressed codominantly in somatic cell hybrids of DRB-resistant and DRB-sensitive cell lines. In vivo treatment of CHO-K1 cells with DRB resulted in specific inhibition of endogenous RNA polymerase II activity in cell lysates. Whereas DRB inhibited RNA polymerase II activity in WTCHO cells by a maximum of 60% at concentrations as low as 60 microM, 300 microM DRB was required to inhibit 60% of the RNA polymerase II activity in DRB10 cells. However, the inhibition of the DRB-sensitive RNA polymerase II activity in DRB10 was biphasic. About half (53 to 56%) of this activity was inhibited by 90 microM DRB and thus showed a DRB sensitivity similar to the wild-type RNA polymerase II activity; the remaining DRB-sensitive RNA polymerase II activity was maximally inhibited by 300 microM DRB. These results indicated that there were two copies of the drbR locus (drb+ and drbR-10) in DRB10 and confirmed that the drbR-10 mutation was expressed codominantly. Somatic cell hybrids of DRB-resistant and alpha-amanitin-resistant cell lines grew in medium containing both DRB and alpha-amanitin, demonstrating that the drbR and amaR mutations were not in the same gene. Thus, the drbR mutations may define an additional component of the RNA polymerase II transcriptional complex in mammalian cells.

Isolation and characterization of cycloheximide-resistant mosquito cell clones

Following treatment of cultured mosquito cells (Aedes albopictus line of Singh) with the chemical mutagen ethyl methanesulfonate, we were able to isolate three cycloheximide-resistant clones. On the basis of growth kinetics, plating efficiency, and protein synthesis, these clones are 10- to 30-fold more resistant to cycloheximide than the parent cells. Cell-free lysates made from these cells retained 30-65% of their endogenous protein synthesizing ability in the presence of cycloheximide concentrations as high as 300 micrograms/ml. Protein synthesis in lysates from the parental cells, however, is reduced to about 10% of the control value (i.e., in the absence of drug) at 14 micrograms of cycloheximide/ml and was completely abolished at 75 micrograms/ml. These results indicate that cycloheximide resistance in these cells is likely due to an alteration in the protein synthetic machinery. This is the first description of cycloheximide-resistant insect cells, and the best example of cycloheximide resistance in cells originating from a higher eukaryotic organism.

Isolation and characterization of puromycin-resistant clones from cultured mosquito cells

We have isolated from an established Aedes albopictus (mosquito) cell line clones which are resistant to the antibiotic puromycin. On the basis of growth and plating efficiency, clones Pur-8026 and Pur-8612 were five- and seven-fold more resistant, respectively, to puromycin than wild-type cells. In vitro protein synthesis was resistant to puromycin only in extracts prepared from Pur-8612 cells. Measurements of puromycin transport, cross-resistance to colchicine, and sensitivity to Tween-80 indicating that resistance in Pur-8026 cells was due to membrane alteration(s) affecting permeability to puromycin. This is the first description of puromycin resistant in insect cells and also the first report of puromycin resistance in an animal cell variant associated with an alteration at the level of protein synthesis.

Isolation and characterization of 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole-resistant mutants of the Chinese hamster ovary cell line

Mutants resistant to the RNA synthesis inhibitor 5,6-dichloro-1-beta-D-ribofurano-sylbenzimidazole (DRB) have been isolated in the Chinese hamster ovary cell line CHO-K1. Three independently isolated mutants, DRB6 DRB10, and DRB13, were 3-, 5-, and 3.5-fold, respectively, more resistant to DRB than the parental cell line WTCHO. The DRB-resistant mutations were expressed codominantly in somatic cell hybrids of DRB-resistant and DRB-sensitive cell lines. In vivo treatment of CHO-K1 cells with DRB resulted in specific inhibition of endogenous RNA polymerase II activity in cell lysates. Whereas DRB inhibited RNA polymerase II activity in WTCHO cells by a maximum of 60% at concentrations as low as 60 microM, 300 microM DRB was required to inhibit 60% of the RNA polymerase II activity in DRB10 cells. However, the inhibition of the DRB-sensitive RNA polymerase II activity in DRB10 was biphasic. About half (53 to 56%) of this activity was inhibited by 90 microM DRB and thus showed a DRB sensitivity similar to the wild-type RNA polymerase II activity; the remaining DRB-sensitive RNA polymerase II activity was maximally inhibited by 300 microM DRB. These results indicated that there were two copies of the drbR locus (drb+ and drbR-10) in DRB10 and confirmed that the drbR-10 mutation was expressed codominantly. Somatic cell hybrids of DRB-resistant and alpha-amanitin-resistant cell lines grew in medium containing both DRB and alpha-amanitin, demonstrating that the drbR and amaR mutations were not in the same gene. Thus, the drbR mutations may define an additional component of the RNA polymerase II transcriptional complex in mammalian cells.

Emetine resistance in chinese hamster ovary cells is associated with an altered ribosomal protein S14 mRNA

Chinese hamster ovary cell mutants resistant to the translational inhibitor emetine (Emt B mutants) express a single genetically altered 40S ribosomal subunit protein, S14. that appears to account for their drug resistance. To determine whether Emt B mutation affect the structural gene for the S14 protein, we isolated mRNAs from several wild-type and Emt B mutant clones. We translated the mRNAs in a cell-free system derived from wheat germ, and we discerned the biosynthesis of 59 ribosomal proteins, including S14. In every case, poly(A)+ mRNA programmed the cell-free system to synthesize an S14 protein electrophoretically identical to the S14 extracted from the ribosomes of the corresponding cell line. Messages from two Emt B mutants (Emr-2 and Emr-2-2) specified S14s that were electrophoretically distinct from the wild-type protein. Thus, Emt B mutations were expressed in mutant cell mRNAs, apparently reflecting mutagen-induced changes in S14 structural genes.

Podophyllotoxin resistance: a codominant selection system for quantitative mutagenesis studies in mammalian cells

Mutants resistant to the microtubule inhibitor podophyllotoxin (PodR), a codominant marker, can be readily selected in various mammalian cell lines such as, CHO, HeLa, mouse L cells, Syrian hamster cells (BHK21) and a mouse teratocarcinoma cell line OC15. In CHO cells, the recovery of PodR mutants is not affected by cell density (up to 1 X 10(6) cells per 100-mm diameter dish), and after treatment with the mutagen ethyl methanesulfonate maximum mutagenic effect is achieved after a relatively short expression time (40-48 h). The frequency of PodR mutants in various cell lines increased in a dose-dependent manner in response to treatment with the mutagens ethyl methanesulfonate and N-methyl-N'-nitro-N-nitrosoguanidine. The PodR selection system thus provides a new genetic marker which should prove useful in studies of quantitative mutagenesis in mammalian cells.

International Commission for Protection against Environmental Mutagens and Carcinogens. ICPEMC working paper 2/5: mutagenesis in mammalian cells

Chemical mutagenesis in animal cells is a complex process. Whereas some chemicals are mutagenic in their original form, others such as the nitrosamines and polycyclic hydrocarbon carcinogens are mutagenic only when enzymatically activated. The active form, or ultimate carcinogen, can interact with proteins and nucleic acids, altering amino acids and producing modified bases in DNA. The modified bases do not usually constitute mutations as produced. Instead they are acted on by the DNA enzymes of the cell, which repair most damaged bases but occasionally insert incorrect base sequences at or near the sites of damage. The frequency at which mutant animal cells are recovered depends upon the selection conditions in culture, upon whether the mutation selected is in a gene present in single or multiple active copies, and upon whether expression is dominant or recessive. Many studies depend on selecting for 8-azaguanine- or 6-thioguanine-resistant mutants, which are due to mutations in the HGPRT locus present in a single active copy on the X-chromosome. Other widely used systems depend on selecting for ouabain resistance, which is dominant and results from a change in the sodium/potassium ATPase activity, or on selecting for thymidine kinase mutants in heterozygous Tk+/Tk- mouse cells. Many other types of mutation including nutritional markers are recessive and express only in cells carrying a single active gene copy, as is sometimes the case in established cell lines. The types of base damage causing mutations have been identified in very few cases only, and little is known about the enzymatic mechanisms of mutagenesis. However, chemical mutagenesis in cultured animal cells provide a practical way of testing chemicals and radiations for mutagenicity directly in animal cells, and much has been learned about the mutagenicity of various carcinogenic substances. To date, there is reasonable qualitative agreement between these results and those obtained in the widely used liver microsome-activated bacterial mutagenesis test systems.

Resistance to the microtubule inhibitor podophyllotoxin: selection and partial characterization of mutants in CHO cells

Stable mutants two- to fourfold more resistant to the microtubule inhibitor podophyllotoxin (PodRI) have been selected in a single step in Chinese hamster ovary (CHO) cells. Mutants showing higher levels of resistance to the drug can be obtained in a second-step selection using the PodRI mutants (PodRII class). Prior treatment of cells with the mutagen ethylmethane sulfonate markedly enhanced the frequency of both PodRI and PodRII mutants in culture. The spontaneous rate of mutation of PodRI as determined by fluctuation analysis was found to be 1 - 2 x 10(-5) and 4 x 10(-7) mutations/cell/generation at 20 and 30 ng/ml podophyllotoxin, respectively. Somatic cell hybrids between PodRI x PodS and PodRII x PodS showed intermediate levels of resistance to the drug, indicating that the PodR phenotype behaves codominantly under these conditions. Cross-resistance studies with various PodRI and PodRII mutants show that they are not cross-resistant to other microtubule inhibitors, e.g., colchicine, colcemid, griseofulvin, vinblastin, that have been examined, or to a compound such as VP16-213, which is a podophyllotoxin derivative which lacks microtubule inhibitory activity. These results indicate that the lesion in PodR mutants is highly specific and probably does not involve a permeability alteration.

Cloning of yeast gene for trichodermin resistance and ribosomal protein L3

Yeast cells sensitive to the eukaryotic protein synthesis inhibitor trichodermin have been transformed with autonomously replicating recombinant plasmids carrying DNA fragments of the genome of a trichodermin-resistant yeast strain. After selection for trichodermin-resistant cells, several transformants yielded a plasmid containing a 13.5-kilobase (kb) DNA fragment that encodes the trichodermin resistance gene, tcm1, and the gene for ribosomal protein L3, the largest of the yeast ribosomal proteins. Cells carrying this plasmid are resistant to trichodermin and to the related drug verrucarin A as well as to the unrelated drug anisomycin. This pattern of resistance is similar to that exhibited by strains carrying a chromosomal copy of tcm1. Moreover, polyribosomes prepared from transformed cells are resistant to trichodermin when tested in an in vitro protein synthesis assay. Subcloning of the 13.5-kb DNA fragment revealed that the gene for tcm1 and the gene for protein L3 are contained within a 3.2-kb segment. These results suggest that the gene for trichodermin resistance in yeast specifies ribosomal protein L3.

Linkage in cultured Chinese hamster cells of two genes, emtB and leuS, involved in protein synthesis and isolation of cell lines with mutations in three linked genes

We have determined via segregation analyses from appropriate hybrids that two genes involved in protein synthesis, one encoding for a ribosomal protein (emtB) and one encoding for leucyl-tRNA synthetase (leuS), cosegregate at a very high frequency and are linked in both Chinese hamster ovary and lung cells. In contrast, the emtA locus, defined by a second complementation group of emetine-resistant mutants which also have alterations affecting protein synthesis and probably the ribosome, is not linked to leuS. In addition, we have determined that a third gene, one that can be altered to give rise to chromate resistance, is syntenic with emtB and leuS. We have selected cell lines with mutations in each of these three linked genes and have shown that the three loci cosegregate at a high frequency. Because the mutations in these three linked genes provide easily distinguishable phenotypes, these cell lines should provide a powerful tool for examining several important questions concerning mitotic recombination in somatic cells.

A novel synergistic effect of alanosine and guanine on adenine nucleotide synthesis in mammalian cells. Alanosine as a useful probe for investigating purine nucleotide metabolism

A novel synergistic effect of the antitumor agent alanosine (2-amino-3-(hydroxynitrosoamino) propionic acid), which specifically inhibits the enzyme adenylosuccinate synthetase (ASS) and guanine on the growth of Chinese hamster ovary (CHO) cells and human diploid fibroblasts (HDF) has been observed. In the presence of subinhibitory concentrations of alanosine, both CHO cells and the HDF show excessive sensitivity to exogenous guanine--a phenotype which closely resembles that seen with some of the mutants containing reduced enzymatic activity of ASS. The growth inhibitory effects of alanosine, or alanosine and guanine, on CHO cells are completely reverted by the addition of adenine to the culture medium, and the synergistic effect of guanine is not observed in mutants which lack the enzyme hypoxanthine-guanine phosphoribosyl transferase. These resuls suggest that guanine nucleotides exert a regulatory effect on the activity of the enzyme adenylosuccinate synthetase. The ability to confer the guanine-sensitive phenotype and its modulation by subinhibitory concentrations of alanosine in different cell types indicates that alanosine provides a useful probe for investigating the regulation of purine nucleotide metabolism in mammalian cells.

Biochemical and genetic evidence for a new class of emetine-resistant Chinese hamster cells with alterations in the protein biosynthetic machinery

We have isolated a number of emetine-resistant mutants from several different clones of CHO and CHL cells. Protein synthesis in extracts derived from each of the mutants is much more resistant to emetine than in the parental, emetine-sensitive cell lines, indicating the lesions affect the protein synthetic machinery directly. However, hybrid cell lines, derived from fusing either of two different emetine-resistant CHO mutants with either one of two different emetine-resistant CHL cells, are much more sensitive to growth inhibition by emetine than either parent. In addition, the incorporation of [3H] amino acids into protein in vivo and protein synthesis in vitro in extracts derived from these hybrids is much more sensitive to emetine inhibition than in either emetine-resistant parent. In contrast, no complementation was observed in hybrids derived from fusing two emetine-resistant CHO mutants or in hybrids derived from fusing two emetine-resistant CHL cell lines. These results indicate the CHO emetine-resistant mutants belong to one complementation group and the CHL emetine-resistant mutants belong to another. The genetic loci represented by these two complementation groups must both encode for gene products involved in protein synthesis.

Diphtheria toxin resistance in Chinese hamster cells: genetic and biochemical characteristics of the mutants affected in protein synthesis

Diphtheria toxin (DT) resistant mutants (Dipr) have been isolated from a number of different Chinese hamster lines. Among mutants affected in protein synthesis (DiprII class), two distinct phenotypes have been identified. In one class, the entire elongation factor-2 (EF-2) activity becomes resistant to DT-catalyzed ADP-ribosylation (DiprIIa class); these mutants behave recessively upon hybridization with sensitive cells. The second kind of protein synthesis mutants contain nearly normal levels of the ADP-ribosylatable EF-2 activity (DiprIIb class). The hybrids the two types of protein synthesis mutants complement each other indicating that mutations in different genes are responsible for them. While the DiprIIa class of mutants are presumably affected in the EF-2, the lesion in DiprIIb mutants seems to have occurred in a yet unidentified protein synthesis factor. Interesting differences are also observed in the characteristics of mutants that presumably are defective in the entry of toxin into cells (DiprI class).

Hydroxyurea-resistant mouse L cells with elevated levels of drug-resistant ribonucleotide reductase activity

We describe the isolation and partial characterization of a mouse L-cell line which is resistant to normally highly cytotoxic concentrations of hydroxyurea. A detailed analysis of the target enzyme ribonucleotide reductase in both wild-type and hydroxyurea-resistant enzyme preparations suggests that the drug-resistant cells form a ribonucleotide reductase enzyme which contains a structural alteration, rendering it less sensitive to inhibition by hydroxyurea. K1 values for hydroxyurea inhibition of ribonucleotide reduction in enzyme preparations from hydroxyurea-resistant cells were significantly higher than corresponding values from preparations from wild-type cells. The Km for CDP reduction in enzyme preparations of drug-resistant cells was approximately threefold higher than the corresponding parental wild-type value. In addition, in vivo enzyme assays detected a major difference between the temperature profiles of ribonucleotide reduction in nucleotide-permeable drug-resistant and wild-type cells. When levels of ribonucleotide reductase activity were measured in vivo, it was found that the drug-resistant cells contained approximately 3 times the wild-type level of CDP reductase activity and twice wild-type level of GDP reductase activity. This combination of enhanced enzyme levels plus an altered sensitivity to drug inhibition can easily account for the drug-resistance phenotype. The properties of these hydroxyurea-resistant cells indicate that they will be useful for genetic and biochemical studies.

DRB resistance in Chinese hamster and human cells: genetic and biochemical characteristics of the selection system

Stable mutants resistant to the nucleoside analog 5,6-dichloro-1-beta-D-ribofuranosyl benzimidazole (DRB), which interferes with RNA synthesis, have been selected in Chinese hamster ovary (CHO) and human diploid fibroblasts. In CHO cells, upon treatment with the mutagen ethyl-methane sulfonate (EMS), a linear dose--response between the concentration of mutagen and the frequency of DrbR mutants was observed in the range of 20--300 micrograms/ml. The selection system did not show cell density or cross-feeding effects, and the optimal expression time following mutagenesis was found to be 2--3 days for CHO cells and 5--6 days for human fibroblasts. The DrbR mutation behaved codominantly in DrbR x DrbS hybrids. Addition of DRB affected nucleoside uptake to a similar extent in both wild-type and mutant cells, indicating that the drug was able to enter the mutant cells. The failure of DrbR mutants to show any cross-resistance to other toxic nucleoside analogs examined suggests that the action of DRB does not involve the initial phosphorylation step. DRB addition did not cause any marked inhibition of either RNA polymerase I or RNA polymerase II activity from both wild-type and mutant cells in vitro, indicating that its effect on RNA synthesis may be indirect.

Pactamycin resistance in CHO cells: morphological changes induced by the drug in the wild-type and mutant cells

Stable mutants resistant to pactamycin (PacR), a polypeptide chain initiation inhibitor, have been selected in a single step in Chinese hamster ovary (CHO) cells. The sensitivity of protein synthesis in mutant cell extracts to pactamycin indicates that resistance involves an alteration in the permeability of this drug. The failure of PacR mutants to show cross-resistance to other compounds provides further indication that the lesion is presumably specific for pactamycin. Cell hybrids formed between PacR X PacS lines show intermediate sensitivity towards pactamycin, suggesting that the PacR lesion behaves codominantly under these conditions. In the presence of subinhibitory concentrations of pactamycin, CHO cells, which are normally short, polygonal and disoriented, became greatly elongated and aligned themselves in parallel fashion to produce highly oriented colony morphologies, reminiscent of normal diploid fibroblasts. This effect of pactamycin on cellular morphology was seen much more clearly with the PacR mutants, although somewhat higher concentrations of the drug were required to produce this change.

Genetic markers for quantitative mutagenesis studies in Chinese hamster ovary cells: characteristics of some recently developed selective systems

Selection conditions have been optimized in the Chinese hamster ovary (CHO) cell system for a number of genetic markers. The genetic systems studied include resistance to the protein-synthesis inhibitors emetine (Emtr) and diphtheria toxin (Dipr), resistance to methylglyoxalbisguanylhydrazone (Mbgr) which affects polyamine transport, resistance to the nucleoside analogs toyocamycin and tubercidin (Toyr), and resistance to thioguanine (Thgr) and ouabain (OuaR). The optimal expression time following mutagenesis for various markers was between 2 and 6 days. A linear dose--response relationship between the concentration of mutagen (ethyl methanesulfonate) and mutation frequency has been observed over the range of 10--700 micrograms/ml, for all of the above markers except Toyr. The response of these markers to other mutagens such as tritium (3H) decay and ICR-191 show some specificity. Since the response of a number of genetic markers can be studied simultaneously in the CHO system, it should prove very useful for studies of quantitative mutagenesis and in assay systems for mutagen detection.

Evidence for variation in the number of functional gene copies at the AmaR locus in Chinese hamster cell lines

The hypothesis of functional hemizygosity has been examined for the alpha-amanitin resistant (AmaR, a codominant marker) locus in a series of Chinese hamster cell lines. AmaR mutants were obtained from different cell lines, e.g., CHO, DHW, M3- 1 and CHO-Kl, at similar frequencies. After fractionation of different RNA polymerase activities in the extracts by chromatographic procedures, the sensitivity of the mutant RNA polymerase II towards alpha-amanitin was determined. While all of the RNA polymerase II activity in mutant CHO and CHO-Kl lines became resistant to alpha-amanitin inhibition, only about 50% of the activity is highly resistant in AmaR mutants of CHW and M3- 1 cell lines. The remaining activity in the latter cell lines shows alpha-amanitin sensitivity similar to that seen with the wild-type enzyme. This behaviour is similar to that observed with a 1:1 mixture of resistant and sensitive enzymes from CHO cells. These results, therefore, strongly indicate that while only one functional copy of the gene affected by alpha-amanitin is present in CHO and CHO-Kl cells, two copies of this gene are functional in the CHW and M3-1 cell lines.

Genetic and biochemical studies with the adenosine analogs toyocamycin and tubercidin: mutation at the adenosine kinase locus in Chinese hamster cells

The pyrrolopyrimidine nucleosides toyocamycin and tubercidin show several unique features of growth inhibition in Chinese hamster ovary (CHO) cells. Stable mutants which are more than 600-fold resistant to these drugs are obtained in CHO cells at a strikingly high frequency of approximately 10(-3), in the absence of mutagenesis. The mutants resistant to toyocamycin (Toyr) and tubercidin (Tubr) exhibit similar cross-resistance patterns to the two selective drugs as well as to adenosine and 6-methyl mercaptopurine riboside, indicating that the same lesion is probably involved in all cases. The mutants examined were found to be deficient in the enzyme adenosine kinase (AK), indicating that the phosphorylation of these analogs is an essential first step in their toxic action. The above mutants (AK-) behaved recessively in cell hybrids, and segregation studies indicate that the AK locus is not linked to the X chromosome. The frequencies of similar Toyr mutants in other Chinese hamster lines, e.g., V79, CHW, M3-1, GM7, and CHO-K1, varied from similar to more than three logs less than that observed for CHO cells, indicating that various cell lines probably differ in the number of functional gene copies for this locus.

Diphtheria-toxin-resistant mutants of CHO cells affected in protein synthesis: a novel phenotype

Stable mutants highly resistant to the protein-synthesis-inhibitor diphtheria toxin have been selected in Chinese hamster ovary (CHO) cells. Protein synthesis in extracts of mutant cells is resistant to the inhibitory action of diphtheria toxin, indicating that the lesion has affected the protein-synthesis machinery. However, about 50% of the elongation factor-2 (EF-2) activity in the mutant cells can still be ADP-ribosylated by diphtheria toxin, and this remaining EF-2 activity is similar to that present in the wild-type cells. We suggest that this result is best explained by assuming that our CHO cells contain two functional copies of the EF-2 gene, and that only one of the copies is altered in the mutants. According to this view, the mutated allele produces EF-2 resistant to ADP-ribosylation which is capable of supporting cell growth in the presence of diphtheria toxin. Although the Dipr marker seems to act dominantly in the parental CHO cells, its behavior in Dipr X Dips hybrids (CHO X CHO) is recessive as measured by cell survival in presence of the toxin. This paradoxical behavior may be due to a gene dosage effect. Segregation studies from hybrids show that the Dipr marker segregates independently of the Emtr and Thgr markers indicating that the Dipr locus is not linked to either the Emtr locus or to the X chromosome.