Selection for temperature-sensitive mutants of diploid and tetraploid mammalian cells

Isolation of a Chinese hamster cell mutant with low intracellular phosphoribosylpyrophosphate concentration

A tritium-adenine suicide procedure was used to select for mutants with reduced uptake of adenine from a population of Chinese hamster V79 cells mutagenized with ethyl methane sulfonate. In one of the mutant lines isolated, designated KC62, the uptake of adenine, hypoxanthine, and guanine was reduced by approximately 70%. The specific activities, Km values, and Vmax values of adenine phosphoribosyltransferase and of hypoxanthine phosphoribosyltransferase were the same in extracts from KC62 and from the parental cell line. Metabolic fate studies of incorporated [3H]adenine and 3[H]hypoxanthine revealed a metabolic block at the level of phosphoribosylation. Determination of phosphoribosylpyrophosphate pool size showed that the mutant contained only 25% of the phosphoribosylpyrophosphate found in the parent. Its reduced availability in KC62 appears to result in a decreased ability to salvage adenine, hypoxanthine, and guaninine via phosphoribosylation. Phosphoribosylpyrophosphate synthetase from KC62 was shown to have an increased sensitivity to inhibition by a variety of nucleotides.

Xanthine oxidase/hydrogen peroxide generates sulfur trioxide anion radical (SO3.-) from sulfite (SO3(2-))

In the presence of hydrogen peroxide (H2O2), xanthine oxidase has been found to catalyze sulfur trioxide anion radical (SO3.-) formation from sulfite anion (SO3(2-)). The SO3.- radical was identified by ESR (electron spin resonance) spin trapping, utilizing 5,5-dimethyl-l-pyrroline-l-oxide (DMPO) as the spin trap. Inactivated xanthine oxidase does not catalyze SO3.- radical formation, implying a specific role for this enzyme. The initial rate of SO3.- radical formation increases linearly with xanthine oxidase concentration. Together, these observations indicate that the SO3.- generation occurs enzymatically. These results suggest a new property of xanthine oxidase and perhaps also a significant step in the mechanism of sulfite toxicity in cellular systems.

Characterization of a Chinese hamster-human hybrid cell line with increased system L amino acid transport activity

We have studied leucine transport in several Chinese hamster-human hybrid cell lines obtained by fusion of a temperature-sensitive line of Chinese hamster ovary cells, ts025C1, and normal human leukocytes. A hybrid cell line exhibiting a twofold increase in L-leucine uptake over that in the parental cell line was found. This hybrid cell line, 158CnpT-1, was temperature resistant, whereas the parental Chinese hamster ovary mutant, ts025C1, contained a temperature-sensitive leucyl-tRNA synthetase mutation. An examination of the different amino acid transport systems in this hybrid cell line revealed a specific increase of system L activity with no significant changes in systems A and ASC. The Vmax for L-leucine uptake exhibited by the hybrid 158CnpT-1 was twice that in the CHO parental mutant, ts025C1. Cytogenetic analysis showed that the hybrid 158CnpT-1 contains four complete human chromosomes (numbers 4, 5, 10, and 21) and three interspecific chromosomal translocations in a total complement of 34 chromosomes. Biochemical and cytogenetic analysis of segregant clones obtained from hybrid 158CnpT-1 showed that the primary temperature resistance and high system L transport phenotypes can be segregated from this hybrid independently. The loss of the primary temperature resistance was associated with the loss of the human chromosome 5, as previously reported by other laboratories, whereas the loss of the high leucine transport phenotype, which is associated with a lesser degree of temperature resistance, was correlated with the loss of human chromosome 20.

Characterization of a Chinese hamster-human hybrid cell line with increased system L amino acid transport activity

We have studied leucine transport in several Chinese hamster-human hybrid cell lines obtained by fusion of a temperature-sensitive line of Chinese hamster ovary cells, ts025C1, and normal human leukocytes. A hybrid cell line exhibiting a twofold increase in L-leucine uptake over that in the parental cell line was found. This hybrid cell line, 158CnpT-1, was temperature resistant, whereas the parental Chinese hamster ovary mutant, ts025C1, contained a temperature-sensitive leucyl-tRNA synthetase mutation. An examination of the different amino acid transport systems in this hybrid cell line revealed a specific increase of system L activity with no significant changes in systems A and ASC. The Vmax for L-leucine uptake exhibited by the hybrid 158CnpT-1 was twice that in the CHO parental mutant, ts025C1. Cytogenetic analysis showed that the hybrid 158CnpT-1 contains four complete human chromosomes (numbers 4, 5, 10, and 21) and three interspecific chromosomal translocations in a total complement of 34 chromosomes. Biochemical and cytogenetic analysis of segregant clones obtained from hybrid 158CnpT-1 showed that the primary temperature resistance and high system L transport phenotypes can be segregated from this hybrid independently. The loss of the primary temperature resistance was associated with the loss of the human chromosome 5, as previously reported by other laboratories, whereas the loss of the high leucine transport phenotype, which is associated with a lesser degree of temperature resistance, was correlated with the loss of human chromosome 20.

Fluctuation analyses of spontaneous mutations to 6-thioguanine resistance in Chinese hamster ovary cells in culture

Fluctuation analyses of the spontaneous appearance of 6-thioguanine (TG)-resistant mutants in cultured Chinese hamster ovary (CHO) cells were performed to investigate (1) whether the resistance is induced by the selective agent or is the result of a mutation which occurs prior to the TG selection and (2) to estimate the spontaneous mutation rate at the hypoxanthine--guanine phosphoribosyl transferase (hgprt) locus. The potential problem of phenotypic delay was minimized by allowing an adequate expression time through maintenance of the cultures in a division-arrested, viable state. The results demonstrate that the TG-resistant (TGr) cells arise randomly in the cultures, independently of the selective agent, which is consistent with spontaneous mutations. The average values for mutation rate +/- standard deviation, based on 4 independent determinations and 2 methods of calculation, are 3.4 +/- 1.2 X 10(-7) (median method) and 5.1 +/- 1.8 X 10(-7) (mean method) mutants/cell/generation.

Assignment of a human genetic locus to chromosome 5 which corrects the heat sensitive lesion associated with reduced leucyl-tRNA synthetase activity in ts025Cl Chinese hamster cells

A heat-sensitive (hs) leucyl-tRNA synthetase (leuRS) deficient CHO mutant, ts025Cl, was fused with human leukocytes and hybrids isolated in HAT medium at the nonpermissive temperature. Nineteen heat-resistant (hr) and 14 hs subclones were isolated from four independent primary hybrids and tested for the expression of 24 human isozymes which have been assigned to 17 human chromosomes. Four hr independent subclones and three hs independent subclones were analyzed for the presence of human chromosomes. A pattern of concordant segregation was noted for the hr phenotype, human hexosaminidase B (EC 3.2.1.30) and human chromosome 5. Based on these results, we have defined the human genetic locus which corrects the heat-sensitive lesion in ts025Cl as hr025Cl and have assigned this locus to human chromosome 5. Two hr hybrids exhibited leuRS activity 2.5 and 4 times the leuRS activity of ts025Cl but a wild-type level of activity was not restored. One hs hybrid had only 73% of the leuRS activity exhibited by ts025Cl while another hs hybrid had 1.8 times the leuRS activity of ts025Cl.

Selection by [3H] amino acids of CHO-cell mutants with altered leucyl- and asparagyl-transfer RNA synthetases

Efficient selection procedures, using [3H]amino acids as the selecting agent, were developed for isolating temperature-sensitive (TS) mutations in CHO cells affecting protein synthesis. After chemical mutagenesis, leucyl-tRNA synthetase mutants were obtained when [3H]leucine was used as the selecting agent in two independent experiments. These mutations seem to involve the same genetic locus as the TSH1 mutant described previously (1). A selection with [3H]valine, in which all amino acids except leucine were at low concentration in the selective medium, resulted in a new class of mutants with reduced asparagyl-tRNA synthetase activity. These results were consistent with the finding that all mutants were phenotypically dependent on the concentration of amino acid, specific to the altered synthetase, in the medium. Our observations suggest that although leucyl synthetase mutations are a relatively common class of TS mutations in CHO cells, the spectrum of mutants obtained can be at least partially manipulated through concentrations of amino acids in selective media. The asparagyl-synthetase mutation was shown to be recessive and to complement the leucyl-synthetase mutation in cell-cell hybrids.