Evidence for structural gene alterations affecting aminoacyl-tRNA synthetases in CHO cell mutants and revertants

An important role for the multienzyme aminoacyl-tRNA synthetase complex in mammalian translation and cell growth

In mammalian cells, aminoacyl-tRNA synthetases (aaRSs) are organized into a high-molecular-weight multisynthetase complex whose cellular function has remained a mystery. In this study, we have taken advantage of the fact that mammalian cells contain two forms of ArgRS, both products of the same gene, to investigate the complex's physiological role. The data indicate that the high-molecular-weight form of ArgRS, which is present exclusively as an integral component of the multisynthetase complex, is essential for normal protein synthesis and growth of CHO cells even when low-molecular-weight, free ArgRS is present and Arg-tRNA continues to be synthesized at close to wild-type levels. Based on these observations, we conclude that Arg-tRNA generated by the synthetase complex is a more efficient precursor for protein synthesis than Arg-tRNA generated by free ArgRS, exactly as would be predicted by the channeling model for mammalian translation.

Temperature-sensitive Chinese hamster fibroblast mutant with a defect in RNA metabolism

We describe a new temperature-sensitive mutant of Chinese hamster cell fibroblasts. After a shift to the nonpermissive temperature of 40.5 degrees C, the rates of DNA, RNA, and protein synthesis declined rapidly (to < or = 50% within 12 h) and the progression of unsynchronized cells through the cell cycle was affected. We believe that DNA synthesis came to a halt after a short time, because cells no longer entered the S phase. The decrease in protein synthesis at 40.5 degrees C was shown to be a consequence of a decrease in the number of polysomes, whereas free 80S ribosomes accumulated. We concluded that the components of the protein biosynthetic machinery were intact (ribosomes and soluble factors), but synthesis was limited by a shortage of mRNA. The decline in mRNA production had a significant effect on the synthesis of proteins (e.g., heat shock proteins) translated from short-lived messages. We observed that both polyadenylated and nonpolyadenylated RNA syntheses declined at 40.5 degrees C, whereas the synthesis of small RNAs (4 to 5S) was less reduced. The argument is made that the temperature-sensitive phenotype is the result of a defect affecting mRNA synthesis.

The tRNA-dependent activation of arginine by arginyl-tRNA synthetase requires inter-domain communication

The tRNA-dependent amino acid activation catalyzed by mammalian arginyl-tRNA synthetase has been characterized. A conditional lethal mutant of Chinese hamster ovary cells that exhibits reduced arginyl-tRNA synthetase activity (Arg-1), and two of its derived revertants (Arg-1R4 and Arg-1R5) were analyzed at the structural and functional levels. A single nucleotide change, resulting in a Cys to Tyr substitution at position 599 of arginyl-tRNA synthetase, is responsible for the defective phenotype of the thermosensitive and arginine hyper-auxotroph Arg-1 cell line. The two revertants have a single additional mutation resulting in a Met222 to Ile change for Arg-1R4 or a Tyr506 to Ser change for Arg-1R5. The corresponding mutant enzymes were expressed in yeast and purified. The Cys599 to Tyr mutation affects both the thermal stability of arginyl-tRNA synthetase and the kinetic parameters for arginine in the ATP-PP(i) exchange and tRNA aminoacylation reactions. This mutation is located underneath the floor of the Rossmann fold catalytic domain characteristic of class 1 aminoacyl-tRNA synthetases, near the end of a long helix belonging to the alpha-helix bundle C-terminal domain distinctive of class 1a synthetases. For the Met222 to Ile revertant, there is very little effect of the mutation on the interaction of arginyl-tRNA synthetase with either of its substrates. However, this mutation increases the thermal stability of arginyl-tRNA synthetase, thereby leading to reversion of the thermosensitive phenotype by increasing the steady-state level of the enzyme in vivo. In contrast, for the Arg-1R5 cell line, reversion of the phenotype is due to an increased catalytic efficiency of the C599Y/Y506S double mutant as compared to the initial C599Y enzyme. In light of the location of the mutations in the 3D structure of the enzyme modeled using the crystal structure of the closely related yeast arginyl-tRNA synthetase, the kinetic analysis of these mutants suggests that the obligatory tRNA-induced activation of the catalytic site of arginyl-tRNA synthetase involves interdomain signal transduction via the long helices that build the tRNA-binding domain of the enzyme and link the site of interaction of the anticodon domain of tRNA to the floor of the active site.

Histidyl-tRNA synthetase

Histidyl-tRNA synthetase (HisRS) is responsible for the synthesis of histidyl-transfer RNA, which is essential for the incorporation of histidine into proteins. This amino acid has uniquely moderate basic properties and is an important group in many catalytic functions of enzymes. A compilation of currently known primary structures of HisRS shows that the subunits of these homo-dimeric enzymes consist of 420-550 amino acid residues. This represents a relatively short chain length among aminoacyl-tRNA synthetases (aaRS), whose peptide chain sizes range from about 300 to 1100 amino acid residues. The crystal structures of HisRS from two organisms and their complexes with histidine, histidyl-adenylate and histidinol with ATP have been solved. HisRS from Escherichia coli and Thermus thermophilus are very similar dimeric enzymes consisting of three domains: the N-terminal catalytic domain containing the six-stranded antiparallel beta-sheet and the three motifs characteristic of class II aaRS, a HisRS-specific helical domain inserted between motifs 2 and 3 that may contact the acceptor stem of the tRNA, and a C-terminal alpha/beta domain that may be involved in the recognition of the anticodon stem and loop of tRNA(His). The aminoacylation reaction follows the standard two-step mechanism. HisRS also belongs to the group of aaRS that can rapidly synthesize diadenosine tetraphosphate, a compound that is suspected to be involved in several regulatory mechanisms of cell metabolism. Many analogs of histidine have been tested for their properties as substrates or inhibitors of HisRS, leading to the elucidation of structure-activity relationships concerning configuration, importance of the carboxy and amino group, and the nature of the side chain. HisRS has been found to act as a particularly important antigen in autoimmune diseases such as rheumatic arthritis or myositis. Successful attempts have been made to identify epitopes responsible for the complexation with such auto-antibodies.

Amino acid-dependent control of p70(s6k). Involvement of tRNA aminoacylation in the regulation

In human T-lymphoblastoid cells, downstream signaling events of mammalian target of rapamycin (mTOR), including the activity of p70(s6k) and phosphorylation of eukaryotic initiation factor 4E-binding protein 1, were dependent on amino acid concentration in the culture media, whereas other growth-related protein kinases were not. Amino acid-induced p70(s6k) activation was completely inhibited by rapamycin but only partially inhibited by wortmannin. Moreover, amino acid concentration similarly affected the p70(s6k) activity, which was dependent on a rapamycin-resistant mutant (S2035I) of mTOR. These data indicate that mTOR is required for amino acid-dependent activation of p70(s6k). The mechanism by which amino acids regulate p70(s6k) activity was further explored: 1) amino acid alcohols, which inhibit aminoacylation of tRNA by their competitive binding to tRNA synthetases, suppressed p70(s6k) activity; 2) suppression of p70(s6k) by amino acid depletion was blocked by cycloheximide or puromycin, which inhibit utilization of aminoacylated tRNA in cells; and 3) in cells having a temperature-sensitive mutant of histidyl tRNA synthetase, p70(s6k) was suppressed by a transition of cells to a nonpermissible temperature, which was partially restored by addition of high concentrations of histidine. These results indicate that suppression of tRNA aminoacylation is able to inhibit p70(s6k) activity. Deacylated tRNA may be a factor negatively regulating p70(s6k).

Cloning and analysis of a cDNA encoding mammalian arginyl-tRNA synthetase, a component of the multisynthetase complex with a hydrophobic N-terminal extension

In mammalian cells, the nine aminoacyl-tRNA synthetases (aaRS) specific for the amino acids (aa) Glu, Pro, Ile, Leu, Met, Gln, Lys, Arg and Asp are associated within a multienzyme complex. Arginyl-tRNA synthetase (ArgRS) is characterized by the occurrence of two structurally distinct forms of that enzyme: a complexed (approximately 74 kDa) and a free (approximately 60 kDa) form. The cDNA encoding the 74-kDa species of ArgRS from Chinese hamster ovary cells has been isolated and sequenced. The deduced aa sequence shows 38% identity to the homologous bacterial enzyme but displays an N-terminal polypeptide extension composed of 73 aa, which is absent in the free form of mammalian ArgRS. Two regions of this extension are predicted to be alpha-helical, leading to the clustering of Leu and Ile residues on one side of the helices. This suggests that the N-terminal domain is involved in the assembly of the 74-kDa species of ArgRS within the multisynthetase complex through hydrophobic interactions. By using the isolated cDNA, a Northern blot analysis showed a single mRNA species. Thus, there is a possibility that the free and complexed forms of ArgRS are encoded by the same gene.

A novel role for aminoacyl-tRNA synthetases in the regulation of polypeptide chain initiation

Exposure of the temperature-sensitive leucyl-tRNA synthetase mutant of Chinese hamster ovary cells, tsH1, to the non-permissive temperature of 39.5 degrees C results in a rapid inhibition of polypeptide chain initiation. This inhibition is caused by a reduced ability of the eukaryotic initiation factor eIF-2 to participate in the formation of eIF-2.GTP.Met-tRNAf ternary complexes and thus in the formation of 43S ribosomal pre-initiation complexes. Associated with this decreased eIF-2 activity is an increased phosphorylation of the eIF-2 alpha subunit. It has previously been shown in other systems that phosphorylation of eIF-2 alpha slows the rate of recycling of eIF-2.GDP to eIF-2.GTP catalysed by the guanine nucleotide exchange factor eIF-2B. We show here that phosphorylation of eIF-2 alpha by the reticulocyte haem-controlled repressor also inhibits eIF-2B activity in cell-free extracts derived from tsH1 cells. Thus the observed increased phosphorylation of eIF-2 alpha at the non-permissive temperature in this system is consistent with impaired recycling of eIF-2 in vivo. Using a single-step temperature revertant of tsH1 cells, TR-3 (which has normal leucyl-tRNA synthetase activity at 39.5 degrees C), we demonstrate here that all inhibition of eIF-2 function reverts together with the synthetase mutation. This establishes the close link between synthetase function and eIF-2 activity. In contrast, recharging tRNALeu in vivo in tsH1 cells at 39.5 degrees C by treatment with a low concentration of cycloheximide failed to reverse the inhibition of eIF-2 function. This indicates that tRNA charging per se is not involved in the regulatory mechanism. Our data indicate a novel role for aminoacyl-tRNA synthetases in the regulation of eIF-2 function mediated through phosphorylation of the alpha subunit of this factor. However, in spite of the fact that cell-free extracts from Chinese hamster ovary cells contain protein kinase and phosphatase activities active against either exogenous or endogenous eIF-2 alpha, we have been unable to show any activation of kinase or inactivation of phosphatase following incubation of the cells at 39.5 degrees C.

Characterization of revertants derived from a mouse DNA temperature-sensitive mutant strain, tsFT20, which contains heat-labile DNA polymerase alpha activity

One spontaneous and four N-methyl-N'-nitro-N-nitrosoguanidine-induced revertants of a mouse FM3A mutant, tsTF20, which has heat-labile DNA polymerase alpha activity and cannot grow at 39 degrees C, were isolated and characterized with respect to the thermolability of their DNA polymerase alpha activity, the intracellular level of enzyme activity, growth rate, cell cycle progression, and frequency of initiation of DNA replication at the origin of replicons. DNA polymerase alpha activity in the extracts from the revertant cells showed partial recovery of heat stability. The intracellular level of enzyme activity of the revertant cells was lower than that of wild-type cells even at 33 degrees C. The level of enzyme activity in the revertant cells decreased considerably after a temperature upshift to 39 degrees C, but the DNA synthesizing ability of these cells did not decrease as much as the level of enzyme activity. The growth rates of the wild-type and revertant lines were almost the same at 33 degrees C. At 39 degrees C, the rate for the wild-type increased considerably compared to that at 33 degrees C, while little difference in the growth rates of the revertant lines was observed at the two temperatures. Therefore, the doubling times of the revertant cells were relatively increased compared to those of wild-type cells cultured at the restrictive temperature. Flow microfluorometric analysis and cell cycle analysis to measure labeled mitosis revealed that the increase in the doubling time was due mainly to the increase in the duration of the S phase. Analysis of the center-to-center distance between replicons by DNA fiber autoradiography indicated that the frequency of replicon initiation per unit length DNA at a given time was reduced in the revertant cells growing at 39 degrees C.

Isolation, structure and expression of mammalian genes for histidyl-tRNA synthetase

A full length cDNA clone that codes for human histidyl-tRNA synthetase (HRS) and cDNA clones that span the full length transcript of hamster HRS have been isolated. The full length human HRS cDNA was expressed after transfection into Cos 1 cells and a CHO ts mutant defective in the gene for HRS. The complete nucleotide sequence of the hamster and human gene were obtained and extensive homologies were observed in three regions on comparing these sequences between themselves and with the sequence of HRS derived from yeast. These results provide unequivocal evidence that we have indeed cloned the hamster and human gene for HRS. Three overlapping phage recombinants containing the complete hamster chromosomal gene for HRS have also been isolated. The genomic HRS is divided into 13 exons. The precise locations of each of the 5' and 3' exon-intron boundaries were defined by sequencing the appropriate regions of the cloned genomic DNA and aligning them with the sequence of HRS cDNAs. These studies provide the basis for future structural and functional analysis of the gene for HRS. In particular, it will be of interest to examine if different exons of HRS correlate to different domains of the HRS polypeptide.

Amplification of the gene for histidyl-tRNA synthetase in histidinol-resistant Chinese hamster ovary cells

Histidinol-resistant (HisOHR) mutants with up to a 30-fold increase in histidyl-tRNA synthetase activity have been isolated by stepwise adaptation of wild-type Chinese hamster ovary (CHO) cells to increasing amounts of histidinol in the medium. Immunoprecipitation of [35S]methionine-labeled cell lysates with antibodies to histidyl-tRNA synthetase showed increased synthesis of the enzyme in histidinol-resistant cells. The histidinol-resistant cell lines had an increase in translatable polyadenylated mRNA for histidyl-tRNA synthetase. A cDNA for CHO histidyl-tRNA synthetase has been cloned, using these histidyl-tRNA synthetase-overproducing mutants as the source of mRNA. Southern blot analysis of wild-type and histidinol-resistant cells with this cDNA showed that the histidyl-tRNA synthetase DNA bands were amplified in the resistant cells. These HisOHR cells owed their resistance to histidinol to amplification of the gene for histidyl-tRNA synthetase.

Amplification of the gene for histidyl-tRNA synthetase in histidinol-resistant Chinese hamster ovary cells

Histidinol-resistant (HisOHR) mutants with up to a 30-fold increase in histidyl-tRNA synthetase activity have been isolated by stepwise adaptation of wild-type Chinese hamster ovary (CHO) cells to increasing amounts of histidinol in the medium. Immunoprecipitation of [35S]methionine-labeled cell lysates with antibodies to histidyl-tRNA synthetase showed increased synthesis of the enzyme in histidinol-resistant cells. The histidinol-resistant cell lines had an increase in translatable polyadenylated mRNA for histidyl-tRNA synthetase. A cDNA for CHO histidyl-tRNA synthetase has been cloned, using these histidyl-tRNA synthetase-overproducing mutants as the source of mRNA. Southern blot analysis of wild-type and histidinol-resistant cells with this cDNA showed that the histidyl-tRNA synthetase DNA bands were amplified in the resistant cells. These HisOHR cells owed their resistance to histidinol to amplification of the gene for histidyl-tRNA synthetase.

Isolation and characterization of revertants from four different classes of aryl hydrocarbon hydroxylase-deficient hepa-1 mutants

Revertants were selected from aryl hydrocarbon hydroxylase (AHH)-deficient recessive mutants belonging to three complementation groups and from a dominant mutant of the Hepa-1 cell line. The recessive mutants had low spontaneous reversion frequencies (less than 4 X 10(-7] that were increased by mutagenesis. The majority of these revertants also had reacquired only partial AHH activity. Revertants of group A mutants were identical to the wild type with respect to both in vivo and in vitro enzyme stability and the Km for the substrate, benzo [alpha]pyrene, and therefore failed to provide evidence that gene A is the AHH structural gene. Group B and group C mutants are defective in the functioning of the Ah receptor required for AHH induction. Revertants of these groups were normal with respect to in vivo temperature sensitivity for AHH induction and for the 50% effective dose for the inducer, 2,3,7,8-tetrachlorodibenzo-p-dioxin, and thus provided no evidence that the B and C genes code for components of the receptor. Two rare group C revertants possessed AHH activity in the absence of induction. The phenotype of one of these was shown to be recessive to the wild type. Spontaneous revertants of the dominant mutant occurred at a frequency 300-fold greater than those of the recessive mutants, and this frequency was not increased by mutagenesis. These revertants all displayed complete restoration of AHH activity to wild type levels. These observations and the results from cell hybridization studies suggest that the dominant revertants arose by a high frequency event leading to functional elimination of the dominant mutation.

Isolation and characterization of revertants from four different classes of aryl hydrocarbon hydroxylase-deficient hepa-1 mutants

Revertants were selected from aryl hydrocarbon hydroxylase (AHH)-deficient recessive mutants belonging to three complementation groups and from a dominant mutant of the Hepa-1 cell line. The recessive mutants had low spontaneous reversion frequencies (less than 4 X 10(-7] that were increased by mutagenesis. The majority of these revertants also had reacquired only partial AHH activity. Revertants of group A mutants were identical to the wild type with respect to both in vivo and in vitro enzyme stability and the Km for the substrate, benzo [alpha]pyrene, and therefore failed to provide evidence that gene A is the AHH structural gene. Group B and group C mutants are defective in the functioning of the Ah receptor required for AHH induction. Revertants of these groups were normal with respect to in vivo temperature sensitivity for AHH induction and for the 50% effective dose for the inducer, 2,3,7,8-tetrachlorodibenzo-p-dioxin, and thus provided no evidence that the B and C genes code for components of the receptor. Two rare group C revertants possessed AHH activity in the absence of induction. The phenotype of one of these was shown to be recessive to the wild type. Spontaneous revertants of the dominant mutant occurred at a frequency 300-fold greater than those of the recessive mutants, and this frequency was not increased by mutagenesis. These revertants all displayed complete restoration of AHH activity to wild type levels. These observations and the results from cell hybridization studies suggest that the dominant revertants arose by a high frequency event leading to functional elimination of the dominant mutation.

Revertants of a trans-dominant S49 mouse lymphoma mutant that affects expression of cAMP-dependent protein kinase

Phenotypic revertants were isolated from an S49 mouse lymphoma tissue culture cell mutant that lacks cAMP-dependent protein kinase (cA-PK) activity (kin-). The mutant phenotype is trans-dominant and results from a lesion that probably lies outside the cA-PK subunit structural genes. The nature of the event that produces the kin- phenotype is unknown. However, the mechanism that is responsible for its behavior is genetically encoded because: spontaneous revertants arise at low frequency; reversion frequency is increased by mutagen treatment; mutagen-specific classes of revertant phenotypes are induced; and some revertants are temperature-sensitive for expression of cA-PK subunit polypeptides. Additional evidence is provided that argues against structural lesions in cA-PK catalytic (C) subunits as explanatory of the kin- phenotype. Kin- cells do not express an immunologically detectable C polypeptide, whereas C expression is restored in revertant cells. Revertants in which phenotype and cA-PK activity levels are only partially restored to that of wild-type cells contain a commensurately reduced amount of C polypeptide. Finally, the structure of C polypeptide in partial revertants is unaltered from that of wild-type C. The evidence supports the hypothesis that the kin- lesion defines a regulatory gene responsible for setting intracellular levels of cA-PK C subunit expression.

Reversion of an S49 cell cyclic AMP-dependent protein kinase structural gene mutant occurs primarily by functional elimination of mutant gene expression

The regulatory subunits of cyclic AMP (cAMP)-dependent protein kinase from a dibutyryl cAMP-resistant S49 mouse lymphoma cell mutant, clone U200/65.1, and its revertants were visualized by two-dimensional polyacrylamide gel electrophoresis. Clone U200/65.1 co-expressed electrophoretically distinguishable mutant and wild-type subunits (Steinberg et al., Cell 10:381-391, 1977). In all 48 clones examined, reversion of the mutant to dibutyryl cAMP sensitivity was accompanied by alterations in regulatory subunit labeling patterns. Some spontaneous (3 of 11) and N-methyl-N'-nitro-N-nitrosoguanidine-induced (2 of 11) revertants retained mutant subunits, but these were altered in charge, degree of phosphorylation, or both. The charge alterations were consistent with single amino acid substitutions, suggesting that reversion was the result of second-site mutations in the mutant regulatory subunit allele that restored wild-type function, although not wild-type structure, to the gene product. The majority of spontaneous (8 of 11) and N-methyl-N'-nitro-N-nitrosoguanidine-induced (9 of 11) revertants and all of the revertants induced by ethyl methane sulfonate (14 of 14) and ICR191 (12 of 12) displayed only wild-type subunits. Dibutyryl cAMP-resistant mutants isolated from several of these revertants displayed new mutant but not wild-type subunits, suggesting that the revertant parent expresses only a single, functional regulatory subunit allele. The mutant regulatory subunit allele can, therefore, be modified in two general ways to produce revertant phenotypes: (i) by mutations that restore its wild-type function, and (ii) by mutations that eliminate its function.

Revertants of an S49 cell mutant that expresses altered cyclic AMP-dependent protein kinase

Dibutyryl adenosine 3',5'-phosphate (Bt2cAMP)-sensitive (Bt2cAMPS) revertants were isolated from a resistant S49 cell mutant carrying a structural gene lesion in the regulatory subunit of cAMP-dependent protein kinase (cA-PK). This was accomplished with a counter-selection in which, first, Bt2cAMP was used to reversibly arrest revertants, and then a sequence of treatments with bromodeoxyuridine, 33258 Hoechst dye, and white light was used to kill cycling mutant cells. Reversion rates in nonmutagenized cultures could not be accurately measured, but spontaneous revertants do occur and with frequencies of less than 10(-7) to 10(-5). The mutagens ethyl methane sulfonate (EMS), N-methyl-N'-nitro-N-nitro-soguanidine (MNNG), and ICR191 increased the reversion frequency. In all cases, reversion to Bt2cAMP sensitivity was associated with restoration of wild-type levels and apparent activation constant for cAMP of cA-PK. MNNG induced revertants whose cell extracts contained cA-PK activity distinguishable from that of wild type by thermal liability. EMS did not. The counter-selection effectively isolates rare phenotypes and is therefore a useful tool in further somatic genetic experiments. The association of reversion with alterations in cA-PK function supports all previous data from this and other laboratories implicating cA-PK as the intracellular mediator of cAMP effects. Reversion is probably the result of a mutational event. Induction of reversion by ICR191 suggests the existence of a novel mechanism for generating revertants in somatic cells.

Reversion of an S49 cell cyclic AMP-dependent protein kinase structural gene mutant occurs primarily by functional elimination of mutant gene expression

The regulatory subunits of cyclic AMP (cAMP)-dependent protein kinase from a dibutyryl cAMP-resistant S49 mouse lymphoma cell mutant, clone U200/65.1, and its revertants were visualized by two-dimensional polyacrylamide gel electrophoresis. Clone U200/65.1 co-expressed electrophoretically distinguishable mutant and wild-type subunits (Steinberg et al., Cell 10:381-391, 1977). In all 48 clones examined, reversion of the mutant to dibutyryl cAMP sensitivity was accompanied by alterations in regulatory subunit labeling patterns. Some spontaneous (3 of 11) and N-methyl-N'-nitro-N-nitrosoguanidine-induced (2 of 11) revertants retained mutant subunits, but these were altered in charge, degree of phosphorylation, or both. The charge alterations were consistent with single amino acid substitutions, suggesting that reversion was the result of second-site mutations in the mutant regulatory subunit allele that restored wild-type function, although not wild-type structure, to the gene product. The majority of spontaneous (8 of 11) and N-methyl-N'-nitro-N-nitrosoguanidine-induced (9 of 11) revertants and all of the revertants induced by ethyl methane sulfonate (14 of 14) and ICR191 (12 of 12) displayed only wild-type subunits. Dibutyryl cAMP-resistant mutants isolated from several of these revertants displayed new mutant but not wild-type subunits, suggesting that the revertant parent expresses only a single, functional regulatory subunit allele. The mutant regulatory subunit allele can, therefore, be modified in two general ways to produce revertant phenotypes: (i) by mutations that restore its wild-type function, and (ii) by mutations that eliminate its function.

Altered aminoacyl-tRNA synthetase complexes in CHO cell mutants

The Chinese hamster ovary (CHO) aminoacyl-tRNA synthetase mutants Gln-2, His-1, and Lys-101 were analyzed for alterations in respective particulate enzyme forms. The mutant Gln-2 showed a preferential loss of the lower molecular weight enzyme form for glutamine. His-1 showed alterations of the enzyme complexes for several other aminoacyl-tRNA activities but only decreased activity for itself. The mutant Lys-101 only showed an altered Lysyl-tRNA synthetase. These results provide evidence for a model of the intracellular role of the aminoacyl-tRNA synthetase complexes wherein the high molecular weight forms utilize amino acids directly from the extracellular pool while the low molecular weight forms utilize intracellular pools.

Temperature-Sensitive Chinese Hamster Fibroblast Mutant with a Defect in RNA Metabolism

We describe a new temperature-sensitive mutant of Chinese hamster cell fibroblasts. After a shift to the nonpermissive temperature of 40.5°C, the rates of DNA, RNA, and protein synthesis declined rapidly (to ≤50% within 12 h) and the progression of unsynchronized cells through the cell cycle was affected. We believe that DNA synthesis came to a halt after a short time, because cells no longer entered the S phase. The decrease in protein synthesis at 40.5°C was shown to be a consequence of a decrease in the number of polysomes, whereas free 80S ribosomes accumulated. We concluded that the components of the protein biosynthetic machinery were intact (ribosomes and soluble factors), but synthesis was limited by a shortage of mRNA. The decline in mRNA production had a significant effect on the synthesis of proteins (e.g., heat shock proteins) translated from short-lived messages. We observed that both polyadenylated and nonpolyadenylated RNA syntheses declined at 40.5°C, whereas the synthesis of small RNAs (4 to 5S) was less reduced. The argument is made that the temperature-sensitive phenotype is the result of a defect affecting mRNA synthesis.

Revertants of an S49 cell mutant that expresses altered cyclic AMP-dependent protein kinase

Dibutyryl adenosine 3',5'-phosphate (Bt2cAMP)-sensitive (Bt2cAMPS) revertants were isolated from a resistant S49 cell mutant carrying a structural gene lesion in the regulatory subunit of cAMP-dependent protein kinase (cA-PK). This was accomplished with a counter-selection in which, first, Bt2cAMP was used to reversibly arrest revertants, and then a sequence of treatments with bromodeoxyuridine, 33258 Hoechst dye, and white light was used to kill cycling mutant cells. Reversion rates in nonmutagenized cultures could not be accurately measured, but spontaneous revertants do occur and with frequencies of less than 10(-7) to 10(-5). The mutagens ethyl methane sulfonate (EMS), N-methyl-N'-nitro-N-nitro-soguanidine (MNNG), and ICR191 increased the reversion frequency. In all cases, reversion to Bt2cAMP sensitivity was associated with restoration of wild-type levels and apparent activation constant for cAMP of cA-PK. MNNG induced revertants whose cell extracts contained cA-PK activity distinguishable from that of wild type by thermal liability. EMS did not. The counter-selection effectively isolates rare phenotypes and is therefore a useful tool in further somatic genetic experiments. The association of reversion with alterations in cA-PK function supports all previous data from this and other laboratories implicating cA-PK as the intracellular mediator of cAMP effects. Reversion is probably the result of a mutational event. Induction of reversion by ICR191 suggests the existence of a novel mechanism for generating revertants in somatic cells.

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.

Relationship between histidyl-tRNA level and protein synthesis rate in wild-type and mutant Chinese hamster ovary cells

A preliminary investigation was carried out to determine how conditional lethal mutants affected in particular aminoacyl-tRNA synthetases may be used to study the role of tRNA charging levels in protein synthesis. The relationship between rate of protein synthesis and level of histidyl-tRNA in wild-type cultured Chinese hamster ovary cells was determined using the analogue histidinol to inhibit histidyl-tRNA synthetase activity. This response was compared with that obtained using a mutant strain with a defective histidyl-tRNA synthetase that phenotypically shows decreased rates of protein synthesis at reduced concentrations of histidine in the growth medium. The approach used was based on measuring the histidyl-tRNA levels in live cells. The percentage charging was estimated by comparing [14C]histidine incorporated into alkali-labile material in paired samples, one of which was treated with cycloheximide, five minutes before terminating during the incubation, to produce maximal aminoacylation. Wild-type cells under histidinol inhibition exhibited a sensitive, sigmoidal relationship between the level of histidyl-tRNA and the rate of protein synthesis. A decrease in the relative percentage of acylated tRNA (His) from 46% to 35% elicited a large reduction in the rate of protein synthesis from 90% to 30% relative to untreated cells. An unpredicted result was that the relationship between protein synthesis and histidyl-tRNA in the mutant was essentially linear. High acylation values for tRNA (His) were associated with rates of protein synthesis that were not nearly as high as in wild-type cells. These findings suggest that the charging charging levels of tRNA (His) isoacceptors could play a regulatory role in determining the rate of protein synthesis under conditions of histidine starvation in normal cells. The mutant appears to be a potentially useful system for studying the pivotal role of tRNA charging in protein synthesis, assuming that the altered response in the mutant is caused by its altered synthetase.