Regulatory significance of transfer RNA charging levels. I. Measurements of charging levels in livers of chow-fed rats, fasting rats, and rats fed balanced or imbalanced mixtures of amino acids

Plasma L-Carnitine and L-Lysine Concentrations in HIV-Infected Patients

Background:

Virus infections are associated with significant alterations in host cells amino acids profiles that support biosynthetic demands necessary for production of viral progeny. Amino acids play an important role in the pathogenesis of all virus-related infections both as basic substrates for protein synthesis and as regulators in many metabolic pathways.

Objective:

Our aim was to determine the changes in plasma L-carnitine levels and its amino acid precursor (L-lysine) in HIV-infected patients.

Methods:

We performed a case-control study of 430 HIV-1 infected males (non-vegetarians) without any restriction in the nourishment, before highly active antiretroviral therapy (HAART) and 125 HIV-1 subjects after the introduction of HAART who were periodically monitored in the Municipal Center of HIV/AIDS prophylaxis, Surgut, Russian Federation

Results:

The plasma total (TC) and free (FC) L-carnitine concentrations markedly decreased with the clinical stages of HIV infection. The mean plasma TC, FC and L-lysine levels were significantly lower in asymptomatic stage (A) and advanced CDC stages (B, C) HIV-infected patients compared with our reference values. The total and free L-carnitine and its amino acid precursor concentrations mild increased in HIV-infected subjects after the introduction of HAART.

Our data revealed that L-lysine amino acid and its derivative (TC) levels were negatively correlated with viral load and inversely with CD4 count lymphocytes in the total cohort.

Conclusion:

The study results show that there was evidence for an association between plasma L-carnitine, L-lysine and HIV-1 RNA levels, immunological markers and clinical stages of HIV infection. The obtained data indicate that level changes of these host essential nutritional elements can play an important role in the HIV life cycle. These findings are important for understanding the pathophysiology of HIV infection and must be considered in further research for the development of new approaches in the treatment of the disease.

Influence of L-lysine amino acid on the HIV-1 RNA replication in vitro

BACKGROUND

Virus replication strongly depends on host metabolic machinery and essential cellular factors, in particular, on amino acid profiles. Amino acids play an important role in the pathogenesis of all virus-related infections both as basic substrates for protein synthesis and as regulators in many metabolic pathways, including gene expression. The inhibitory effects of deficiency or excess of these essential elements on virus replication are widely appreciated. Although the same interrelationship between host cellular factors and HIV have been recognized for a long time, the effects of amino acids on HIV-1 RNA replication dynamic is not yet well documented. Our aim was to determine in this pilot study the direct effect of L-lysine amino acid on HIV-1 RNA replication in vitro in HIV-infected patients.

METHODS

A total of 100 HIV-1-infected males without highly active antiretroviral therapy (HAART) were monitored in our center. The patients were in stage A of the disease according to the 1993 Centers for Disease Control (CDC) classification system for HIV-infection. Patients with HIV were enrolled in one stage (A) of the disease with the average amount CD4 lymphocytes in the range of 200-300 cells/µL at the time of sample acquisition. For evaluation of the effects of essential L-lysine amino acid on HIV-1 RNA replication level, we used a model of amino acid-excess system in vitro following incubation of plasma samples for 24 h at 25 °C. Quantitative HIV-1 RNA assay was performed using (RT-PCR) reverse-transcriptase polymerase chain reaction (Rotor-Gene Q, QIAGEN, Germany).

RESULTS

The mean HIV-1 RNA levels were significantly higher in the enriched peripheral blood mononuclear cells plasma samples HIV-infected subjects after 24 h incubation at 25 °C temperature than in the plasma samples the same patients studied on the date of blood tests (p < 0.0001). The number of HIV-1 RNA copies increased in 1.5 times. We observed that in plasma of the same HIV-infected patients after adding L-lysine and following incubation in vitro, viral load increased significantly in comparison with standard samples (p < 0.0001). The increased viral load was found in 100/92 (92%) of HIV-infected subjects. The average number of HIV-1 RNA copies in samples had increased by 4.0 times. However, we found no difference in HIV-1 RNA levels after replacement of L-lysine for L-arginine in comparison samples in the same HIV-infected patients. It is obvious that the addition of L-arginine does not increase viral replication in vitro as L-lysine amino acid supplement does. Additionally, no increase in viral load was determined after adding L-lysine and non toxic doses of its inhibitor (L-lysine alpha-oxidase) in plasma samples.

CONCLUSIONS

The results show that L-lysine amino acid excess is characterized by significant increased of HIV-1 RNA copies in enriched peripheral blood mononuclear cells plasma samples of HIV-infected patients. There was evidence for an association between L-lysine supplementation and HIV-1 RNA replication and the level changes of this host essential nutritional element play a key role in the synthesis of the virus proteins and in transcription initiation of the retrovirus life cycle. High intake of L-lysine amino acid may increase the risk of high viral load, subsequent acceleration of immunosuppression and HIV progression. Overall results demonstrate that the simple L-lysine-related model in vitro can be widely used for practical purposes to evaluate HIV-1 RNA replication dynamic, disease prognosis and new approaches in treatment of the patients with human immunodeficiency virus. Although the impact mechanism of L-lysine amino acid on the viral load in the pathogenesis of HIV-infection is at present conjectural and requires further development, the results highlight an interesting target in antiviral therapy, and this statement remains to be proved in further research and clinical trials.

The protein synthesis machinery operates at the same expense in eurythermal and cold stenothermal pectinids

Translationally active cell-free systems from gills of the Antarctic scallop Adamussium colbecki and the European scallop Aequipecten opercularis were developed, characterised, and optimised for an analysis of translational capacity. The aim was to determine the energetic cost of protein synthesis in the in vitro cell-free system by directly measuring the required energy equivalents in the lysates. Protein synthesis rate in assays conducted with lysates of A. colbecki (1.029+/-0.061 micromol Phe min(-1) at 15 degrees C; Phe=phenylalanine) were higher compared with lysates of A. opercularis (0.087+/-0.013 micromol Phe min(-1) at 15 degrees C and 0.156+/-0.023 micromol Phe min(-1) at 25 degrees C). This can in part be attributed to the naturally occurring higher RNA content in lysates of A. colbecki (0.883+/-0.037 mg RNA mL(-1) lysate) compared with A. opercularis (0.468+/-0.013 mg RNA mL(-1) lysate). There was no significant difference in the energetic costs of protein synthesis in cell-free systems of gill lysates of the cold stenothermal A. colbecki with 4.3+/-0.7 energy equivalents per peptide bond formed and the eurythermal A. opercularis with 5.6+/-0.6 energy equivalents, indicating that there are no differences in the efficiency of the translation machinery. The energetic costs specified for protein synthesis correspond with the generally accepted theoretical value of four energy equivalents per peptide bond formed, especially in gill lysates of A. colbecki, whereas the value for gill lysates of A. opercularis was slightly higher.

F-actin sequesters elongation factor 1alpha from interaction with aminoacyl-tRNA in a pH-dependent reaction

The machinery of eukaryotic protein synthesis is found in association with the actin cytoskeleton. A major component of this translational apparatus, which is involved in the shuttling of aa-tRNA, is the actin-binding protein elongation factor 1alpha (EF-1alpha). To investigate the consequences for translation of the interaction of EF-1alpha with F-actin, we have studied the effect of F-actin on the ability of EF-1alpha to bind to aa-tRNA. We demonstrate that binding of EF-1alpha:GTP to aa-tRNA is not pH sensitive with a constant binding affinity of approximately 0.2 microM over the physiological range of pH. However, the sharp pH dependence of binding of EF-1alpha to F-actin is sufficient to shift the binding of EF-1alpha from F-actin to aa-tRNA as pH increases. The ability of EF-1alpha to bind either F-actin or aa-tRNA in competition binding experiments is also consistent with the observation that EF-1alpha's binding to F-actin and aa-tRNA is mutually exclusive. Two pH-sensitive actin-binding sequences in EF-1alpha are identified and are predicted to overlap with the aa-tRNA-binding sites. Our results suggest that pH-regulated recruitment and release of EF-1alpha from actin filaments in vivo will supply a high local concentration of EF-1alpha to facilitate polypeptide elongation by the F-actin-associated translational apparatus.

Essential amino acids regulate fatty acid synthase expression through an uncharged transfer RNA-dependent mechanism

To better understand the regulation of gene expression by amino acids, we studied the effects of these macronutrients on fatty acid synthase (FAS), an enzyme crucial for energy storage. When HepG2 cells were fed serum-free media selectively deficient in each amino acid, the omission of any single classic essential amino acid as well as Arg or His (essential in some rapidly growing cells) resulted in FAS mRNA levels that were about half of those in complete medium. Control message levels were unaffected and omission of nonessential amino acids did not alter FAS expression. FAS mRNA levels peaked 12-16 h after feeding complete and Ser (nonessential)-deficient media but did not increase in cells fed Lys (essential)-deficient medium. With Lys, FAS mRNA increased over the physiologic concentration range of 15-150 microM, and low concentrations of lysine decreased FAS but not apoB protein mass. Transcription inhibitors mimicked treatment with Lys-deficient media, and nuclear run-off assays showed that Lys-deficient media abolished FAS but not apoB transcription. After treatment with Lys-deficient media, the intracellular Lys pool was rapidly depleted in association with an increase of uncharged (deacylated) tRNA Lys from < 1 to 64% of available tRNA Lys. Even in the presence of the essential amino acid His, increasing the level of uncharged tRNA His with histidinol, a competitive inhibitor of the histidinyl-tRNA synthetase, blocked FAS expression. Tyrosinol treatment did not alter FAS mRNA levels. These results suggest that essential amino acids regulate FAS expression by altering uncharged tRNA levels, a novel mechanism for nutrient control of gene expression in mammalian cells.

Influence of valine flooding on channeling of valine into tissue pools and on protein synthesis

Rates of valine incorporation into protein were measured under control and valine-"flooding" conditions and included correction for the degree of recycling of unlabeled valine derived from the steady-state breakdown of tissue protein into the precursor pool (tRNA bound). The correction factor lambda, which is the ratio of the steady-state specific activity of valine in the tissue tRNA-bound pool to that in the arterial plasma, was determined for each of the tissues. In controls, values of lambda ranged from 0.31 in adrenals to 0.54 in heart; in flooded animals, values were higher, but only in liver was the value of lambda close to 1.0. In control and flooded rats, rates of protein synthesis were highest in liver and adrenals and lowest in skeletal muscle, with intermediate values in brain and heart. Flooding resulted in increased rates of protein synthesis in liver and decreased rates in adrenals. Rates of protein synthesis in brain, heart, and skeletal muscle were not statistically significantly affected by flooding.

Isolation of aminoacyl-tRNA and its labeling with stable-isotope tracers: Use in studies of human tissue protein synthesis

We isolated aminoacyl-tRNA (60-70% yield) from human and rat tissues and measured, by GC/MS, its labeling in vivo by [15N]- and [13C]leucine. Tracer dilution artifacts seemed unlikely since, after infusion of L-[1-13C,15N]leucine into rats, (i) muscle leucyl-tRNA labeling exceeded tissue free leucine labeling, (ii) values were largely unaffected by storing over 5 min at 22 degrees C, and (iii) L-[2,4,5-methyl-13C]leucine was not incorporated into leucyl-tRNA during homogenization. Leucyl-tRNA labeling in liver and muscle suggested charging from extra- and intracellular pools: e.g., after infusing L-[1-13C,15N]leucine, rat muscle tissue free leucine 13C labeling (8.97 +/- 0.30 atom % excess) exceeded that by 15N (3.37 +/- 0.33 atom % excess), and both were significantly lower (P less than 0.02) than venous plasma (13C, 12.1 +/- 1.8; 15N, 5.54 +/- 0.6 atom % excess) indicating tracer dilution by transamination and by proteolysis; however, leucyl-tRNA labeling by either isotope (13C, 10.26 +/- 0.50; 15N, 4.72 +/- 0.72 atom % excess) was significantly above mixed tissue free leucine (P less than 0.05). Labeling of leucyl-tRNA in human erector spinae muscle (obtained after preoperative L-[1-13C]leucine infusion) was, at 4.98 +/- 0.43 atom % excess, lower (27%) than venous plasma leucine (P less than 0.05) and intermediate between muscle free leucine (9% lower; P less than 0.01) and venous alpha-ketoisocaproate (11% higher; P less than 0.02). Human placental leucyl-tRNA labeling (after predelivery tracer infusion) was 37% lower (P less than 0.05) than maternal uterine vein labeling but not significantly different from placental free leucine or umbilical arterial leucine.

Effect of elective surgical procedures on tissue protein synthesis

The purpose of the study was to examine whether an early decrease in protein synthesis rates occurred in any tissues after abdominal surgery in postprandial rats. Leucine-specific radioactivity in mixed protein and on tRNA was determined after continuous infusion of L-[3H]leucine. Synthesis rates of mixed protein were significantly decreased in the gastrocnemius muscle, but not in the jejunum, liver, or heart, of rats 1-2 hr after splenectomy and ovariectomy or after combining the groups that had various types of abdominal surgery. These results suggest that a very early decrease in the protein synthesis rate of the gastrocnemius muscle occurs after laparotomy.

Actin synthesis rate and mRNA level increase during early recovery of atrophied muscle

The purpose of this study was to correlate actin synthesis rate and alpha-actin mRNA level in the gastrocnemius-plantaris muscles of limbs during their recovery from 7 days of immobilization in 200- to 280-g female rats. The fractional synthesis rate of actin in control muscle was 1%/day. Actin synthesis rate was 33% of control level at the 7th day of hindlimb immobilization, returned to control value at the 2nd recovery day, and was three times higher than control on the 4th day of recovery. The alpha-actin mRNA was 53% of control at the 7th day of immobilization, and its increase during the 1st 2 recovery days paralleled the increase in actin synthesis rate; this suggests that pretranslational mechanisms caused the initial increase in actin synthesis. Further increases in actin synthesis from the 2nd to the 4th day appear to be under translational control, since actin synthesis was 300% of control on the 4th recovery day and alpha-actin mRNA was only 128% of control.

A positive relationship between protein synthetic rate and intracellular glutamine concentration in perfused rat skeletal muscle

During muscle-protein wasting associated with injury and disease the distribution ratio of free glutamine between muscle and blood falls. In pursuing possible consequences of this, we investigated the relationship between the rate of muscle protein synthesis and intramuscular glutamine concentration, manipulated acutely in the isolated perfused rat hindquarter. Increasing perfusate glutamine from 0.67 to 5.0 mM caused a 200% increase in intracellular glutamine and a 66% increase in protein synthesis in the absence of insulin; in the presence of insulin a 30% increase in intramuscular glutamine was accompanied by an 80% increase in protein synthesis. Analysis of variance of the results confirmed the existence of positive relationships between intramuscular glutamine and protein synthesis in the presence or absence of insulin. Control of the size of the intramuscular free pool of glutamine may be important in determining the muscle protein mass.

Rate-limiting steps for protein synthesis in isolated rat liver cells. Role of aspartate availability

Amino-oxyacetate (carboxymethoxylamine) was found to inhibit protein labelling in isolated liver cells. A similar degree of inhibition (about 70%) was observed of basal and substrate-stimulated rates of protein labelling, ruling out an action on the cellular energy state. Its effect does not seem to be related either to a perturbation of the reduction state of the NAD system or to rate changes in the gluconeogenic pathway. The following observations indicate that amino-oxyacetate inhibits protein labelling by limiting aspartate supply. Amino-oxyacetate was ineffective in a postmitochondrial supernatant under non-limiting amino acid supply conditions. The aspartate cellular content decreases in the presence of amino-oxyacetate, although most other amino acids tend to accumulate. L-Cycloserine was unable to decrease aspartate content and was ineffective in decreasing protein labelling. The inhibitory action of amino-oxyacetate was specifically reversed by incubating cells with amino acids that increase the cellular content of aspartate.

Cytochrome c protein-synthesis rates and mRNA contents during atrophy and recovery in skeletal muscle

It is known that immobilization of the rat hindlimb by plaster casting leads to muscle atrophy and loss of muscle protein. In the present study, immobilization of the rat hindlimb for 6 h resulted in a significant 27% decrease in the absolute rate of cytochrome c synthesis in the red quadriceps muscle, without any change in the relative amount of cytochrome c mRNA. Cytochrome c mRNA in normal red quadriceps muscle was observed to be of four different lengths (1400, 1050, 650 and 580 bases). After 7 days of immobilization, the absolute rate of cytochrome c synthesis remained depressed and cytochrome c mRNA decreased by 40%; each of the cytochrome c mRNAs decreased, with a preferential disappearance of the 1050- and 1400-base lengths. Immobilization was ended on day 7, and the atrophied muscle was allowed to recover. At day 4 of recovery, the absolute rate of cytochrome c synthesis was 92% higher and the amount of cytochrome c mRNA had returned to control values. The abundances of the 1050- and 1400-base cytochrome c mRNAs had increased more than the shorter cytochrome c mRNAs, so that they were higher than control values. It appears that acute decreases in contractile activity of the red quadriceps muscle alter cytochrome c synthesis rates via translational or post-translational mechanisms, whereas chronic periods of modified contractile activity alter its synthesis rate via pre-translational mechanisms.

Transfer ribonucleic acid populations in concanavalin-A-stimulated bovine lymphocytes

Transfer RNA isolated from lymphocytes stimulated by concanavalin A and that from resting cells were compared with respect to amino-acid acceptance, integrity of the CCA-terminus, extent of modification and isoacceptor distribution. Following growth stimulation the overall amino-acid acceptance of the tRNA is elevated, in particular the relative acceptor activities for threonine and arginine are increased. The reduced acceptor activity of the tRNA from the quiescent cells is not due to a preferential degradation of the CCA-end, since it persists even in the presence of ATP(CTP):tRNA nucleotidyltransferase. We therefore conclude that this reduced activity is caused by structural differences of the tRNAs. The content of modified nucleotides in newly synthesized tRNA from lymphocytes cultured in the presence and absence of concanavalin A was determined. tRNA from resting cells was found to be undermodified with respect to pseudouridine and dihydrouridine. Upon monitoring the tRNA isoacceptor distribution by affinity chromatography on immobilized elongation factor Tu and subsequent two-dimensional gel electrophoresis, a preferential synthesis of particular lysine- and threonine-accepting tRNAs was observed upon mitogenic stimulation. Evidently, a specific tRNA population is needed by the proliferating cells. These results are discussed in view of the hypothesis that the commitment of lymphocytes to proliferation is at least in part under translational control.

Effects of liver regeneration on tRNA contents and aminoacyl-tRNA synthetase activities and sedimentation patterns

The tRNA content and aminoacyl-tRNA synthetases of regenerating liver in the phase of rapid growth were compared with those of livers from both intact and sham-operated rats. At 48 h after hepatectomy, the amount of active tRNA (called 'total acceptor capacity') is significantly higher in regenerating liver than in control livers, owing to a general, possibly not uniform, increase in the various tRNA families, which suggests that it may contribute to the increased protein synthesis and to decreased protein degradation as well. The activities of most, but not of all, aminoacyl-tRNA synthetases in cell sap of regenerating liver tend to be greater than normal. Increased activity of histidyl-tRNA synthetase fits in with the possibility that the mechanisms that control the rate of protein degradation through aminoacylation of tRNAHis in cultured cells [Scornik (1983) J. Biol. Chem. 258, 882-886] also operate in the liver and play a role in regeneration. Sedimentation analysis of cell sap in sucrose density gradients shows a shift of prolyl-tRNA synthetase activity toward the high-Mr form in regenerating liver. This change might be related to the positive protein balance and to growth in vivo, since it is also observed in the anaplastic Yoshida ascites hepatoma AH 130.

Quantities of individual aminoacyl-tRNA families and their turnover in Escherichia coli

The cellular content of all 20 aminoacyl-tRNA species was determined in small cultures of Escherichia coli by labeling cells with 3H-amino acids and extraction of 3H-amino acid-labeled nucleic acid by standard procedures. Of 3H-amino acid-labeled material, 25 to 90% was identified as 3H-aminoacyl-tRNA by the following criteria: sensitivity to base hydrolysis with expected kinetics; association of 3H counts released by base treatment of the 3H-amino acid-labeled nucleic acid with amino acid standards upon paper chromatography of the hydrolysate; and changes in the amount of 3H-amino acid-labeled nucleic acid recovered from cells as a function of time. Individual aminoacyl-tRNA content was determined with as few as 8 X 10(7) to 4 X 10(8) E. coli cells. Although the total number of aminoacyl-tRNA molecules per cell varied only by 10 to 20% among various strains of E. coli, some individual aminoacyl-tRNA families varied two- to threefold among strains. For a given amino acid, the number of aminoacyl-tRNA molecules per cell in E. coli strain K38 growing with a doubling time of 60 min varied from 730 (glutamyl-tRNA) to 7,910 (valyl-tRNA) with a mean value of 3,200. The total number of aminoacyl-tRNA molecules per cell (6.4 X 10(4)) in E. coli K38 was 5.5-fold higher than the number of ribosomes and was equal to 84% of the amount of elongation factor Tu molecules per cell. The ratio of aminoacyl-tRNA to synthetase for 10 amino acids varied from about 1 to 15 with a mean value of 4.7. The turnover of individual aminoacyl-tRNA families in E. coli cells was estimated to be in the range of 1.7 to 8.1 s-1 with a mean value of 3.7 s-1. An estimate of minimum in vivo molecular activity of aminoacyl-tRNA synthetases gives values of 2 to 48 s-1 for individual enzymes.

Mechanism of inhibition of polypeptide chain initiation in heat-shocked Ehrlich ascites tumour cells

The rate of polypeptide synthesis is inhibited by 80% in Ehrlich cells incubated at 43 degrees C compared to those at 37 degrees C. The regulatory site of translation resides at polypeptide chain initiation. Polypeptide synthesis does not recover at the higher temperature; however, the inhibition is reversed by returning the cells to 37 degrees C. Neither new RNA synthesis or protein synthesis is required for recovery at 37 degrees C, eliminating degradation of mRNA and irreversible denaturation of a protein essential for polypeptide chain initiation. The concentration of 40-S initiation complexes was found to be reduced markedly in heat-shocked cells compared to controls. This was confirmed in the cell-free protein-synthesizing systems prepared from heat-shocked and control cells. Reversible alteration in the activity of components affecting eIF2 function is, therefore, a likely mechanism of regulation in heat-shocked Ehrlich cells. In extracts from heat-shocked cells, Met-tRNA synthetase activity was unaltered compared to control extracts.

Utilization of tyrosine and tryptophan for protein synthesis by undernourished developing rat brain

Incorporation of tracer doses of radiolabeled tryptophan and tyrosine into brain proteins was investigated in rats malnourished during gestation and lactation. Age and time dependent increases in the radioactivity was observed in the whole homogenate and in the TCA insoluble fraction. Protein malnourished rats showed increased incorporation of tryptophan and tyrosine. However the diet restricted (Pair-fed) animals showed increased incorporation of tyrosine only. The increased incorporation may probably be due to changes in the pool size of the amino acids and effective recycling of the amino acids. The enhanced utilization in protein synthesis may also probably offer a mechanism for conservation of these amino acids.

Function and fidelity of aging tRNA: in vivo acylation, analog discrimination, synthetase binding, and in vitro translation

The liver transfer RNAs for valine and lysine were completely acylated in vivo, as judged by periodate oxidation, at 4 and 24 months of age in male Sprague-Dawley rats. In vitro acylation capacity for whole tRNA populations from rat livers is decreased, but this is evidently not deleterious in vivo. Several halogenated phenylalanines were synthesized and their effects upon acylation capacity for phenylalanine were examined. Synthetases bound to young (3 month) and old (24 month) tRNAs discriminated differently between p-chlorophenylalanine and authentic phenylalanine; synthetase with young tRNA was less able to discriminate than with old tRNA. Purified tRNAphe from old rats did not form ultraviolet-induced crosslinks to purified phenylalanyl tRNA synthetase as well as young tRNAphe. In vitro translation of encephalomyocarditis virus, hemoglobin, and ovalbumin mRNAs was effective, using tRNAs of young or old Sprague-Dawley or Fischer 344 rat livers, although, when the old tRNA was supplied, the product synthesized per unit tRNA was reduced. All of the protein products were synthesized with all tRNAs, as shown by sodium dodecyl sulfate polyacrylamide gel electrophoresis. We conclude that tRNA is capable of normal functions in livers of aging rats, is probably modification deficient, and is unlikely to produce protein errors.

Equilibration of leucine between the plasma compartment and leucyl-tRNA in the heart, and turnover of cardiac myosin heavy chain

By 30min continuous infusion of [3H]leucine into rats, the specific radioactivities of plasma leucine and tissue-free and tRNA-bound leucine in heart were equal. The specific radioactivity of leucyl-tRNA in heart therefore follows a time course identical with that of plasma leucine soon after the start of infusion. The half-life of cardiac myosin heavy chain (5.5 days) was the same as that reported by other investigators who used the pulse-labelling protocol.

Control of protein synthesis in mammalian cells by aminoacylation of transfer ribonucleic acid

The dependence of protein synthesis on the intracellular content of aminoacylated tRNA has been studied in mouse ascites tumor cells deprived for various amino acids. A remarkable reduction in net protein synthesis has been found only after a drastic decrease in aminoacylation of tRNA. The quantitative correlation of protein synthesis with the degree of aminoacylation suggests that a moderate amino acid starvation primarily influences the rate of elongation at the codon concerned. These results are in contrast to the findings previously reported for HeLa cells. Some crucial steps during the determination of intracellular aminoacyl-tRNA have been investigated. The reliability of the method employed has been discussed on a theoretical basis.

Biochemical research on oogenesis. Transfer RNA is fully charged in the 42-S storage particles of Xenopus laevis oocytes

1. Transfer RNA makes up 30-40% of total RNA in previtellogenic oocytes of Xenopus laevis. The bulk of tRNA is associated with 5-S RNA and two proteins in a high-molecular-weight complex sedimenting at 42S. 2. We show here that all kinds of tRNA are present in the 42-S particles and all of them sediment coincidently. Particle tRNA is fully charged in vivo. During purification of the 42-S particles tRNA becomes partially uncharged. When purified particles are incubated in vitro with amino acids and ATP a charging reaction occurs without disruption of the nucleoprotein complex. Many aminoacyl-tRNA synthetases can be shown to co-sediment with the 42-S particles. We conclude that complete aminoacylation of tRNA within the storage particles results from the activity of particle-bound aminoacyl-tRNA synthetases.

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.

L-Tryptophan action on hepatic RNA synthesis and enzyme induction

L-Tryptophan increases the activity of hepatic amino acid metabolizing enzymes, affects gluconeogenesis and displays a modulatory effect on several enzymes connected with RNA synthesis. The underlying mechanism differ in individual cases and result in both an increase of enzyme synthesis de novo and a decrease of enzyme degradation. Tryptophan displays a unique effect causing aggregation of hepatic polyribosomes connected with enhanced protein synthesis and preceded by a higher transport of poly (A) messenger RNA from the nucleus to the cytoplasm. The variety of rather specific effects mediated by tryptophan brings to mind hormonal action and the existence of specific tryptophan receptors is predicted.

Effect of starvation on tRNA synthesis, amino acid pool, tRNA charging levels and aminoacyl-tRNA synthetase activities in the posterior silk gland of Bombyx mori L

Changes in the translational machinery components of the Bombyx mori posterior silk gland were analysed during starvation and refeeding and compared to the regularly fed larvae. During starvation, tRNA and ribosomal RNA synthesis are stopped. The amounts of different RNA classes and of the different tRNA species slow down at the same rate. Thus various tRNA show similar half-lifes and the preexisting tRNA adaptation to fibroin mRNA translation persists during starvation. Similarly, the tRNA/rRNA ratio is constant during starvation and refeeding (12 tRNA molecules for one ribosome) as in silk glands of control animals. Aminoacyl-tRNA synthetases and tRNA charging levels are decreased during starvation. The maximal tRNA charging level obtained during maximal protein synthesis in control animals is regained after 24 h refeeding of starved larvae. Changes observed in the free amino acid pool are not similar from one amino acid to another and levels reached after starvation do not differ strongly from the controls. Our results suggest that the production of translation apparatus components is coordinated and adjusted to the protein synthesis activity. Whether this coordination occurs in the silk gland is discussed on the basis of the "metabolic regulation", primarily described in prokaryotes and Yeast. Transfer RNA charging levels seem to play a key role in the process of regulation and could be implicated in the mechanism of tRNA adaptation if this phenomenon results as expected from a transcriptional control.

Accelerated breakdown of reticulocyte protein formed under conditions of amino acid depletion

1. Labile protein is formed when rat or rabbit reticulocytes are incubated in medium deficient in individual amino acids, especially histidine, valine or alanine. The fraction of unstable protein is increased to about 35% of the total protein synthesized when the histidinyl-tRNA-charging inhibitor, histidinol, is added to histidine-deficient media. 2. The molecular weights of the labile proteins measured by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis in the presence of urea are less than haemoglobin and probably represent prematurely terminated haemoglobin chains. 3. Although protein synthesis is always lower under conditions that produce labile protein, inhibition of protein synthesis by fluoride or cycloheximide does not give an effect similar to amino acid depletion. 4. The synthesis of protein in deficient medium does not alter the degradation rate of pre-existing protein in reticulocytes and is thus unrelated to the stringent response in bacteria. 5. We propose that amino acid-deficient medium leads to a decreased charging of the appropriate tRNA, a concomitant decrease in protein synthesis and the degradation of nascent peptides.

Uncoupling of amino acid turnover on transfer RNA from protein synthesis in HeLa cells

The aminoacylation of tRNA has been investigated in relation to protein aynthesis in living HeLa cells. In cells growing normally, the rates of tRNA charing are compatible with the observed entry of amino acids into protein. In contrast, when protein synthesis is inhibited 95--98% by either reduced temperature or cycloheximide, aminoacylation of tRNA is relatively unaffected. We conclude that, under these conditions, the aminoacylation of tRNA is uncoupled from subsequent steps in protein synthesis. These results provide for the first time a possible biological role for the observed aminoacyl-tRNA hydrolase activities of the tRNA synthetases.

The effects of hyperphenylalaninaemia on the concentrations of aminoacyl-transfer ribonucleic acid in vivo. A mechanism for the inhibition of neural protein synthesis by phenylalanine

An acute administration of phenylalanine to neonatal animals has been reported to result in large decreases in the intracellular concentrations of several essential amino acids in neural tissue, as well as an inhibition of neural protein synthesis. The present report evaluates the effects of the loss of amino acids on the concentrations of aminoacyl-tRNA in vivo, with the view that an alteration in the concentrations of specific aminoacyl-tRNA molecules could be the rate-limiting step in brain protein metabolism during hyperphenylalaninaemia. tRNA was isolated from saline- and phenylalanine-injected mice 30-45 min after injection, by using a procedure designed to maintain the concentrations of aminoacyl-tRNA present in vivo. Periodate oxidation of the non-acylated tRNA and aminoacylation with radioactively labelled amino acids was used to determine the proportion of tRNA that was present in vivo as aminoacyl-tRNA. Although decreases in the intracellular concentrations of alanine, lysine and leucine were observed after phenylalanine administration, the concentrations of alanyl-tRNA, lysyl-tRNA and leucyl-tRNA actually increased by 15%. Although tryptophan has been suggested to be rate-limiting during hyperphenylalaninaemia, the proportion of tryptophan tRNA that was acylated was maximal in both normal and hyperphenylalaninaemic animals. This unexpected increase in aminoacyl-tRNA concentration is discussed as perhaps a secondary effect resulting from the phenylalanine-induced inhibition of protein synthesis. In contrast, the proportion of methionine tRNA that was acylated in vivo after phenylalanine administration was demonstrated to be decreased by approx. 17%. When the isoaccepting species of methionine tRNA were separated by reverse-phase column chromatography, three species were separated, one of which was demonstrated to be the initiator species, tRNAfMet, by the selective aminoacylation and formylation with Escherichia coli enzymes. After the administration of phenylalanine, the acylation of each of the three methionine tRNA species was decreased, with the initiator species being lowered by 10%. This effect on aminoacylation of tRNAfMet may be the primary step by which phenylalanine affects neural protein synthesis, and this is consistent with previous reports that re-initiation may be inhibited during hyperphenylalaninaemia.

Biosynthesis of mammalian transfer RNA. Evidence for regulation by deacylated transfer RNA

The rate of tRNA synthesis in cultured Friend leukemia cells has been examined as a function of the variation in polyribosome structure produced by treatment with a variety of inhibitors of protein synthesis. The results indicate, in contrast to the conclusions of Bölcsföldi (Bölcsföldi, G. (1974) Exp. Cell Res., 88, 231--240), that no necessary relationship exists between the ribosome distribution and the rate of tRNA synthesis. Alternatively, it is observed that inhibitors of tRNA aminoacylation cause, in all cases, a decrease in the rate of tRNA synthesis whereas drugs which may stimulate the aminoacylation of tRNA cause, in all cases, an elevation of the rate of tRNA synthesis. It is concluded that tRNA synthesis in mammalian cells may be regulated by the relative levels of acylated and deacylated tRNA.

Studies on the role of uncharged tRNA in pleiotypic response of animal cells

Experiments were carried out to assess the physiological significance of the charging level of tRNA. Histidinol, a competitve inhibitor of charging of tRNAHis, was used to induce uncharged tRNA in mammalian cells. It is demonstrated that both in the presence of histidinol and under histidine depletion about 40% of the tRNAHis is uncharged. Concomitant with this appearance of uncharged tRNA(a) the pools of GTP and ATP are decreased rapidly by 25--30%; (b) the synthesis of both protein and ribosomal RNA is inhibited, whereas that of nucleoplasmic RNA is not affected; (c) the uptake of 2-deoxyglucose, phosphate, Ca2+; uridine and adenosine is inhibited; and (d) the growth of 3T6 fibroblasts is arrested. It is suggested that the appearence of uncharged tRNA is one of the earliest events occurring under conditions of amino acid starvation, which in turn causes the various metabolic changes observed.

Requirement for protein synthesis in the regulation of protein breakdown in cultured hepatoma cells

The modes of action of insulin and of inhibitors of protein synthesis on the degradation of labeled cellular proteins have been studied in cultured hepatoma (HTC) cells. Protein breakdown is accelerated upon the deprivation of serum (normally present in the culture medium), and this enhancement is inhibited by either insulin or cycloheximide. An exception is a limited class of rapidly turning over cellular proteins, the degradation of which is not influenced by insulin or cycloheximide. Alternative hypotheses to explain the relationship of protein synthesis to the regulation of protein breakdown, viz., control by the levels of precursors of protein synthesis, regulation by the state of the ribosome cycle, or requirement for a product of protein synthesis, have been examined. Protein breakdown was not influenced by amino acid deprivation, and measurements of valyl-tRNA levels in HTC cells subjected to various experimental conditions showed no correlation between the levels of charged tRNAVal and the rates of protein degradation. Three different inhibitors of protein synthesis (puromycin, pactamycin, and cycloheximide) suppressed enhanced protein breakdown in a similar fashion. A direct relationship was found between the respective potencies of these drugs to inhibit protein synthesis and to block enhanced protein breakdown. When cycloheximide and insulin were added following a prior incubation of HTC cells in a serum-free medium, protein breakdown was maximally suppressed within 15-30 min. Actinomycin D inhibited protein breakdown only after a time lag of about 90 min. It is suggested that the regulation of protein breakdown in hepatoma cells requires the continuous formation of a product of protein synthesis, in a manner analogous to the mode of the control of this process in bacteria.

Control of hepatic protein synthesis. Differential effects of ATP levels on the initiation and elongation steps

The effect of nucleotide energy levels in vivo on the different steps of protein synthesis has been studied. Hepatic anoxia was induced by interrupting the blood portal-vein flow. At 5 min of anoxia ATP fell to 59% of the control values and the amino acid incorporation into protein was inhibited by more than 70%. This strong inhibition was not paralleled by polyribosomal breakdown. On the contrary, when fasted rats were used, at 5 min of anoxia the ribosomal state of aggregation was found to increase. Longer periods of anoxia resulted in a further decrease in triphosphonucleoside content and polyribosomal breakdown. Based on these results and other reports from the literature it is concluded that the Km for the GTP of the peptide-chain-elongation mechanism must be higher than the Km of the initiation step. This finding implies that variations of nucleotide levels in vivo within the physiological range may control protein synthesis at the elongation step without apparent changes in the polyribosomal profiles.