Genotoxic Effects of Amplitude-Modulated Microwaves on Human Lymphocytes Exposed in Vitro under Controlled Conditions

Peripheral human blood from 23 healthy donors aged between 23 and 95 years was exposed to continuous wave (CW) or 50 Hz amplitude modulated (AM) microwave radiation and was cultured for 72 h. Other exposure parameters were: frequency 9 GHz, specific absorption rate (SAR) 90 mW/g, exposure duration 10 min. The possible genotoxic effect was evaluated by means of cytokinesis-block micronucleus method. A significant (p < 0.05) increase in micronuclei was found following AM microwave exposure.     

Effect on the immune system of mice exposed chronically to 50 Hz amplitude-modulated 2.45 GHz microwaves

The effect of continuous (CW; 2.45 GHz carrier frequency) or amplitude-modulated (AM; 50 Hz square wave) microwave radiation on the immune response was tested. CW exposures (6 days, 3 h/day) induced elevations of the number of antibody-producing cells in the spleen of male Balb/c mice (+37%). AM microwave exposure induced elevation of the spleen index (+15%) and antibody-producing cell number (+55%) in the spleen of male mice. No changes were observed in female mice. It is concluded that both types of exposure conditions induced moderate elevation of antibody production only in male mice. © 1996 Wiley-Liss, Inc.     

Frequency-dependent alterations in enolase activity in Escherichia coli caused by exposure to electric and magnetic fields

Some neurochemical effects of low-intensity electric and magnetic fields have been shown to be nonlinear functions of exposure parameters. These effects occurred within narrow ranges of frequency and intensity. Previous studies on membrane-associated endpoints in cell culture preparations demonstrated changes in calcium efflux and in acetylcholinesterase activity following exposure to radiofrequency radiation, amplitude modulated (AM) at 16 and at 60 Hz, at a specific absorption rate of 0.05 W/kg. In this study, these modulation frequencies were tested for their influence on the activity of a cytoplasmic enzyme, enolase, which is being tested clinically for detection of neoplasia. Escherichia coli cultures containing a plasmid with a mammalian gene for enolase were exposed for 30 min, and cell extracts were assayed for enolase activity by measuring absorbance at 240 nm. The enolase activity in exposed cultures was compared to the activity in paired control cultures. Exposure to 147 MHz carrier waves at 0.05 W/kg, AM at 16 Hz showed enolase activity enhanced by 62%, and AM at 60 Hz showed enolase activity reduced by 28%. Similarly, exposure to 16 Hz fields alone, at 21.2 V/m_rms (electric) and 97 nT_rms (magnetic), showed enhancement in enolase activity by 59%, whereas exposure to 60 Hz fields alone, at 14.1 V/m_rms (electric) and 65 nT_rms (magnetic), showed reduction in activity by 24%. Sham exposures as well as exposure to continuous-wave 147 MHz radiation at 0.05 W/kg showed no change in enolase activity. Although the underlying basis for these field effects in the cytoplasmic compartment has not been established, differential sensitivities to 16 Hz and to 60 Hz signals provide a clear focus for additional research to determine the responsible mechanism. © 1994 Wiley-Liss, Inc.     

Effects of low level microwave radiation on carcinogenesis in Swiss Albino mice

This study concerns with the multiple treatment of the target site to potent carcinogen and the super imposition of low level radiofrequency and microwave radiation. Swiss albino mice (male) were used for this investigation. The study has been divided in two parts, part A: a single dose of 7,12-dimethylbenz(a)anthracene (DMBA) 100 μg/animal was applied topically on the skin of mice and were exposed to 112 MHz amplitude modulated (AM) at 16 Hz (power density 1.0 mW/cm(2), specific absorption rate (SAR) 0.75 W/kg). Similarly after a single dose of DMBA, mice were exposed to 2.45 GHz radiation (power density of 0.34 mW/cm(2), SAR, 0.1 W/kg), 2 h/day, 3 days a week for a period of 16 weeks. The two sets of experiments were carried out separately. Part B: mice were transplanted intraperitoneally (ip) with ascites 8 × 10(8) (Ehrlich-Lettre ascites, strain E) carcinoma cells per mouse. These mice were exposed to 112 MHz amplitude modulated at 16 Hz and 2.45 GHz radiation separately for a period of 14 days. There was no tumor development in mice exposed to RF and MW. Similarly a topical application of single dose of DMBA followed by RF/MW exposure also did not produce any visible extra tumor on the skin of mice. On the other hand mice were transplanted intraperitoneally with ascites (8 × 10(8) cell/ml) and subsequently exposed to above mentioned fields for 14 days showed a slight increase in the cell numbers as compared to the control group. However, the increase is insignificant. There were insignificant differences either in the mortality or cell proliferation among the control and exposed group. This results show that low level RF or MW do not alter tumor growth and development as evidenced by no observable change in tumor size.     

Subunit structure and tRNA-binding properties of Bombyx mori Glycyl-tRNA synthetase

Large amounts of glycyl-tRNA synthetase were purified from the posterior silk glands of Bombyx mori. The synthetase was estimated to be a dimer with a molecular weight of 180,000. When the enzyme solution was diluted, the dimer dissociated into monomers which were inactive in tRNA aminoacylation. The aminoacylation was investigated with two isoaccepting tRNAsGly isolated from the posterior silk glands. Transfer RNA1Gly was aminoacylated 2-fold faster than tRNA2Gly. Transfer RNA-binding experiments revealed that tRNA1Gly binds with the enzyme in a molar ratio of 2:1, whereas tRNA2Gly formed a 1:1 complex with the enzyme. Based on these experimental results, we proposed that the Bombyx mori glycyl-tRNA synthetase has two active sites for tRNA aminoacylation and that the number of tRNA molecules bound on the synthetase closely correlates with the velocity of aminoacylation.     

Influence of 2.45-GHz CW microwave radiation on spontaneously beating rat atria

The chronotropic and inotropic effects of 2.45-GHz continuous wave (CW) microwave radiation were investigated in the isolated spontaneously beating rat atria. Isolated atria were placed in specially designed tubes inserted into a waveguide exposure system. The atria were then irradiated for a period of 30 min, followed by a 30-min recovery period. The control atria were prepared simultaneously and sham exposed. Experiments were conducted at two temperatures, 22 and 37 °C, and two specific absorption rates, 2 mW/g and 10 mW/g. At both temperatures the rate of atrial contraction was not altered by a 30-min exposure at either 2 or 10 mW/g. The average rate (beats per min) was approximately 100 for both the control and exposed atria at 22 °C and 215 beats per min for both the control and exposed atria at 37 °C. In addition, no inotropic effects on the spontaneously beating atria were noted at any exposure level. These data suggest that 2.45-GHz CW microwave radiation at these intensities has no overt effect on these variables in isolated rat atria.     

Calcium-ion movement and contractility in atrial strips of frog heart are not affected by low-frequency-modulated, 1 GHz electromagnetic radiation

Calcium efflux from electrically stimulated, ⁴⁵Ca²⁺-preloaded atrial strips of the frog heart was measured from samples of the rinsing perfusate collected at 2-min intervals for 32 min in a continuous perfusion chamber. Contractile force was simultaneously monitored. The specimen chamber was located in a stripline apparatus in which the atrial strips were exposed for 32 min to constant (CW) or amplitude-modulated (AM), 1 GHz electromagnetic (EM) fields at specific absorption rates (SAR) ranging from 3.2 μW/kg to 1.6 W/kg. Amplitude modulation was either at 0.5 Hz, in synchrony with the electrical stimulus applied to the preparation, or at 16 Hz. Neither unmodulated nor 0.5 Hz or 16 Hz modulated 1 GHz waves affected the movement of calcium ions or the contractile force in isolated atrial strips of the frog heart. © 1993 Wiley-Liss, Inc.     

50Hz正弦交变电磁场促进体外培养成骨细胞分化成熟的双“强度窗”效应

为研究频率为50 Hz不同强度正弦交变电磁场对体外培养成骨细胞分化及基因表达的影响,体外分离培养大鼠颅骨成骨细胞,传代后随机分为15组,分别用频率50 Hz、强度为0 mT(对照)和0.9～4.8 mT(每组间隔0.3 mT)的正弦交变电磁场处理。发现:正弦交变电磁场处理3～5 d后,成骨细胞呈漩涡样分布;第9 d,1.8和3.6 mT组的碱性磷酸酶活性极显著高于对照组;第0、12、24和96 h,1.5、1.8、3.0和3.6 mT组碱性磷酸酶、骨形态发生蛋白-2和Osterix基因表达水平显著高于对照组;第10 d,1.8和3.6 mT组钙化结节数明显多于对照组。说明50 Hz、0.9～4.8 mT的正弦交变电磁场能促进体外培养成骨细胞分化成熟,并且具有较明显的双强度窗效应,其中1.8和3.6 mT作用最为明显。     

Effects of microwave exposure and Gemcitabine treatment on apoptotic activity in Burkitt’s lymphoma (Raji) cells

Abstract

We investigated the effects of 1.8?MHz Global System for Mobile Communications (GSM)-modulated microwave (MW) radiation on apoptotic level and cell viability of Burkitt’s lymphoma (Raji) cells with or without Gemcitabine, which exhibits cell phase specificity, primarily killing cells undergoing DNA synthesis (S-phase). Raji cells were exposed to 1.8?GHz GSM-modulated MW radiation at a specific absorption rate (SAR) of 0.350?W/kg in a CO₂ incubator. The duration of the exposure was 24?h. The amount of apoptotic cells was analyzed using Annexin V-FITC and propidium iodide (PI) staining with flow cytometer. The apoptotic activity of MW exposed Raji cells was increased significantly. In addition, cell viability of exposed samples was significantly decreased. Combined exposure of MW and Gemcitabine increased the amount of apoptotic cells than MW radiation alone. Moreover, viability of MW?+?Gemcitabine exposed cells was lower than that of cells exposed only to MW. These results demonstrated that MW radiation exposure and Gemcitabine treatment have a synergistic effect on apoptotic activity of Raji cells.     

Leucyl-transfer ribonucleic acid synthetase from a wild-type and temperature-sensitive mutant of Salmonella typhimurium

Leucyl-transfer ribonucleic acid (tRNA) synthetase was purified 100-fold from extracts of Salmonella typhimurium. The partially purified enzyme had the following K(m) values: leucine, 1.1 x 10(-5)m; adenosine triphosphate, 6.5 x 10(-4)m; tRNA(I) (Leu), 4.1 x 10(-8)m; tRNA(II) (Leu), 4.3 x 10(-8)m; tRNA(III) (Leu), 5.3 x 10(-8)m; and tRNA(IV) (Leu), 2.9 x 10(-8)m. The tRNA(Leu) fractions were isolated from Salmonella bulk tRNA by chromatography on reversed-phase columns and benzoylated diethylaminoethyl cellulose. The enzyme had a pH optimum of 8.5 and an activation energy of 10,400 cal per mole, and was inactivated exponentially at 49.5 C with a first-order rate constant of 0.064 min(-1). Strain CV356 (leuS3 leuABCD702 ara-9 gal-205) was isolated as a mutant resistant to dl-4-azaleucine and able to grow at 27 C but not at 37 C. Extracts of strain CV356 had no leucyl-tRNA synthetase activity (charging assay) when assayed at 27 or 37 C. Temperature sensitivity and enzyme deficiency were caused by mutation in the structural gene locus specifying leucyl-tRNA synthetase. A prototrophic derivative of strain CV356 (CV357) excreted branched-chain amino acids and had high pathway-specific enzyme levels when grown at temperatures where its doubling time was near normal. At growth-restricting temperatures, both amino acid excretion and enzyme levels were further elevated. The properties of strain CV357 indicate that there is only a single leucyl-tRNA synthetase in S. typhimurium.     

2.45-GHz microwave irradiation adversely affects reproductive function in male mouse,Mus musculus by inducing oxidative and nitrosative stress

Abstract

Electromagnetic radiations are reported to produce long-term and short-term biological effects, which are of great concern to human health due to increasing use of devices emitting EMR especially microwave (MW) radiation in our daily life. In view of the unavoidable use of MW emitting devices (microwaves oven, mobile phones, Wi-Fi, etc.) and their harmful effects on biological system, it was thought worthwhile to investigate the long-term effects of low-level MW irradiation on the reproductive function of male Swiss strain mice and its mechanism of action. Twelve-week-old mice were exposed to non-thermal low-level 2.45-GHz MW radiation (CW for 2 h/day for 30 days, power density = 0.029812 mW/cm² and SAR = 0.018 W/Kg). Sperm count and sperm viability test were done as well as vital organs were processed to study different stress parameters. Plasma was used for testosterone and testis for 3β HSD assay. Immunohistochemistry of 3β HSD and nitric oxide synthase (i-NOS) was also performed in testis. We observed that MW irradiation induced a significant decrease in sperm count and sperm viability along with the decrease in seminiferous tubule diameter and degeneration of seminiferous tubules. Reduction in testicular 3β HSD activity and plasma testosterone levels was also noted in the exposed group of mice. Increased expression of testicular i-NOS was observed in the MW-irradiated group of mice. Further, these adverse reproductive effects suggest that chronic exposure to nonionizing MW radiation may lead to infertility via free radical species-mediated pathway.     

Arginyl-tRNA synthetase from baker's yeast. Purification and some properties.

Arginyl-tRNA synthetase from baker's yeast (Saccharomyces cerevisiae, strain 836) was obtained pure by a large-scale preparative method, which involves four chromatographic columns and one preparative polyacrylamide gel electrophoretic step. The enzyme has a high specific activity (9000 U/mg) and consists of a single polypeptide chain of molecular weight approximately 73000 as determined by polyacrylamide gel electrophoresis in the presence of sodium dodecylsulphate. Amino acid analysis of the enzyme permitted calculation of the absorption coefficient of arginyl-tRNA synthetase (A(1 mg/ml 280 nm)=1.26). Concerning kinetic parameters of the enzyme we found the following Km values: 0.28 muM, 300 muM, 1.5 muM for tRNA(Arg III), ATP and arginine in the aminoacylation reaction, and 1400 muM, 2.5 muM, and 50 muM for ATP, arginine and PP(i) in the ATP-PP(i) exchange reaction. Arginyl-tRNA synthetase required tRNA(Arg III) to catalyse the ATP-PP(i) exchange reaction.     

Dose dependence of acetylcholinesterase activity in neuroblastoma cells exposed to modulated radio-frequency electromagnetic radiation.

Radio-frequency electromagnetic radiation (RFR) at 915 and 147 MHz, when sinusoidally amplitude modulated (AM) at 16 Hz, has been shown to enhance release of calcium ions from neuroblastoma cells in culture. The dose-response relation is unusual, consisting of two power-density "windows" in which enhanced efflux occurs, separated by power-density regions in which no effect is observed. To explore the physiological importance of these findings, we have examined the impact of RFR exposure on a membrane-bound enzyme, acetylcholinesterase (AChE), which is intimately involved with the acetylcholine (ACh) neurotransmitter system. Neuroblastoma cells (NG108), exposed for 30 min to 147-MHz radiation, AM at 16 Hz, demonstrated enhanced AChE activity, as assayed by a procedure using 14C-labeled ACh. Enhanced activity was observed within a time window between 7.0 and 7.5 h after the cells were plated and only when the exposure occurred at power densities identified in a previous report as being effective for altering the release of calcium ions. Thus RFR affects both calcium-ion release and AChE activity in nervous system-derived cells in culture in a common dose-dependent manner.     

Comparison of dose dependences for bioeffects of continuous-wave and high-peak power microwave emissions using gel-suspended cell cultures

The study compared bioeffects of continuous wave (CW) microwaves and short, extremely high power pulses (EHPP) at the same carrier frequency (9.3 GHz) and average power (1.25 W). The peak transmitted power for EHPP was 250 kW (0.5-micro s pulse width, 10 p.p.s.), producing the E field of 1.57 MV/m in the waveguide. A biological endpoint was the density of yeast cells, achieved after a 6 h growth period in a solid nutrient medium (agarose gel) during EHPP or CW exposure. Owing to power losses in the medium, the specific absorption rate (SAR) ranged from 3.2 kW/kg at the exposed surface of the sample to 0.6 mW/kg at 24 mm depth. Absorption and penetration of EHPP was identical to CW, producing peak SAR values 200 000 times higher than the average SAR, as high as 650 MW/kg at the surface. CW and EHPP exposures produced highly nonuniform but identical heating patterns in exposed samples. Following the exposure, the samples were sliced in a plane perpendicular to the wave propagation, in order to separate cell masses exposed at different SAR levels. Cell density in the slices was determined by nephelometry and compared to unexposed parallel control samples. Cell density was strongly affected by irradiation, and the changes correlated well with the local temperature rise. However, the data revealed no statistically significant difference between CW and EHPP samples across the entire studied range of SAR levels (over six orders of magnitude). A trend (P<0.1) for such a difference was observed in slices that were exposed at a time average SAR of 100 W/kg and higher, which corresponded to peak SAR above 20 MW/kg for the EHPP condition. These numbers could be indicative of a threshold for a specific (not merely thermal) exposure effect if the trend is confirmed by future studies.     

Effects of continuous and pulsed 2450-MHz radiation on spontaneous lymphoblastoid transformation of human lymphocytes in vitro.

Normal human lymphocytes were isolated from the peripheral blood of healthy donors. One-ml samples containing (10(6)) cells in chromosome medium 1A were exposed for 5 days to conventional heating or to continuous wave (CW) or pulsed wave (PW) 2450-MHz radiation at non-heating (37 degrees C) and various heating levels (temperature increases of 0.5, 1.0, 1.5, and 2 degrees C). The pulsed exposures involved 1-microsecond pulses at pulse repetition frequencies from 100 to 1,000 pulses per second at the same average SAR levels as the CW exposures. Actual average SARs ranged to 12.3 W/kg. Following termination of the incubation period, spontaneous lymphoblastoid transformation was determined with an image analysis system. The results were compared among each of the experimental conditions and with sham-exposed cultures. At non-heating levels, CW exposure did not affect transformation. At heating levels both conventional and CW heating enhanced transformation to the same extent and correlate with the increases in incubation temperature. PW exposure enhanced transformation at non-heating levels. This finding is significant (P less than .002). At heating levels PW exposure enhanced transformation to a greater extent than did conventional or CW heating. This finding is significant at the .02 level. We conclude that PW 2450-MHz radiation acts differently on the process of lymphoblastoid transformation in vitro compared with CW 2450-MHz radiation at the same average SARs.     

Ultracentrifugation studies of yeast valyl-tRNA synthetase and of its interaction with tRNAVal.

Yeast valyl-tRNA synthetase and its complexes with yeast tRNAVal were investigated by means of analytical ultracentrifugation. A molecular weight of 125 700 +/- 1500 and a sedimentation coefficient (SO 20, w) of 6.3 +/- 0.3 were found for the native enzyme. When the enzyme (3--60 muM) was mixed with its cognate tRNA, several types of complex were observed, depending on the relative amounts of the two macromolecules. In the presence of equimolecular amounts of tRNA and enzyme, a complex formed by the association of one of each molecule was observed with a sedimentation coefficient of about 7.3 S. However, for tRNA/enzyme stoichiometries lower than one, beside the 1 : 1 complex, a complex of higher molecular weight was observed, with a sedimentation coefficient of about 10.0 S which fits with the association of two valyl-tRNA synthetase molecules with one tRNA molecule. This 2 : 1 complex was predominant from tRNA/enzyme stoichiometries lower than 0.3. It dissociated into the 1 : 1 complex upon addition of monovalent salts or MgCl2, suggesting the electrostatic nature of the interaction in this association. All these association and dissociation phenomena were detected over a large range of pH (6.0--7.5) and in various buffers.     

Effects of prenatal exposure to 50 Hz magnetic fields on development in mice: II. Postnatal development and behavior

To investigate the potential of magnetic fields to act as a behavioral teratogen, pregnant CD1 mice were exposed or sham-exposed for all of gestation to a 50 Hz/20 mT magnetic field. Maturation of offspring was assessed using a range of standard developmental indices (eye opening, pinna detachment, hair coat, tooth eruption, sexual maturity, and weight) and simple reflexive behaviors (air righting, surface righting, forepaw grasp, cliff avoidance, and negative geotaxis). Activity and coordination levels were explored in juvenile and adult mice using an open field arena, a head-dip board, an accelerating Rotarod, and a residential activity wheel. All assessments were carried out without knowledge of exposure condition. Results from 168 sham-exposed mice from 21 litters and from 184 exposed mice from 23 litters were compared using survival analysis techniques and multivariate regression methods. Three possible field-dependent effects were found: Exposed animals performed the air righting reflex earlier (P < 0.01); exposed males (but not females) were significantly lighter in weight (P = 0.008) at 30 days of age; and exposed animals remained on a Rota-rod for less time as juveniles (P = 0.03). Some of these results have not been reported in other studies and may reflect spurious statistical significance, although some effect of magnetic field exposure cannot be ruled out. Overall, these results suggest that prenatal exposure to a 50 Hz magnetic field does not engender any gross impairments in the postnatal development or behavior of mice. This does not preclude such exposure affecting more subtle aspects of behavior. Published 1994 by Wiley-Liss, Inc.     

DNA strand breaks are not induced in human cells exposed to 2.1425 GHz band CW and W-CDMA modulated radiofrequency fields allocated to mobile radio base stations

We conducted a large-scale in vitro study focused on the effects of low level radiofrequency (RF) fields from mobile radio base stations employing the International Mobile Telecommunication 2000 (IMT-2000) cellular system in order to test the hypothesis that modulated RF fields may act as a DNA damaging agent. First, we evaluated the responses of human cells to microwave exposure at a specific absorption rate (SAR) of 80 mW/kg, which corresponds to the limit of the average whole body SAR for general public exposure defined as a basic restriction in the International Commission on Non-Ionizing Radiation Protection (ICNIRP) guidelines. Second, we investigated whether continuous wave (CW) and Wideband Code Division Multiple Access (W-CDMA) modulated signal RF fields at 2.1425 GHz induced different levels of DNA damage. Human glioblastoma A172 cells and normal human IMR-90 fibroblasts from fetal lungs were exposed to mobile communication frequency radiation to investigate whether such exposure produced DNA strand breaks in cell culture. A172 cells were exposed to W-CDMA radiation at SARs of 80, 250, and 800 mW/kg and CW radiation at 80 mW/kg for 2 and 24 h, while IMR-90 cells were exposed to both W-CDMA and CW radiations at a SAR of 80 mW/kg for the same time periods. Under the same RF field exposure conditions, no significant differences in the DNA strand breaks were observed between the test groups exposed to W-CDMA or CW radiation and the sham exposed negative controls, as evaluated immediately after the exposure periods by alkaline comet assays. Our results confirm that low level exposures do not act as a genotoxicant up to a SAR of 800 mW/kg.     

GSM modulated radiofrequency radiation does not affect 6-sulfatoxymelatonin excretion of rats

In this study, the effect of exposure to 900 and 1800 MHz GSM-like radiofrequency radiation upon the urinary 6-sulfatoxymelatonin (6SM) excretion of adult male Wistar rats was studied. Seventy-two rats were used in six independent experiments, three of which were done with 900 MHz and the other three with 1800 MHz. The exposures were performed in a gigahertz transverse electromagnetic mode (GTEM) cell. The power densities of radiation were 100 and 20 microW/cm(2) at 900 and 1800 MHz frequency, respectively. The carrier frequency was modulated with 218 Hz, as in the GSM signal. The animals were exposed for 2 h between 8:00 AM and noon daily during the 14 day exposure period. The urine of rats was collected from 12:00 AM to 8:00 AM, collecting from exposed and control animal groups on alternate days. The urinary 6SM concentration was measured by (125)I radioimmunoassay and was referred to creatinine. The combined results of three experiments done with the same frequency were statistically analyzed. Statistically significant changes in the 6SM excretion of exposed rats (n = 18) compared to control group (n = 18) were not found either at 900 or 1800 MHz.     

Interrelation between transfer RNA and amino-acid-activating sites of methionyl transfer RNA synthetase from Escherichia coli.

Binding of tRNA(Met/f) to the monomeric trypsin-modified methionyl-tRNA synthetase turns off the methionine-dependent isotopic ATP--PPi exchange. In the case of the dimeric native methionyltRNA synthetase, one anticooperatively bound tRNA(Met/f) inhibits the exchange by only 50%. These behaviours of tRNA do not require the integrity of the 3'-terminal adenosine. Esterification by methionine of the 3' end of tRNA reinforces the affinity of tRNA(Met/f)for the enzymes. In the case of the native enzyme, due to this effect, a second binding mode for methionyl-tRNA may be demonstrated through the isotopic exchange. This additional binding of tRNA corresponds to the expression of the anticooperatively blocked tRNA binding site. Methionine reverses competitively the reinforcing effect of the esterified methionyl moiety on tRNA binding. It is concluded that after esterification of tRNA, the aminoacyl residue still binds the enzyme, probably within the methionine activating site. The latter behaviour may account for the observation that excess methionine accelerates the aminoacylation turnover rate of tRNA(Met/f).