Sample records for azaguanine

  1. X-ray-induced mutants resistant to 8-azaguanine

    International Nuclear Information System (INIS)

    Carver, J.H.; Dewey, W.C.; Hopwood, L.E.


    Asynchronous Chinese hamster ovary cells were irradiated and colony survival in Alpha MEM medium with dialyzed serum was determined with or without 15 μg/ml 8-Azaguanine (AG). Data indicated that a reproducible assay for the system was dependent upon controlling cell density at least two days prior to induction as well as throughout the expression period. Generally, spontaneous and radiation-induced mutant frequencies decreased when cell densities exceeded a critical density of 3-6 x 10 4 cells/cm 2 . Infrequently, the critical density was exceeded by a factor of two with no observed decrease, possibly correlated with a longer cell doubling time. Drug depletion artifacts can occur because of drug degradation, or because wild-type cells utilize the drug or produce conditions which reduce uptake of the drug. Thus, as the effective drug concentration is lowered, the observed mutant frequency increases because a spectrum of mutants resistant to only low concentrations can now survive. In fact, refeeding with AG at intervals during the incubation period lowered spontaneous and radiation-induced frequencies approx. 5-fold. Therefore, to standardize conditions, cells were trypsinized at the end of the expression time and replated at a constant cell number for mutant selection by AG. Over two generations of growth during the expression period were required for optimal manifestation of induced mutants, and when densities were kept below 4 x 10 4 cells/cm 2 at all times, observed mutant frequencies did not change significantly over a period between 80 and 140 h post-induction (over 4 generations for irradiated cells and over 6 generations for controls). Previous reports of observed mutant frequencies decreasing beyond three generations may be due to cell interaction prior to mutant selection

  2. Stimulation of cytolytic T lymphocytes by azaguanine-resistant mouse tumor cells in selective hat medium

    International Nuclear Information System (INIS)

    Snick, J. van; Uyttenhove, C.; Pel, A. van; Boon, T.


    Primed syngeneic or umprimed allogeneic mouse spleen cells were stimulated with azaguanine-resistant P815 tumor cells that were killed by the addition of aminopterin to the stimulation medium. The recovery of lymphocytes and their cytolytic activity and specificity were similar to those obtained after stimulation with irradiated cells. This method conveniently replaces the inactivation of stimulatory cells by irradiation or mitomycin treatment. Moreover, it has the advantage of inactivating not only the stimulatory cells but also the tumor cells that often contaminate the spleens of tumor-bearing animals, provided these animals have been inoculated with azaguanine-resistant tumor cell mutants. (Auth.)

  3. Protective action of DNA preparations on the survival of cells and yield of 8-azaguanine resistant mutations in X-irradiated cell culture of chinese hamsters

    International Nuclear Information System (INIS)

    Kuznetsova, N.N.; Feoktistova, T.P.


    A DNA preparation (molecular weight 19.6-21.0x1O 6 daltons) administered to cell culture of Chinese hamsters in concentrations of 100 to 122 μg/ml 60 minutes before and in the course of 3 days after X-irradiation (600 R) decreased the lethality of irradiated cells and reduced induction of 8-azaguanine resistant genic mutations. DNA preparations with the concentrations under study had no toxic action on cells and were not mutagenous

  4. Study of the interaction between 8-azaguanine and bovine serum albumin using optical spectroscopy and molecular modeling methods. (United States)

    Gong, Qiao-Ling; Hu, Xin-Gen; Fang, Guo-Yong; Li, Xin-Hua


    The interaction between 8-azaguanine (8-Azan) and bovine serum albumin (BSA) in Tris-HCl buffer solutions at pH 7.4 was investigated by means of fluorescence and ultraviolet-visible (UV-Vis) spectroscopy. At 298 K and 310 K, at a wavelength of excitation (λ (ex)) of 282 nm, the fluorescence intensity decreased significantly with increasing concentrations of 8-Azan. Fluorescence static quenching was observed for BSA, which was attributed to the formation of a complex between 8-Azan and BSA during the binding reaction. This was illuminated further by the UV-Vis absorption spectra and the decomposition of the fluorescence spectra. The thermodynamic parameters ∆G, ∆H, ∆S were calculated. The results showed that the forces acting between 8-Azan and BSA were typical hydrophobic forces, and that the interaction process was spontaneous. The interaction distance r between 8-Azan and BSA, evaluated according to fluorescence resonance energy transfer theory, suggested that there is a high possibility of energy transfer from BSA to 8-Azan. Theoretical investigations based on homology modeling and molecular docking suggested that binding between 8-Azan and BSA is dominated by hydrophilic forces and hydrogen bonding. The theoretical investigations provided a good structural basis to explain the phenomenon of fluorescence quenching between 8-Azan and BSA.

  5. Untitled

    Indian Academy of Sciences (India)

    Broadly speaking, the above observation suggests that the dissolved oxygen complexes slowly with 8-azaguanine and also its binding is loosened slowly due to the presence of nitrogen. Oxygen set free due to nitrogen may be activated again due to which the original situation (higher and lower intensities near 275 and ...

  6. 40 CFR 798.5300 - Detection of gene mutations in somatic cells in culture. (United States)


    ... the mutant form which gives rise to a change in an enzymatic or functional protein. (2) Base pair mutagens are agents which cause a base change in the DNA. (3) Frameshift mutagens are agents which cause... selected by resistance to 8-azaguanine (AG) or 6-thioguanine (TG) and cells with altered Na=/K= ATPase are...

  7. Enzymatic Incorporation of Modified Purine Nucleotides in DNA. (United States)

    Abu El Asrar, Rania; Margamuljana, Lia; Abramov, Mikhail; Bande, Omprakash; Agnello, Stefano; Jang, Miyeon; Herdewijn, Piet


    A series of nucleotide analogues, with a hypoxanthine base moiety (8-aminohypoxanthine, 1-methyl-8-aminohypoxanthine, and 8-oxohypoxanthine), together with 5-methylisocytosine were tested as potential pairing partners of N 8 -glycosylated nucleotides with an 8-azaguanine or 8-aza-9-deazaguanine base moiety by using DNA polymerases (incorporation studies). The best results were obtained with the 5-methylisocytosine nucleotide followed by the 1-methyl-8-aminohypoxanthine nucleotide. The experiments demonstrated that small differences in the structure (8-azaguanine versus 8-aza-9-deazaguanine) might lead to significant differences in recognition efficiency and selectivity, base pairing by Hoogsteen recognition at the polymerase level is possible, 8-aza-9-deazaguanine represents a self-complementary base pair, and a correlation exists between in vitro incorporation studies and in vivo recognition by natural bases in Escherichia coli, but this recognition is not absolute (exceptions were observed). © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

  8. Mutagenic activity of phthalate esters in bacterial liquid suspension assays.


    Seed, J L


    The mutagenic activities of several phthalate esters have been evaluated in an 8-azaguanine resistance assay in Salmonella typhimurium. Three phthalate esters were found to be mutagenic: dimethyl phthalate, diethyl phthalate and di-n-butyl phthalate. A number of other phthalate esters were not found to be mutagenic, including di(2-ethylhexyl) phthalate, di-n-octyl phthalate, diallyl phthalate, diisobutyl phthalate and diisodecyl phthalate. A metabolite of di(2-ethylhexyl) phthalate, 2-ethylhe...

  9. Characterisation of methionine adenosyltransferase from Mycobacterium smegmatis and M. tuberculosis

    Directory of Open Access Journals (Sweden)

    Knodel Marvin H


    Full Text Available Abstract Background Tuberculosis remains a serious world-wide health threat which requires the characterisation of novel drug targets for the development of future antimycobacterials. One of the key obstacles in the definition of new targets is the large variety of metabolic alterations that occur between cells in the active growth and chronic/dormant phases of tuberculosis. The ideal biochemical target should be active in both growth phases. Methionine adenosyltransferase, which catalyses the formation of S-adenosylmethionine from methionine and ATP, is involved in polyamine biosynthesis during active growth and is also required for the methylation and cyclopropylation of mycolipids necessary for survival in the chronic phase. Results The gene encoding methionine adenosyltransferase has been cloned from Mycobacterium tuberculosis and the model organism M. smegmatis. Both enzymes retained all amino acids known to be involved in catalysing the reaction. While the M. smegmatis enzyme could be functionally expressed, the M. tuberculosis homologue was insoluble and inactive under a large variety of expression conditions. For the M. smegmatis enzyme, the Vmax for S-adenosylmethionine formation was 1.30 μmol/min/mg protein and the Km for methionine and ATP was 288 μM and 76 μM respectively. In addition, the enzyme was competitively inhibited by 8-azaguanine and azathioprine with a Ki of 4.7 mM and 3.7 mM respectively. Azathioprine inhibited the in vitro growth of M. smegmatis with a minimal inhibitory concentration (MIC of 500 μM, while the MIC for 8-azaguanine was >1.0 mM. Conclusion The methionine adenosyltransferase from both organisms had a primary structure very similar those previously characterised in other prokaryotic and eukaryotic organisms. The kinetic properties of the M. smegmatis enzyme were also similar to known prokaryotic methionine adenosyltransferases. Inhibition of the enzyme by 8-azaguanine and azathioprine provides a starting

  10. Extracellular polysaccharidases synthesized by the epiphytic lichen Evernia prunastri (L.) Ach. (United States)

    Yagüe, E; Orus, M I; Estevez, M P


    Evernia prunastri Ach., an epiphytic lichen growing on Quercus rotundifolia Lam., produces a β-1,4-glucanase (EC and a polygalacturonase (EC The activity of these polysaccharidases increases as a response to incubation of the lichen with carboxymethylcellulose or sodium polygalacturonate, respectively. This increase in activity is thought to be the result of enzyme induction because it is inhibited by both cycloheximide and 8-azaguanine. Both polysaccharide-degrading enzymes are partially secreted into the incubation media.

  11. Haplopappus gracilis cell strains resistant to pyrimidine analogues. (United States)

    Jones, G E; Hann, J


    Strains of Haplopappus gracilis (Nutt.) Gray cells resistant to 6-azauracil have been isolated from cultures of diploid cells. These strains are also resistant to 8-azaguanine, as is their parent. The variants are 100- to 125-fold more resistant to 6-azauracil than their parent, and they exhibit different spectra of cross resistance to other pyrimidine analogues. The phenotype of each variant is stable in the absence of selection. The majority of cells in cultures of the variants are diploid; all others examined were tetraploid. Initial rates of uptake of uracil are not reduced in the variants. Fluorouracil, to which two variants are resistant, is taken up by one of them as well as by the parent. Responses of the other two to fluorouracil are not correlated with decreased ability to accumulate this analogue.

  12. Synthesis and antiviral activity of 8-aza analogs of chiral [2-(phosphonomethoxy) propyl]guanines. (United States)

    Franchetti, P; Sheikha, G A; Cappellacci, L; Grifantini, M; De Montis, A; Piras, G; Loi, A G; La Colla, P


    (R)- And (S)-8-aza-9(-)[2-(phosphonomethoxy)propyl]guanine [(R)-and (S)-8-aza-PMPG] were synthesized and tested in vitro for anti-human immunodeficiency virus (HIV) activity. The synthesis of the above compounds and of (R)-9(-)[2-(phosphonomethoxy)propyl]guanine [(R)-PMPG] was carried out through the alkylation of 8-azaguanine or guanine with (R)- and (S)-2-O(-)[(diisopropylphosphono)methyl]-1-O-(tolylsulfonyl) -1,2-propanediol followed by deprotection of the phosphonic moiety. A different, even more convenient synthesis of (R)-8-aza-PMPG starting from 2-amino-6-chloro-5-nitro-4(3H)-pyrimidinone and (R)(-)[2(-)[(diisopropylphosphono)-methoxy]propyl]amine is also reported. Both (R)-8-aza-PMPG and (R)-PMPG demonstrated anti-HIV activity in the MTT assay with EC50 values of 12 and 4.5 microM, respectively. The corresponding S enantiomers were found to be less potent. When evaluated in combination with AZT, ddI, or DABO 603, (R)-8-aza-PMPG gave additive, additive, and synergistic anti-HIV-1 effects, respectively.

  13. Cytotoxicity and mutagenicity of Kevlar: an in vitro evaluation. (United States)

    Wening, J V; Marquardt, H; Katzer, A; Jungbluth, K H; Marquardt, H


    Toxicity and mutagenicity of Kevlar 49 (PPPT; poly-para-phenylene-terephthalamide) was tested in six strains of Salmonella typhimurium (Ames test; TA97, TA98, TA100, TA102, TA1535, TA1537) with and without an external metabolic activation system (S9), as well as in a mammalian cell mutagenesis assay using V79 Chinese hamster cells. For the Ames test, liquid preincubation, which is considered particularly sensitive, was used. The cells were incubated for 24 h at a temperature of 37 degrees C either directly with Kevlar49 or with ethanol- or chloroform-extracted Kevlar49. The experiments were performed at least twice. The Ames test with six different Salmonella typhimurium strains featuring either base pair substitution or frameshift mutations revealed no cytotoxic or mutagenic activity of Kevlar49. In the mammalian cell mutagenesis assay, using 8-azaguanine (AG) as a selective agent, Kevlar49 was also devoid of cytotoxic or mutagenic activity. Both tests have to be regarded as an initial exploratory screening due to the chosen testing conditions and should be supplemented by tests at different temperatures.

  14. Metabolism of resistant mutants of Streptococcus faecalis. IV. Use of a biophotometer in growth-curve studies. (United States)



    Coultas, M. Katharine (Sloan-Kettering Institute for Cancer Research, Rye, N.Y.) and Dorris J. Hutchison. Metabolism of resistant mutants of Streptococcus faecalis. IV. Use of a biophotometer in growth-curve studies. J. Bacteriol. 84:393-401. 1962. - Analysis of data obtained using the Bonet-Maury and Jouan biophotometer has shown it to be an accurate, automatic device for recording an entire bacterial growth curve, from which quantitative measurements of growth rate can be calculated. Growth rate values obtained in this manner agree with those obtained by conventional methods. The instrument is described in detail and reference is given to other uses of this type of photometer. The growth curves of Streptococcus faecalis ATCC 8043 and three mutants resistant to purine analogues were compared in media with and without a purine. Differences in these growth curves were evident. In the purine-deficient medium SF/AZAG, the 8-azaguanine-resistant mutant had the shortest doubling time, 61 min, followed by the two 6-mercaptopurine-resistant mutants (SF/MPcc, 72 min; SF/MP, 91 min), and finally the wild strain with a doubling time of 135 min. In the medium supplemented with xanthine, the respective doubling times were 52, 61, 68, and 68 min.

  15. Selective fluorescent and fluorogenic substrates for purine-nucleoside phosphorylases from various sources, and direct fluorimetric determination of enzyme levels in human and animal blood

    International Nuclear Information System (INIS)

    Wierzchowski, Jacek; Ogiela, Maria; Iwanska, Beata; Shugar, David


    The decrease in fluorescence of 7-methylguanosine (m 7 Guo) accompanying its phosphorolysis is readily applicable to monitoring the level of purine nucleoside phosphorylase (PNP) in mammalian whole blood lysates, without prepurification of the latter. A 10-fold higher sensitivity is attained with the use of 8-azaguanine (8-azaGua), a non-reversible, highly fluorogenic, substrate in the reverse, synthetic pathway. Although its affinity for human PNP is low (K m =250 μM at pH 7), its high reaction rate and fluorescence quantum yield of the product, 8-azaguanosine (φ=0.55 at pH 9), permit direct fluorometric monitoring of nucleoside synthesis in 1000-fold diluted hemolysates. It is also quite selective for mammalian enzymes, 8-azaGua being a poor substrate for bacterial (E. coli) PNP. Specificity of the assay was confirmed by the competitive effects of the natural purines hypoxanthine and adenine, with IC 50 values comparable to their K m values. PNP levels in animal blood, assayed with both substrates, gave comparable results. With the exception of mice, PNP blood levels in animals were lower than in humans. We have also characterized specific fluorescent/fluorogenic substrates for bacterial (E. coli) PNP. The best of these is 2,6-diaminopurine riboside. Kinetic parameters for all the foregoing substrates are listed, and shown to be favorable for analytical applications

  16. Cryptic purine transporters in Aspergillus nidulans reveal the role of specific residues in the evolution of specificity in the NCS1 family. (United States)

    Sioupouli, Georgia; Lambrinidis, George; Mikros, Emmanuel; Amillis, Sotiris; Diallinas, George


    NCS1 proteins are H + or Na + symporters responsible for the uptake of purines, pyrimidines or related metabolites in bacteria, fungi and some plants. Fungal NCS1 are classified into two evolutionary and structurally distinct subfamilies, known as Fur- and Fcy-like transporters. These subfamilies have expanded and functionally diversified by gene duplications. The Fur subfamily of the model fungus Aspergillus nidulans includes both major and cryptic transporters specific for uracil, 5-fluorouracil, allantoin or/and uric acid. Here we functionally analyse all four A. nidulans Fcy transporters (FcyA, FcyC, FcyD and FcyE) with previously unknown function. Our analysis shows that FcyD is moderate-affinity, low-capacity, highly specific adenine transporter, whereas FcyE contributes to 8-azaguanine uptake. Mutational analysis of FcyD, supported by homology modelling and substrate docking, shows that two variably conserved residues (Leu356 and Ser359) in transmembrane segment 8 (TMS8) are critical for transport kinetics and specificity differences among Fcy transporters, while two conserved residues (Phe167 and Ser171) in TMS3 are also important for function. Importantly, mutation S359N converts FcyD to a promiscuous nucleobase transporter capable of recognizing adenine, xanthine and several nucleobase analogues. Our results reveal the importance of specific residues in the functional evolution of NCS1 transporters. © 2016 John Wiley & Sons Ltd.

  17. MMS induction of different types of genetic damage in Aspergillus nidulans: a comparative analysis in mutagenesis. (United States)

    Gualandi, G; Bellincampi, D; Puppo, S


    Methyl methanesulphonate (MMS) was used to test the induction of gene mutation, somatic crossing-over and mitotic non-disjunction in A. nidulans. Gene mutation was tested by inducing mutants resistant to 8-azaguanine and revertants of methG1 in a haploid strain. Somatic crossing-over was tested in heterozygous diploids, both with a selective method, i.e. inducing homozygosis to FPA resistance in a heterozygous fpa A1/+ strain, and with a non-selective method, i.e. identifying the frequencies of colour sectors. This latter method was also used to estimate the induction of non-disjunction because additional markers were present which permitted us to distinguish the two types of colour segregant. Generally, 3 different experimental procedures were used, namely the "plate test", i.e. plating of conidia in agar media containing MMS, and two types of "liquid test", i.e. brief treatment of quiescent or pre-germinated conidia in MMS solution before they were plated on agar media. Point mutations were induced with about equal efficiency with each method, whereas crossing-over was induced preferentially when germinating conidia were exposed to MMS. On the other hand, non-disjunction was induced in germinating and quiescent spores with equal efficiency, but such segregants were not recovered with the selective (fpa) method. The results are discussed for both their practical use in the mutagenic testing procedure and their theoretical implication.

  18. Mutagenicity of cyclopenta-fused polynuclear aromatic hydrocarbons and a non-polar fraction from a fuel combustion sample in a Salmonella forward mutation assay without exogenous metabolic activation. (United States)

    Busby, W F; Smith, H; Plummer, E F; Lafleur, A L; Mulder, P P; Boere, B B; Cornelisse, J; Lugtenburg, J


    A series of cyclopenta-fused polynuclear aromatic hydrocarbons (PAH) were tested for mutagenicity in a bacterial forward mutation assay based on resistance to 8-azaguanine (8-AG) in Salmonella typhimurium TM677 in the absence of Aroclor-induced rat liver postmitochondrial supernatant (PMS). All of the aceanthrylenes tested were mutagenic in the absence of PMS, whereas none of the acephenanthrylenes were active. The following mutagenic potency series expressed as the minimum detectable mutagen concentration (MDMC) in nmol/ml was obtained: aceanthrylene (AA) (5.5); cyclopent[h,i]aceanthrylene (CPAA)(18.2); 6-methylaceanthrylene (6-MeAA)(112); 1,2,6,7-tetrahydrocyclopent[h,i]aceanthrylene (THCPAA) (166); 1,2-dihydroaceanthrylene (DHAA) (298). Saturation of the cyclopenta rings or methylation at the 6-position of AA reduced, but did not eliminate, mutagenicity measured in the absence of PMS. AA was unusual because it was approximately 4-fold more mutagenic in the absence of PMS than in its presence. The other aceanthrylenes tested were 1.3-10.7 times more mutagenic in the presence of PMS than in its absence to give an MDMC potency series of: CPAA (3.8); 6-MeAA (10.5); AA (19.9); THCPAA (52.9); DHAA (229). Approximately 20% of the PMS-independent mutagenicity in a combustion sample from ethylene burned under fuel-rich conditions was found in a fraction containing only non-polar, 4-7 ring PAHs, widely attributed to be mutagenic only in the presence of PMS. None of this mutagenicity could be attributed to aceanthrylenes, thus other non-polar PAHs appear to possess significant PMS-independent mutagenicity as well.

  19. Leptospira mayottensis sp. nov., a pathogenic species of the genus Leptospira isolated from humans. (United States)

    Bourhy, Pascale; Collet, Louis; Brisse, Sylvain; Picardeau, Mathieu


    A group of strains representing species of the genus Leptospira, isolated from patients with leptospirosis in Mayotte (Indian Ocean), were previously found to be considerably divergent from other known species of the genus Leptospira. This was inferred from sequence analysis of rrs (16S rRNA) and other genetic loci and suggests that they belong to a novel species. Two strains from each serogroup currently identified within this novel species were studied. Spirochaete, aerobic, motile, helix-shaped strains grew well at 30-37 °C, but not at 13 °C or in the presence of 8-azaguanine. Draft genomes of the strains were also analysed to study the DNA relatedness with other species of the genus Leptospira. The new isolates formed a distinct clade, which was most closely related to Leptospira borgpetersenii, in multilocus sequence analysis using concatenated sequences of the genes rpoB, recA, fusA, gyrB, leuS and sucA. Analysis of average nucleotide identity and genome-to-genome distances, which have recently been proposed as reliable substitutes for classical DNA-DNA hybridization, further confirmed that these isolates should be classified as representatives of a novel species. The G+C content of the genomic DNA was 39.5 mol%. These isolates are considered to represent a novel species, for which the name Leptospira mayottensis sp. nov. is proposed, with 200901116(T) ( = CIP 110703(T) = DSM 28999(T)) as the type strain. © 2014 IUMS.

  20. Factors effecting the efficiency of purine analogues as selective agents for mutants of mammalian cells induced by ionising radiation

    International Nuclear Information System (INIS)

    Thacker, J.; Stephens, M.A.; Stretch, A.


    In the Chines hamster cell line V79-4, the frequencies of cells selected for their resistance to purine analogues do not always reflect the true frequencies of resistant mutants. The frequency of cells resistant to 8-azaguanine varied widely, especially when different sources of serum were used in the selective medium. Even with the more efficient analogue, 6-thioguanine, small colonies arose in the selective medium at a frequency which was strongly dependent upon analogue concentration and viable cell seeding density. These colonies were shown to have a phenotype which was indistinguishable from wild type. Hence with irradiated cells, where the viability of the cell population is reduced to an extent varying with the dose and the interval allowed for mutant expression, the counting of all colonies arising in selective medium can lead to spuriously variable, and sometimes very high, 'mutation frequencies'. Although the frequency of wild type colonies selected in thioguanine was diminished by the use of high concentrations of the analogue, a loss of induced mutants also occurred at these concentrations. Further, the V79-4 line contained two distinct types of mutant with different levels of hypoxanthine-guanine phosphoribosyl transferase (HGPRT) activity, and only one of these types (HGPRT-negative mutants) increased in frequency with radiation dose. These results can account for many of the anomalies encountered in previous studies with purine analogues as selective agents, and show that some care has to be taken to characterize the mutants selected by resistance to purine analogues before meaningful dose-response relationships can be established

  1. Transformation and mutation of golden hamster embryo cells induced by low doses of x-rays

    International Nuclear Information System (INIS)

    Watanabe, M.; Suzuki, N.; Nikaido, O.


    A new cell system which makes quantitative analysis possible in both mutation and transformation induced by low doses of X-rays was described and the frequencies of both mutation and transformation were compared in relation to DNA repair which takes place in X-irradiated cells. Golden hamster embryo (GHE) cells were employed to show the availability of the system for the efficient detection of both mutants and transformants concomitantly. The mutation frequency of the cell population irradiated with various doses of X-rays was expressed as the ratio of the number of 8-azaguanine resistant colonies to the 10 5 colonies formed in normal medium. A linear increase in mutation frequency with increasing dose was observed at doses ranging from 100 to 600 rad. There was no significant increase in mutation frequency with doses below 100 rad. On the other hand, the transformation frequency of the cells was expressed as the ratio of the number of the transformed colonies to the total number of colonies counted. A drastic increase in the transformation frequency was observed when cells were irradiated with less than 100 rad of X-rays. DNA repair might be involved in modifying transformation frequency and survivals of GHE cells, and DNA synthesis might be involved in inducing transformation in GHE cells. It seems that the repair of potentially lethal damage taking place in density-inhibited GHE cells within 24 hours after X-irradiation decreases the frequencies of both transformation and mutation. Furthermore, it is evident that the system using GHE cells is sensitive enough to assess the transformational effect of low doses of X-rays. (Namekawa, K.)