Summary of Symposium on Transfer RNA and Transfer RNA Modification in Differentiation and Neoplasia1

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[CANCER RESEARCH 31, 716-721, May 1971] Summary of Symposium on Transfer RNA and Transfer RNA Modification in Differentiation and Neoplasia1

P. C. Zamecnik The John Collins Warren Laboratories of the Huntington Memorial Hospital of Harvard University at the Massachusetts General Hospital, Boston, Massachusetts 02114

It is over a decade since the discovery of tRNA and 5 years other or toward the -CCA aminoacyl acceptor site. Thus there since the elucidation of the first primary sequence. The is no assurance that alteration of a single base (outside of the complete primary structures of approximately 19 tRNA's are anticodon triplet) has a direct effect on its aminoacyl acceptor now known, and without exception the clover leaf or ribosomal association functions. There is now no evidence configuration has provided a representation for the secondary to rule out the suggestion that the aminoacyl synthetase structure which is theoretically satisfying. The tertiary recognition site may be constituted from more than 1 loop structure is still, however, a hazy subject; we await results of area, with tertiary folding of the tRNA molecule bringing the X-ray diffraction of crystalline tRNA's, with the use of active site into being (87). isomorphous replacements, for clarification and possible The data of Shugart et al. (66), Stulberg and Shugart (72), decision among the several structures proposed. and Peterkofsky et al. (57) help to clarify the role of tRNA has long been known to be rich in minor base methylation of tRNA in aminoacyl synthetase recognition. modifications (6, 20), of which the most common is Using enzyme-saturating concentrations of tRNA^e and methylation, and this occurs on all 4 major bases. Data initial aminoacylation rates, Stulberg reports that under- presented at this symposium by Stulberg and Shugart (72) methylated tRNA cannot be aminoacylated as well as support the logical inference that these base modifications normally methylated tRNA can be, a conclusion with which change the tertiary structure of the tRNA's and modulate its the work of Peterkofsky (using tRNA^t) agrees. Peterkofsky several functions. The data of Carbon and Squires (15), using also finds that, while normal tRNA^H is acylated by both tRNA^'^3 and the glycine-activating enzyme, and of Gefter Escherichia coli and yeast aminoacyl-tRNA ligases, methyl- and BikotT (29), using the tRNATyr system of the Cambridge deficient tRNA^v is acylated only by the E. coli enzyme. group (1, 2, 30), implicate the anticodon in amino acid Furthermore, while normal tRNA^*^ recognizes both poly UC recognition as well as in the established interaction with the and poly UG, methyl-deficient tRNA^JÃ recognizes only poly codon triplet of the mRNA. These studies add substance to UC. Finally, he reports that undermethylation results in earlier data pointing in this direction, including the suggestion decreased ability of tRNAcoh to bind E. coli ribosomes. from our laboratory ( 13) that the 2nd position in the anticodon Stulberg and Shugart (72) report that overall protein synthesis, may be used as part of the aminoacyl synthetase recognition as reflected by poly U-stimulated phenylalanine incorporation, information. is also decreased in rate in the undermethylated species. The evidence of Shugart and Stulberg (67), using Studies on physical parameters of tRNA by the same borohydride reduction of intact tRNAÂ£Â£,et,points to the investigators show a change in circular dichroic and optical dihydrouracil loop as being essential for aminoacyl synthetase rotatory dispersion patterns and in melting profiles of recognition. In this connection, it is important to note, as undermethylated tRNAÂ£Â£,eÂ¡ascompared with the fully these investigators did, that this effect may be either direct or methylated tRNA^e. indirect. The 2-dimensional projection of tRNA in the clover The use of bacterial mutants with defects in leaf configuration fails to show the rotational translation tRNA-methylating enzymes would appear to be helpful in which occurs in each of the double-stranded stem regions. In pinpointing the role of methyl groups in the functioning of general, these 4 double-stranded stems (of the major loops plus tRNA. BjÃ¶rk(8, 9), now working with Neidhardt (56) on the â€”CCAend of the molecule) are quite free of modifications growth control mechanisms involving tRNA, has in fact of the major bases. In fact, there are no minor bases in the isolated an E. coli mutant lacking 5-methyluridine in its tRNA. double-stranded heptanucleotide of the -CCA region. Only small but significant decreases from normal were Chemically or mutationally induced minor base changes in the observed in the rate of growth of this mutant and in the rate double-stranded regions, by impairing hydrogen bonding or of overall protein synthesis and in translation efficiency. It still base stacking, would be likely to produce a topological change remains a possibility that the conditions used for these in the relationship of the loops of the molecule toward each experiments were not such as to make the rate of the reaction being scrutinized the limiting one. Nevertheless, at present BjÃ¶rk'sfindings fail to support those mentioned previously on 'This is Publication 1387 of the Cancer Commission of Harvard University. Supported by grants-in-aid from the United States Atomic the role of methylation in the biological functioning of tRNA. Energy Commission, the Department of Health, Education, and The work of Carbon and of Gefter in this symposium and Welfare, and the American Cancer Society. elsewhere (14-16, 29, 30, 41) points up the potential power

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Downloaded from cancerres.aacrjournals.org on October 2, 2021. © 1971 American Association for Cancer Research. tRNA in Differentiation and Neoplasia of the use of mutational changes in tRNA in the study of these found to charge tRNAyg^t well as a substrate. Whether it structure-function relationships. Carbon and Squires (15), in dissociates into subunits has not been determined. particular, emphasize the greater specificity and freedom from confusing side reactions of mutational alteration of tRNA as compared with chemically induced changes. It is clear that this Alterations of tRNA during Differentiation approach, i.e., genetic engineering, holds promise for solution of the recognition reaction problem. At this point, the symposium moved from a biochemical Hall et al. (35) mentioned that from their work, that of characterization of tRNA's to a consideration of alterations of Skoog and the Wisconsin-Illinois group, and that of others (cf. tRNA during differentiation. Separate data of Taylor [Holland Ref. 68), it appears that for codons with uracil in the 1st et al. (42); Taylor et al. (73, 74)], Mittelman (53), and Yang position (i.e., UNN codons, where N refers to any 1 of the 4 (85, 86) support the conclusion that there are more ribonucleotide bases) that an isopentenyladenylyl residue or a isoacceptor tRNA species in embryonic forms of a tissue than modification thereof is in the position adjacent to the in the adult tissue. Taylor [Taylor et al. (73, 74)] compared anticodon base which pairs with the uracil residue in position tRNA's from unfertilized sea urchin eggs with those from 1 of the codon triplet. Newer data from Dyson et al. (21) 24-hr blÃ¡stulas and tRNA's from hepatomas. Using methyl indicate that 7V-(purin-6-ylcarbamoyl)threonine is located in ated albumin-Kieselguhr columns and reversed phase column the residue adjacent to the anticodon base which pairs with an chromatography, as introduced by Weiss and Kelmers (84), he A situated in the first position of a codon triplet. Incidentally, found both quantitative and qualitative changes in specific ./V-[9-(/3-D-ribofuranosyl)purine-6-ylcarbamoyl] glycine has also tRNA elution patterns. He was led to consider that genetic recently been isolated from enzymatic digests of redundancy and differential gene transcription may be unfractionated yeast tRNA (65). Both isopentenyladenosine involved, in explanation of the above results, and in agreement and modification of the carbamoylthreonine adenosine residue with a conclusion of Russell et al. (64). His experiments also are powerful cytokinins. The work of Gefter and Bikoff (29) suggested, in line with recent work of Burdon and Clason (12) with tRNA mutants also indicates that the modification of the and of Bernhardt and Darnell (7), and as presented here also base adjacent to the anticodon in tRNA is related to the code by Gefter and Bikoff (29), the presence in his cellular RNA words used. Removal of the modification impairs separations of RNA molecules which are precursors to tRNA. codon-anticodon interaction by reducing binding of the tRNA HatfÃ¬eldet al. (39) found differences in elution profiles to ribosomes (30). Hall hinted that it is conceivable that there (using Kelmers-type columns) of methionyl-, arginyl-, and may also be particular base modifications adjacent to seryl-tRNA's from bovine brain and liver. No differences in elution profiles of the phenylalanyl-, lysyl-, and leucyl-tRNA's anticodons which pair with cytosine and guanine in the 1st codon positions, although there is presently no firm from brain, liver, and kidney nor in elution profiles of methionyl-, arginyl-, and seryl-tRNA's of liver and kidney were experimental support for this suggestion. There is, however, sufficient evidence from the above-mentioned data to point observed. Furthermore, codon responses of fractionated out that positioning of an amino acid in a growing peptide seryl-tRNA were reported. One species of seryl-tRNA chain, with the high degree of fidelity which is characteristic responded specifically to the codon, UGA. These organ- of protein synthesis, may involve more than a triplet specific differences in Chromatographie elution pattern, and codon-anticodon interaction. This point of view was forecast the codon response to UGA have no ready explanation. Great in the early days of the coding era by Loftfield (49), who pains were taken to rule out or minimize possible artifactual based his opinion on the energetics of the selectivity process. differences that might be related to isolation procedures. Fidelity of translation, as has been mentioned in this Mittelman (53, 55) extracted tRNA from spleens of symposium by Borek (10), may be a function of tertiary patients with malignant lymphoma and leukemia and from structure of the tRNA molecule. nonneoplastic diseases. Fractionation of these tRNA's on Little direct attention was paid at this symposium to the benzoylated diethylaminoethyl cellulose columns (33) nature of the interaction between an aminoacyl-tRNA ligase revealed the presence of 2 phenylalanine acceptor tRNA's in and its cognate tRNA. It is surprising how little progress has nonneoplastic spleens, while from the neoplastic spleens there been made toward elucidation of the molecular nature of the were at least 3 phenylalanine acceptor tRNA peaks. When code which must exist between a few key side chains of the these tRNA's were charged with aminoacyl synthetase amino acids of the aminoacyl-tRNA synthetase and certain extracted from human embryo livers, many isoaccepting bases or modified bases of the cognate tRNA. tRNAPhe subspecies could be observed. There was suggestive Tchou et al. (75) described their purification of evidence that the number of aminoacyl synthetases and their tryptophan-tRNA synthetase from human leukemic blood. activities might vary with the age of the embryonic tissue. One pint of blood may provide an amount of starting material Yang (85, 86) extracted tRNA from 2 mammalian tumor (around 200 g of lymphoblasts) sufficient for enzyme systems, L-m tumor cells (both in tissue culture and in the purification comparable to the scale used for E. coli. By means mouse), and Reuber hepatomas; and from 3 control tissues, of column chromatography, a 240-fold purification was regenerating, fetal, and adult normal livers. Using reversed obtained, and another 10-fold purification was achieved by phase Chromatographie columns, he found that the elution isoelectric focusing. The enzyme was partially characterized patterns for the tRNA's from tumors were very different from with respect to stability, kinetics, and specificity, and a those of the control tissues for the aspartyl-, histidyl-, tyrosyl-, molecular weight of the order of 90,000 was obtained. It was and threonyl-tRNA's. Isoaccepting tRNA's of tumor tissues

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Downloaded from cancerres.aacrjournals.org on October 2, 2021. © 1971 American Association for Cancer Research. P. C. Zamecnik such as aspartyl- or tyrosyl-tRNA's could be divided into 2 in tRNA methylase activity in the brain tumors and also sets. One tRNA similar to the normal, differentiated tissue and marked changes in the percentages of certain minor bases one tRNA not found in the differentiated tissue were also present in the tumors as compared with their percentages in found in embryonic tissue (term fetus liver), although in lower normal brain tissue. The question was raised by Weinstein (82) quantity in the latter. as to whether such differences might reflect, as mentioned Results parallel to these have also been reported in previously, histochemical variability, as for example between differentiating plant tissues (18), and in sporulating bacteria the amount of gray matter versus white matter of the (48). These various findings agree with the hypothesis (71) malignant as compared with the normal tissue. Here again, that differentiation and loss of synthetic capacities there seemed to be no ready answer to the query. characteristic of cell growth are associated with loss of specific Randerath (61, 62) then reported details of a sensitive isoacceptor tRNA subspecies, with consequent codon technique for identifying and determining quantitatively the restriction. minor base constituents of tRNA. This procedure depends on degradation of the tRNA enzymatically by nucleases and phosphomonoesterases to the mononucleoside level. Next, the Alterations in tRNA and tRNA Methylases in Neoplasia mononucleosides are oxidized by periodate; then they are reduced to the trialcohols by tritiated borohydride of very Mandel [Hacker and Mandel (34); Mandel Ã©tal.(52)] found suggestive evidence of /zypermethylation of tRNA in Marek's high specific activity (cf. also, Refs. 47 and 59). These trialcohols of the mononucleosides are next separated by disease (the cell-associated herpes virus oncogenic disease) in 2-dimensional thin-layer chromatography on cellulose. They infected chicken livers as compared with normal livers. He did are then located by fluorography, the latter procedure being find aberrant methylation in vitro with the neoplastic liver enhanced (60) in sensitivity by addition of scintillator directly extracts as compared with the normal liver, to whit, in 5 to the cellulose plate and by exposure to sensitive X-ray film methyl uracil, 7 methyl guanosine, and 3 methyl-cytosine. In at â€”80Â°.Usingthis procedure, Randerath found no differences vitro tRNA methylase activity was elevated 3-fold in the in minor base contents between several tumors and normal tumors. tissues, thus introducing a disquieting note in the hitherto The question was raised here, and in subsequent papers generally agreeable preceding reports. There was no ready involving comparisons of methylases and methylating patterns reconciliation of these findings with those of Viale (79, 80) in malignant and control normal tissues, as to what role the nor with certain other published data on minor base change in histological cell type distribution between the constituency of tRNA from tumors as compared with normal tumorous tissue and the tissue from which it was derived tissues (6, 55). On the other hand, in some published reports might play in the change in methylase and in methylation (3, 43), base constituency data of tumors are in agreement pattern. There was no easy answer to this question, which has with the data of Randerath. There was also a conflict between of course troubled investigators in the cancer field for years, the report of a high A/'-methylguanosine content of hamster whenever a comparison is made between constituents of liver tRNA by Mittelman et al. (54) and the value obtained by malignant and normal tissues. This point was also made in a Randerath (61). study of enzymatic methylation of tRNA by Gantt (27, 28) as In the presence of some of the uncertainties mentioned reported at this meeting. The necessity for careful histological above, due in reality to the complexity of the biological controls in comparisons of methylase and 14C-methylating system under study and to the difficulties of isolating minor activities in normal and tumor tissues has likewise been bases, the reports of Kerr (45) and Kerr et al. (46) were both stressed recently by Breier and Holley (11). very interesting and an added source of worry to those Work of Gallo (25, 26), Turkington (78), Pillinger and studying methylases. At this meeting, we seemed to be in the Wilkinson (58), Hay et al. (40), and Yang (85, 86) then position of one taking an automobile off the assembly line pointed out that the methylating activity of neoplastic liver before completion and driving off while trying to attach a was more similar to that of fetal liver than it was to adult liver. wheel. We are trying to explore the biochemical anatomy of Put another way, the methylase activity and methylation the machinery of growth and at the same time to apply it to patterns appear to be more nearly alike between neoplastic the problems of differentiation and neoplasia. and fetal tissues than between fetal and adult tissues. This Kerr (45) found 2 methylase-inhibitory factors in the observation suggests the possibility that a neoplastic cell may 100,000 X g, pH 5 supernatant fraction of rabbit liver. be unmasking genetic information which was present earlier in Together, these factors complexed with rabbit liver methylase the embryonic or fetal state of the same species, information and inhibited its action. One of these factors was a protein which has become repressed and unavailable for transcription with a molecular weight of 100,000, and the other was a small and translation in the highly differentiated cell. peptide with a molecular weight of below 700. Fetal livers and Is derepression of embryonic information in a differentiated certain tumors were found to lack the large-molecular-weight adult tissue sufficient in itself to account for the neoplastic protein-inhibitory factor. It is thus possible that these 2 state? This is an uneasy question for which there may be no factors (cf., also Ref. 17) and a somewhat different direct answer at present. methylase-inhibiting factor, which Wainfan (81) mentioned in Viale (79, 80) compared tRNA methylases and minor base her bacterial methylase studies, may play a role in the control contents of tRNA's of a large number of human brain tumors mechanisms involved in differentiation. and of samples of a considerable number of normal brain There were several papers on chemical carcinogenesis. The tissues obtained from frontal lobectomies. He found a change presentation of Weinstein et al. (83) was a nice description of

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molecular oncology, in pointing out a rather specific and Smith, J. D. Mutant Tyrosine Transfer Ribonucleic Acids. J. attachment of the carcinogen acetylaminofluorene to position Mol. Biol., 47: 15-28, 1970. 8 of a guanine residue in tRNA. Model studies reinforced the 3. Baguley, B. C., and Staehelin, M. Substrate Specificity of Adenine-specific Transfer RNA Methylase in Normal and Leukemic impression that the attachment of the carcinogen to this Tissues. European J. Biochem., 6. 1-7, 1968. position would interfere both with the hydrogen bonding and 4. Baliga, B. S., Srinivasan, P. R., and Borek, E. Changes in the tRNA with the single chain base stacking of the affected base. Methylating Enzymes during Insect Metamorphosis. Nature, 208: Magee (50, 51) studied the relation of chemical methylation 555-557, 1965. to carcinogenesis and concluded that, while there is no clear 5. Baltimore, D. Viral RNA-dependent DNA Polymerase. Nature, indication of which bases of nucleic acids are critical for 226: 1209-1211, 1970. carcinogenesis, tRNA may be a crucial target. 6. Bergquist, P. L., and Matthews, R. E. F. Occurrence and In relationship to chemical carcinogenesis, Hancock Distribution of Methylated Purines in the Ribonucleic Acids of (36-38) studied details of the methylation reaction itself, Subcellular Fractions. Biochem. J., 85: 305-313, 1962. described an ammonium ion enhancement effect on methylase 7. Bernhardt, D., and Darnell, J. E., Jr. tRNA Synthesis in HeLa (cf. also, Refs. 44 and 63), and endeavored to define the Cells: A Precursor to tRNA and the Effects of Methionine Starvation on tRNA Synthesis. J. Mol. Biol., 42: 43-56, 1969. conformational requirements for methylase activity. He also 8. Bjork, G. R., and Isaksson, L. A. Isolation of Mutants of posed the rather bold question, suggested likewise by Escherichia coli Lacking 5-Methyluracil in Transfer Ribonucleic Weinstein et al. (83), of whether all carcinogenesis might be Acid or 1-Methylguanine in Ribosomal RNA. J. Mol. Biol., 51: due to alterations of minor bases of transfer RNA. Weinstein 83-100, 1970. had presented a diagram suggesting that there may be some 9. BjÃ¶rk,G. R., and Neidhardt, F. C. Analysis of 5-Methyluridine normal feedback from tRNA to the genetic material. Such a Function in the Transfer RNA of Escherichia coli. Cancer Res., 31: possibility does not seem as remote as it might have appeared 706-709,1971. before the Temin-Baltimore (5, 76) enzyme became well 10. Borek, E. Introduction. Cancer Res., 31: 596-597, 1971. recognized. Incidentally, since the term "retroscription" has 11. Breier, B., and Holley, R. W. Enzymatic Methylation of Transfer been used in parlance to describe the action of this RNA-DNA RNA by Extracts of Polyoma Virus-transformed Cells. Biochim. polymerase, the colloquial name "retroscriptase" may be Biophys. Acta, 213: 365-370, 1970. 12. Burdon, R. H., and Clason, A. E. Intracellular Location and suggested for it. Molecular Characteristics of Tumor Cell Transfer RNA Precursors. It may be interesting to recall that we have recently had a J. Mol. Biol., 39: 113-124, 1969. defense of the Central Dogma presented nicely by Crick (19). 13. Burton, K., Varney, N. F., and Zamecnik, P. C. Action of Osmium According to the current formulation of the directions of flow Tetroxide on Amino Acid-Transfer Ribonucleic Acid: Correlations of information in the living cell, protein is not synthesized between the Genetic Code and the Sensitivity of Acceptor when protein is used as a template, but recent work of the Activity. Biochem. J., 99: 29C-31C, 1966. Lipmann (31, 32) and the Laland (24) groups shows that the 14. Carbon, J., and Curry, J. B. Genetically and Chemically Derived Missense Suppressor Transfer RNA's with Altered Enzymic peptide antibiotic, gramicidin S, containing 10 amino acids, can be synthesized on an enzyme surface without the Aminoacylation Rates. J. Mol. Biol., 38: 201-216, 1968. intervention of an RNA template. This finding raises thoughts 15. Carbon, J., and Squires, C. Studies on Genetically Altered Transfer RNA Species in Escherichia coli. Cancer Res., 31: 663-666, 1971. about the time in evolutionary molecular biology when tRNA 16. Carbon, J., Squires, C., and Hill, C. W. Glycine Transfer RNA of may have been inserted. If one searched hard enough among Escherichia coli. II. Impaired GGA-Recognition in Strains primitive living species, there might be found not only Containing a Genetically Altered Transfer RNA: Reversal by a precursor molecules of tRNA which are 20 residues or so Secondary Suppressor Mutation. J. Mol. Biol., 52: 571-584, 1970. longer than tRNA itself, as mentioned in this symposium, but 17. Chancy, S. Q., Halpern, B. C., Halpern, R. M., and Smith, R. A. possibly simpler, shorter models of tRNA, sparse in minor base Transfer RNA Methylase Inhibitors in Neoplastic and Normal Rat constituents. These perhaps would be incapable of the Tissue. Biochem. Biophys. Res. Commun., 40: 1209-1215, 1970. sophistication which tRNA's now have in translating the 18. Cherry, J. H., and Osborne, D. J. Specificity of Leucyl-tRNA and message of the nucleotide to that of the protein. Synthetase in Plants. Biochem. Biophys. Res. Commun., 40: 763-769,1970. Let us mention that Aelius Donatus said in the 4th century A.D., "Perish those who said our good things before we did." 19. Crick, F. Central Dogma of Molecular Biology. Nature, 277: 561-563,1970. The last speaker at this symposium is, contrariwise, indebted 20. Dunn, D. B. Additional Components in Ribonucleic Acid of to the preceding speakers for helping to clarify the role of Rat-Liver Fractions. Biochim. Biophys. Acta, 34: 286-288, 1959. methyl groups in the functioning of tRNA. Finally, we are 21. Dyson, W. H., Chen, C. M., Alam, S. N., Hall, R. H., Hong, C. I., grateful to Dr. Borek for bringing this symposium together and and Chheda, G. B. Cytokinin Activity of Ureidopurine Derivatives for pointing his finger so early (4, 22, 23, 69, 70, 77) at the Related to a Modified Nucleoside Found in Transfer RNA. Science, relationship of methylation of tRNA to problems of growth 770:328-330,1970. and neoplasia. 22. Fleissner, E., and Borek, E. A New Enzyme of RNA Synthesis: RNA Methylase. Proc. Nati. Acad. Sei. U. S., 48: 1199-1203, 1962. REFERENCES 23. Fleissner, E., and Borek, E. Studies on the Enzymatic Methylation of Soluble RNA. I. Methylation of the s-RNA Polymer. 1. Abelson, J., Barnett, L., Brenner, S., Gefter, M., Landy, A., Biochemistry, 2: 1093-1100, 1963. Russell, R., and Smith, J. D. Mutant Tyrosine Transfer Ribonucleic 24. Fr0shov, 0., Zimmer, T. L., and Laland, S. G. The Nature of the Acids. Federation European Biol. Soc. Letters, 3: 1-4, 1969. Enzyme Bound Intermediates in Gramicidin S Biosynthesis. 2. Abelson, J. N., Gefter, M. L., Barnett, L., Landy, A. Russell, R. L., Federation European Biol. Soc. 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25. Gallo, R. C. Transfer RNA and Transfer RNA Methylation in 46. Kerr, S. J., Sharma, O. K., and Borek, E. Agents of Modulation of Growing and "Resting" Adult and Embryonic Tissues and in the Transfer RNA Methylases. Cancer Res., 31: 633-636, 1971. Various Oncogenic Systems. Cancer Res., 31: 621-627, 1971. 47. Khym, J. X., and Cohn, W. E. Characterizations and Some 26. Gallo, R. C., and Pestka, S. Transfer RNA Species in Normal and Chemical Reactions of Periodate-Oxidized Nucleosides. J. AM. Leukemic Human Lymphoblasts. J. Mol. Biol., 52: 195-219, Chem. Soc.,82: 6380-6386, 1960. 1970. 48. Lazzarini, R. A. Differences in Lysine-sRNA from Spore and 27. Gantt, R., and Evans, V. J. Comparison of Soluble RNA Methylase Vegetative Cells of Bacillus Subtilis. Proc. Nati. Acad. Sei. U. S., Capacity in Paired Neoplastic and Nonneoplastic Cell Lines in 56. 185-190, 1966. Vitro. Cancer Res., 29: 536-541, 1969. 49. Loftfield, R. B. The Frequency of Errors in Protein Biosynthesis. 28. Gantt, R. R. In Vitro Transfer RNA Methylation in Paired Biochem. J., 89: 82-92, 1963. Neoplastic and Nonneoplastic Cell Cultures. Cancer Res., 31: 50. Magee, P. N. The Possible Role of Nucleic Acid Methylases in the 609-612,1971. Induction of Cancer. Cancer Res., 31: 599-604, 1971. 29. Gefter, M. L., and Bikoff, E. Studies on Synthesis and Modification 51. Magee, P. N., and Barnes, J. M. Carcinogenic Nitroso Compounds. of Transfer RNA. Cancer Res., 31: 667-670, 1971. Advan. Cancer Res., 10: 163-246, 1967. 30. Gefter, M. L., and Russell, R. L. Role of Modifications in Tyrosine 52. Mandel, L. R., Hacker, B., and Maag, T. A. Altered Transfer RNA Transfer RNA: A Modified Base Affecting Ribosome Binding. J. Methylase Patterns in Marek's Disease Tumors. Cancer Res., 31: Mol. Biol., 39: 145-157, 1969. 613-616,1971. 31. Gevers, W., Kleinkauf, H., and Lipmann, F. The Activation of 53. Mittelman, A. Patterns of Isoaccepting Phenylalanine Transfer Amino Acids for Biosynthesis of Gramicidin S. Proc. Nati. 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Downloaded from cancerres.aacrjournals.org on October 2, 2021. © 1971 American Association for Cancer Research. Summary of Symposium on Transfer RNA and Transfer RNA Modification in Differentiation and Neoplasia

P. C. Zamecnik

Cancer Res 1971;31:716-721.

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