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The 1989 in Chemistry Goes to Sidney Alhmm and Thomas R. Cech for the Discovery of Enzymatic RNA

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The 1989 has been awarded to Sidney Altrnan and Thomas R. Cech for their discovery that RNA in living cells can function not onfy as a molecule of heredity, but also as a biocatalyst. A brief historical overview of the laureates’ work is presented, and citation data for their most-cited publications are examined. Research-front analysis highlights both scientists’ cen- trafit y to the field of RNA enzymology studies. The effect of the finding of RNA catafysis on origin- of-iife theories is also covered. In the late 1970s and early 1980s, two and cell timction have had to be rethought. researchers, working independently, made In the words of the Swedish Academy, a discovery that reversed a 50-year-old “Many chapters in our textbooks have to be dogma in biochemistry-that the triggering revised. ” 1 and acceleration (catalysis) of chemicrd reac- Future applications of catalytic RNA tions within living cells were the exclusive could include its use as gene shears to domain of protein molecules called en- destroy RNA molecules involved in viral zymes. Biophysicist Sidney Altman, Yale infections such as the common cold or University, New Haven, Connecticut, and AIDS, in producing genetically engineered, Thomas R. Cech, University of disease-resistant agricultural plants, and, at Colorado, Boulder, found that ribonucleic some date, correcting certain hereditag dis- acid (RNA), traditionally considered to be eases in both animals and humans. only a passive intracelktkw carrier of genetic The chemistry prize carries an award of information, can rdso act as an or three million Swedish kronor, or about biocatalyst. This unexpected and unprece- 470,000 US dollars. The 1989 award to the dented discovery has been hailed as one of two Americans continued a trend of the the most important scientific breakthroughs chemistry prize throughout the 1980s, where of the 1980s, and, in recognition of this, 12 out of the 19 awards were given to US Altman and Cech were awarded the 1989 scientists. Altman and Cech became the 35th Nobel Prize in chemistry by the Royal and 36th Americans to win the Nobel Prize Swedish Academy of Sciences. in chemistry since it was first instituted in Cech showed that the RNA molecule 1901. There have been 114 chemistry prize- could cut and rejoin itself and thereby alter wimers through 1989. the material it produces, an operation that had previously been thought to be the sole Bbgraphical Data property of proteins. Altmatr showed that Sidney Akman RNA could have an independent catalytic function-that is, it could alter the makeup Sidney AJtrnan was born May 8, 1939, in of other than itself, proving that RNA , , . He spent his was indeed as much a catalyst as proteins. childhood in the west end Montreal district The work of Altrnart and Cech has led to a of Notre-Dame-de-Grace, where his father new scientific field called RNA enzymol- ran a grocery store. Altman left Montreal ogy, and old theories of the origin of life in 1956 to attend the Massachusetts Institute

266 Sidney Ahnan Thomas R. Cech of Technology (MIT), Cambridge, where he University, New York (1990); and honorary received a BS in physics in 1960. Seven degrees from the University of Montreal and years later, Altman earned a PhD in bio- York University, Toronto, Ontario, Canada physics from the University of Colorado. (1990). He has also been an associate editor Subsequently, he obtained research fellow- of Cell, 1983-1987, and a consulting editor ships from (1%7-1969), of Amen’can Scientist, 1972-1990. Through Cambridge, and the Medical Research early 1990, Altman has authored 89 Council Laboratory of publications. (1969-1971), , UK, where he worked with Sydney Bremer and Zhomas R. Cech Nobelist Francis H.C. Crick. Ahrnan joined the faculty of Yrde University as an assistant Thomas Robert Cech was born in Chi- professor in 1971 and has been a professor cago, Illinois, on December 8, 1947. He of biology there from 1980 onwards. He received a BA in chemistry from Grimell rdso acted as a dean at Yale College from College, Iowa, in 1970 and a PhD in chem- 1985 through last year. In 1984 Altman istry from the University of California, became a US citizen while retaining his Berkeley, in 1975. After a stint at MIT as Canadian citizenship. a National Cancer Institute fellow in molec- Altman’s honors and awards include ular biology (1975-1977), he joined the membership in the US Academy of Arts and faculty of the University of Colorado in Sciences (1988), the US National Academy 1978. Cech became a full professor there of Sciences (NAS) (1990), and the American in 1983. From 1988 onwards he has been Philosophical Society (1990); the Lewis S. an investigator of tie Howard Hughes Medi- for Distinguished Work crd Institute at the university. in Basic Medical Research (given by Bran- Cech received the 1985 Pfizer Award in deis University, Walthasn, Massachusetts) Enzyme Chemistry and was elected to 1989; the Yale Science and Engineering membership in NAS in 1987. In 1988 he Association Medal (1990); the Distinguished received the American Association for the Service Medal, Teachers College, Columbia Advancement of Science Newcombe-Cleve-

267 Tab}e 1: The mmst-rikd works of Sidney Attrnan and Thomas R. Ceck Data sre from the SCP /SSCl@, 1945-1989.A. numberof citations. Works thst appear as core papain the ftistoriograph in Figure 2 are indicated with an asterisk (*); research-front numbers appear in parentheses after the bibliographic &ta.

A Bfblfographic Data

273 ●Kruger K, Grabowsfd P J, Zaug A J, Smrds J, G@tacbfing D E & Ceeh T R. Self-splicing RNA: autoexcision and autmycfization of the ritrosonud RNA intervening sequence of Tetrahymena. Cell 31:147-57, 1982. (84-1651, 85-3838, 86-1923, 87-7297) 252 Aftman S & Smith J D. Tyrosine tRNA precursor molecule polynucleotide sequence. Narure New Biol. 233:35-9, 1971. 208 ●Guerrier-Takada C, Gardiner K, Marsh T, Pace N & AttrtmrsS. The RNA moiety of is the catalytic subunit of the enzyme. Cell 35:849-57, 1983. (84-1651, 85-3838, 86-1923, 87-6317, 88-3760) 191 *Ceeh T R, Tamrer N K, Tmoco 1, Weir B R, Zuker M & Perhoan P S. Secondary structure of the Tetmbymena rihosormd-RNA intervening sequence: structursf homology with fungal mitochondrial intervening sequences. Proc. Na[. Acad. Sci. USA SO:3903-7, 1983. (85-3838, 86-1923) 172 *Ceeh T R & Boas B L. Biological cata.fysis by RNA, Annu. Rev. Biorhem. 55 :59S-629, 1986. (87-1678, 88-3760) 172 Cech T R, Zaug A J & Grafmwdd P J. /n vitro splicin8 of the ritroaomsl RNA precursor of Tetrahymena: involvement of a guanosine rurcleotide in the excision of the intervening sequence. Cell 27:487-%, 1981. 142 *Ceeh T R. RNA splicing. three themes with variations. Ce[[ 34:713-6, 1983. (85-3838) 125 Akrnasr S. Isolatinn of tyrosine tRNA precursor molecules. Nafure New Bio/. 229:19-20, 1971. 116 Ceeh T R. The generality of self-splicing RNA: relationship to nuclear messenger RNA splicing, Celf 44:207-10, 1986, 116 Stark B C, Kole R, BowmanE J & Aftman S. ROJOnuclmse P: an enzyme with an essential RNA compunent. Proc. Naf. Acad Sci. USA 75:3717-21, 1978, 109 Ceeh T & Pardue M L. Cross-linking of DNA with trimethylpsorafen is a probe for chromatin structure, Cef/ 11:631-40, 1977, 88 Aftmars S. Biosynthesis of trsnsfer RNA in Escherichia coli. Cell 4:21-9, 1975. 81 Ceeh T R & Hearst J E. Electron microscopic study of mouse foldback DNA. Cell 5:42946, 1975. 76 Gutfrrie C, Seidman J G, Aftmmr S, Barren B G, Smftb J D & MeClahs W H. Identification of tRNA precursor molecules made by phage T4. Naoere New Bid, 246:6-11, 1973. 72 Cech T R & Pardue M L. Electron microscopy of DNA crnsslinked with trimethylpsoralerx test of secondary structure of eukaryotic inverted repest sequences, Pror. Nat, Acd. Sci. USA 73:2644-8, 1976, 65 Aftman S & Lerman L S. Kinetics and intermediates in intracellular synthesis of bacteriophage T4 deoxyribonucleic acid. J. .Mol, Bid. 50:235-61, 1970. 61 Ceeh T R, Roaerrfeld A & Hearst J E. Characterization of the most rapidly denaturing squences in mouse main-band DNA. J, Mol. Biol, 81:299325, 1973. 58 Ceeh T R & Rio D C. Localization of transcribed rezions on exrrachromcwmnaf ribmomal RNA 8enes of Tetmhymena therrnophila by R-lwp ma.ppi;g, Proc. Nat. Acad. Sci. USA 76:5051-5, 1979,

I land Award, the Royal Netherlands Sidney Akman: RNase-P and the Academy of Sciences Heineken Prize, the Overturning of a Dictum Gairdner International Award for Outstand- ing Achievement in Medical Science (given by the Gairdner Foundation, Willowdale, Altman’s early research focused on RNA Ontario), and the Albert Lasker Basic Medi- processing, in particular that of transfer cal Research Award (given by the Akrt and RNA (tRNA). The tRNAs are the nucleic Mary Lasker Foundation, New York). In acid molecules responsible for delivering in- 1989 Cech received the Lewis S. Rosenstiel dividual amino acids to the protein-synthe- Award, which he shared with Altman. Cech sizing machinery of a cell. In 1970 Altman rdso ha8 been an associate editor for Ceif and his colleagues isolated a tRNA-pro- (1986-1987) and serves on the editorial cessing enzyme found both in bacteria and board of Genes and Development ( 198’7on- higher cells and named it RNw-P.Z The wards). Cech has authored 105 publications characteristics of the enzyme are unusual in through early 1990. that it contains RNA and protein in a single

268 package. This enzyme is responsible for one ing that RNA was a necessary, if not suffi- of the reactions that produces tRNA. cient, component of RNase-P. In his Nobel RNase-P is found in human intestinal lecture, he recalled: “When Stark’s experi- bacteria, Escherichia coli, and participates ments were published we did not have the in the activation of tRNA by removing a por- temerity to suggest, nor did we suspett, that tion of the precursor to tRNA, which is un- the RNA component alone of RNase-P could necessary for the function of the mature be responsible for its catrdytic activity. The tRNA molecule. Altman’s eady work in fact that a simple enzyme had an essential observing and characterizing enzymatic RNA subunit, in itself, seemed heretical activity affecting tRNA resulted in a series enough.”1 of papers in the early 1970s. Several of these The paper reported the authors’ findings appear in Table 1, a listing of the two Nobel in cautious and circumspect language. We laureates’ highly cited works. Altman’s stating that’ ‘the presence of a dkcrete RNA most-cited work, ‘‘l$osine tRNA precur- component in RNase-P appears to be essen- sor molecule polynucleotide sequence,”3 tial for enzymatic function, ” the authors coauthored with J.D, Smith, Medical Re- acknowledged that “we do not know if the search Council Laboratory of Molecular RNA component... is needed for stabiliza- Biology, and published in Nature New tion of the protein moiety [of RNase-P] or Bi@ogy in 1971, dates from this period. if it plays a more active role in substrate In 1976 one of Altman’s graduate students recognition.’ ‘c Although the authors of- at , Ben C. Stark, showed fered a “model.. for enzyme-substrate r&- that both the RNA and protein were essen- ognition in which this RNA component tial components of the active enzyme.z plays an important role, ” they also stressed Akrnan and his colleagues at Yale also that “this scheme is hypothetical: wc have found that RNase-P seemed to lose its no direct evidence that the critical position- enzymatic abilities when the RNA portion ing of RNase-P on precursor tRNA sub- was destroyed or removed. Originally, strates is determined by RNA-RNA rather Altman thought that the RNA component of than by protein-RNA interactions.’ ‘c the enzyme RNase-P did not serve any According to Science Citation Indexa active enzymatic function because of the (SCl@ ) data, the paper excited significant– well-established “fact” that only proteins but not extraordinary-initial interest as could serve as catalysts. judged by citations. It was cited once in 1978 The finding that RNA plays an essential and 13 times in 19’79. Citations nearly role in an enzyme directly challenged the doubled to 23 in 1980 and 21 in 1981. They long-held biochemical dogma of’ ‘proteins declined sharply to six in 1982, and the only as catalysts. ” Not surprisingly, paper continued to be cited at about this level Altman’s work met with some resistance in through 1988. Clearly, the paper reported the biochemical community.z.d,s Altman’s an intriguing and even sensational hypoth- colleague, Yale biologist Joel Rosenbaum, esis. But the impact of this hypothesis was observed to a Washington Post reporter: “It perhaps limited by the lack of hard experi- was heresy .... What Sidney was saying was mental evidence to convince researchers of so revolutionary he had trouble getting his RNA’s enzymatic properties. Experimental papers published.”~ verification was indeed reported four years Indeed, it was two years after the Yale later in 1982, and it came about serendipi- group’s experiments were completed that the tously from the work of Cech. paper’ ‘Ribonuclease P: an enzyme with an essential RNA component” was published Thomas R. Cech: Ribosomal RNA and in the Proceedings of the National Academy Serendipitous Discovery of Sciences of the USA (PNAS’) in 1978. c Altman himself acknowledged personal Cech’s research concentrated on under- reluctance to accept the revolutionary find- standing how introns (or intervening se-

269 quences) are removed from RNA. Introns Art Zaug and the painstaking follow-up ex- were first seen in the mid- 1970s, when periments: ‘‘ ‘Well Art, this looks very methods for determining nucleotide se- encouraging, except you must have made quences were introduced. g Researchers some mistake rnizing up the control sample.’ found that genetic materird in some animal Yet several careful repetitions [five times] viruses as well as genes encoding proteins of the experiment gave the same restdt: in hemoglobin and antibody molecules have [RNA splicing had] occurred independent extra stretches of DNA in their interiors. of the addition of nuclear extract, and there- In 1977 Cech became interested in the fore apparently independent of any regulation of gene expression. He initiated enzyme. ”11 efforts to isolate the protein enzyme that was The experimental data made Cech realize presumed to control a splicing reaction in that RNA was itself closely associated with which certain extraneous segments are selec- RNA splicing activity. Further experiments tively removed from RNA molecules. established that no protein was required for Cech and his group of researchers at the the splicing reaction to take place. When a University of Colorado developed an in vitro nonsense segment is removed from the RNA transcription system using a ribosomal RNA molecule, the loose ends thus formed are (rRNA) gene from the single-celled micro- then joined together. Still, the self-splicing organism Tetrohymena thermophila. The RNA molecule could not be considered a gene from the microorganism happens to true enzyme because it could perform the contain a 400 base pair intron sequence.2 operation only once and was itself changed Some of Cech’s early results within vitro in the process. (True can catalyze splicing of Te~rahymena rRNA appared in a reaction repeatedly while emerging from a paper entitled “In vitro splicing of the the activity unchanged. ) Later, in 1986, ribosomal RNA precursor of Tetrahymena: Cech and his colleagues reported that the involvement of a guanosine nucleotide in the portion of the RNA molecule that had been excision of the intervening sequence.’ ‘g removed then modifies itself subsequently This 1981 Cell paper, Cech’s third most- so that it can function as an RNA-synthe- cited publication, can be seen as the penulti- sizing enzyme-meaning that catalytic RNA mate work—’ ‘on the doorstep” as it were— could also make new RNA.2 to the experimental verification of RNA as In 1982, when Cech and his group of re- a catalyst that was to be published the searchers published their findings in Cell following year by Cech and colleagues.’0 that RNA indeed was a biochemical catalyst, During development of the in viiro tran- it created a sensation, as well as instant scription system, the Boulder group made the acclaim for Cech from his peers. According surprising discovery that RNA splicing also to SCI data, “Self-splicing RNA: autoexci- was occurring.2 This unexpected result led sion and autocyclization of the ribosomal Cech to attempt the isolation and definition RNA intervening sequence of Tetrahy - of the enzyme responsible for the splicing mena” has garnered over 270 citations reaction. An experiment was set up where through 1989, making the paper Cech’s precursor rRNA was to be used with cell- most-cited work. 10 nuclei extract-containing enzymes, as was The following year Altman and ~s group an enzyme-free control without the extract. at Yrde, in collaboration with Norman Pace After incubation, the enzyme with cell- and his group at the University of Colorado nuclei extract experiment worked. But so did Medical Center, Denver, published a pa~r the control experiment-it, too, showed that that amounted the discovery of the enzy- splicing was occurring. Cech’s initial reac- matic activity of the RNA molecule tion was to suspect that the result was due associated with RNase-P, The 1983 Cell to a methodological error that may have con- paper, entitled “The RNA moiety of riho- taminated the control experiment. He re- nuclease P is the catalytic subunit of the ,, 1z demonsmat~ that under non- called the comment he made to his colleague enzyme,

270 Figure 1: Distribution of year-by-year citations to three of the papers in Table t.

60- — Krug@r et M, 1982 --- s~afk et~/c, 1978 50- ...... Guerrier-Takada et 8/., 1983

,4 -9-* \ # \ 0 9 lo- 4 \ \, #’ ------4 4 \ ● @ 0- , 1978 1979 1980 1981 198; 1983 1984 1985 1986 1987 1988 198 Year

physiological conditions, the RNA subunit Historiographic Data Reveal Centrality of of RNase-P alone cleaves tRNA precursors. Laureates’ Research It received 14 citations in 1984,26 in 1985, and 51 in 1986. Through 1989, the paper Figure 2 is a historiograph of ISI@ re- accumulated over 200 explicit citations, search fronts on RNA catalysis from 1984 making it Altman’s second most-cited paper. through 1988. A research front (SI cialty) The graph in FiWre 1 shows the citation is formed by the comections made by scien- records of these two landmark papers, which tists in their referencing patterns. Using a provide convincing experimental verifica- method called co-citation clustering, it is tion of RNA’s catalytic properties. Also possible to order automatically the scientific shown are annual citations to Altman’s 1978. literature into bibliographically distinct and PNAS paper, discussed earlier, that first intellectually coherent units. Articles that are hypothesized an essential role for RNA in frequently cited together by current papers an enzymatic reaction. constitute the “core” of the specialty. The Catalytic RNA segments quickly came to research front, in part, is composed of citing be known as’ ‘, ” and researchers articles and is named from phrases co-occur- all over the world turned to finding out how ring in these citing titles. widespread these ribozymes really are, The historiograph in Figure 2 is a graphic Today, nearly 100 ribozymes have been display of citation data that can be used to identified. Work condrmm on identitjing the show key scientific events: their chronology, structure of enzymatic RNA, where there their interrelationships, and their relative im- are clear indications that catalytic RNA portance. 13 Of special interest in the his- possesses a specific three-dimensional toriograph is the linkages between fronts, framework in the same manner as proteins. showing antecedent and follow-up research. According to Altman, efforts are under way These linkages are based on the percentage to crystallize RNase-P for X-ray crystallo- of similarity of the core documents that each graphic studies.z front cites.

271 F~re 2: Historiograph of work in hiologicsl catdysk by RNA. The number of core/citing pspera is given at M bottom of each bx. An asterisk (*) next to the research-front number iodicares that Thonrss R, Cech snd/or Sidney Altman are core authors

I “8W7S0 klttechO!Wtal RNA and evoturlon Of cafatytk Wwtkn W46 I i-----J “87+317 Chemlatw 01 ssws~king m4A 3152 I

“87-7s97 SSIf-SpllCIng RNA and LRNA enzyme Seven of the nine research fronts in Fig- It should also be pointed out that both ure 2 contain core papers by Cech and laureates appear as core authors in research Ahmars-specifically, the pathbreaking Cell fronts prior to tlrose shown in Figure 2. Two papers that experimentally demonstrated that of Altman’s most-cited worlm—” Tyrosine RNA is an enzyme, which are discussed tRNA precursor molecule polynucleotide se- earlier. Both appear in research fronts quence,”3 and “Biosynthesis of transfer #84-165 1, “Biological catalysis by RNA,” RNA in Eseherichia coli” 14—appeared in #85-3838, “Self-splicing RNA, ” and research fronts during the years 1975-1977 #86- 1923, “RNA catalyzed replication of and 1980; one of Cech’s papers, entitled RNA. ” In addition, Cech’s 1982 Cell paper “Characterization of the most rapidly de- is a core document in research front naturing sequences in mouse main-band #87-7297, “Self-splicing RNA and RNA DNA, ” is a core paper in a 1976 research enzyme, ” while Akman’s 1983 Cell paper front. 15 also appears in research front #88-3760, ‘‘Mitoehondfid RNA and evolution of Conclusion: Catalytic RNA and the catalytic function:’ and #87-6317, Origins of Life “Chemistry of self-splicing RNA.” Other The scientific community has long been papers by the authors are also core fascinated by the origins of life and has doetsments in research fronts #85-3838, attempted to elucidate the conditions and #86- 1923, and #88-3760 as well as mechanisms that gave rise to the incrdlble #87- 1678, ‘‘Rlxrnucleoprotein particles in diversity of living organisms. For some time pre-messenger RNA splicing. ” there had been a dilemma-how could life It is interesting to note that two of the have arisen if the DNA molecules of the linked research fronts, #86- 1923 and genetic material could only multiply and be #87- 1678, indicate an explosion of research deciphered with the help of proteins, on RNA enzymology. The number of core whereas proteins can ordy be built up with papers increased fivefold, from 10 in 1986 genetic information from DNA? In the to 50 in 1987. The number of citing works words of Cech, “Which came first, the increased 12-fold during this two-year nucleic acid or the protein, the information period, from 150 to more than 1,800. or the function?”11

272 With the discovery that RNA can be both molecule, and some life scientists feel that genetic material and enzyme at the same it is too elaborate to come into being just time, the dilemma appears to have been by random reactions, so researchers are solved-neither DNA nor proteins came looking for more likely candidates. There first. RNA is the “grandfather” molecule have been discussions recently in the scien- from which DNA and proteins descended. 1 tific press as to what chemical processes or There appears to be some corroborating alternative molecules to RNA might have evidence for this. Ribozymes are excellent “kick-started” life on Earth. 17 catalysts, although at present scientists find This concludes our examination of the that enzymatic RNA remains limited in ver- 1989 Nobel Prize in chemistry. In a forth- satility. There are also indications that coming essay, as is our custom, we will look ribozymes can recognize and manipulate at 1989 Nobelists in physics from a cita- amino acids directly (RNA can form a tionist perspective. specific binding site for at least one aino acid 16,. More rewarch needs to be done ***** before a consensus can be reached, but all these findings point to a prebiotic, primor- dial soup on Earth four bfion years ago that was ruled by RNA. My thanks to C.J. Fiscas and Peter But a big question remains: From what Pesavento for their help in the preparation did RNA itself evolve? RNA is not a simple of rhis essay, $m$Om

REFERENCES

1. Royal Swedish Academy of Sciences. Ribmmcleic acid (RNA)-a biomolecule of manyfunctions. 12 October 1989.4 p. (Press release.) 2. Brmro R M. Nobel Prize for catalytic RNA fmurd winners on separate tracks, C/rem, Eng. News 67(49):3 1-4, 4 December 1989. 3. Altmarr S & Sasdth J D. Tyrosine rRNA precursor molecule polynucleotide sequence. Nafure Nw BioL 233:35-9, 1971. 4. Gfadweff M. Genetic breakthrough could affect the undcrstarrding of life, viruses. Wa.rhingion Posf 13 October 1989. p. A3. 5. l.ewin R. Prizewinners came by separate path- to understarrd RNA. New Sci. 124( 1687) :3WI, 21 October 1989. 6. Stark B C, Kole R, Buwmrm E J & Aftrrmn S. RibonucleaseP: an enzyme with m essentiafRNA comprent. Proc. Not. Acad. Sci. USA 75:3717-21, 1978. 7. Aftsrmn S. E_”c cleavage of RNA by RNA. Stockholm, Sweden: Nobel Foundation, 1990. 25 p. (Nobel kCNr&) 8. Hesdkurm R. Enzymatic RNA. 1989 yearbook of science and the fiture. Chicago, U Encyclopedia Britarmics,1988.P, 357-9. 9. Ceeh T R, Zaug A J & GrabowsfdP J. In vitrosplicing of the ribusonud RNA precursur of Tetmhymenz involvement of a guanosirre nuclwtide in tk excision of the intervening sequence. Cell 27:487-%, 1981. 10. Kruger K, GrabowsfdP J, Zaug A J, SnndsJ, GottschMrtgD E & CechT R. self-splicing RNA: autuexcision and autocyctization of the ribosorusl RNA intervening sequence of Tetmhymem. Cell 31:147-57, 1982. 11. Cech T R. Se~+piicirtg ad enzymatic? activity of arr brtervenirrg sequence RNA @m Tetrafrymens. Stockholm, Sweden: Nobel Fmurdation, 1990.40 p. (Nobel lecture.) 12. Guerrier-Takado C, Garttiner K, Msrstr T, Pace N & Attsrmo S. The RNA moiety of ribonuclesse P is the eatsfytic subunit of tbe enzyme, Ccl/ 35:849-57, 1983. 13. Garfield E. Historiographs, Iibrariarrsbip, arrdthe history of science. Essays of an iny%rrratimr scbmfisr. PhiksdelpIrk 1S1 Press, 1977. Vol. 2. p. 134-50. 14. Altnrsrr S. Biosynthesis of transfer RNA in Eacherichia wfi. Cell 4:21-9, 1975. 15. Cech T R, Roaanfeld A & Hearst J E. Characterization of the nrnst rapidty rerraturirrgaequertces br mouse msirr-barrdDNA. J, Mol. BioL 81:299-325, 1973. 16. Yarrra M. A specific smirro-acid binding site composed of RNA, Science 20.1751-8, 1988. 17. Wskfrop M M. Dld life really start out in an RNA world? Science 246:1248-9, 1989.

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