current eammsnts’EUGENE GARFIELD

IFJSI(TUTE FOR SC16NT(F(L lNf C)HMATl ON~

3501 MA FIK6T ST PHILADELPHIA PA 19104

The 1989 Nobel Prize in Chemistry Goesto Sidney Alhmm and Thomas R. Cech

for the Discovery of Enzymatic RNA

Numt3er zy JUIY lb, IYYU

The 1989 Nobel Prize in chemistry has been awarded to Sidney Altrnan and Thomas R. Cech fortheir discovery that RNA in living cells can function not onfy as a molecule of heredity, but alsoas a biocatalyst. A brief historical overview of the laureates’ work is presented, and citation datafor 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, tworesearchers, working independently, madea discovery that reversed a 50-year-olddogma in biochemistry-that the triggeringand acceleration (catalysis) of chemicrd reac-tions within living cells were the exclusivedomain of protein molecules called en-zymes. Biophysicist Sidney Altman, YaleUniversity, New Haven, Connecticut, andchemist Thomas R. Cech, University ofColorado, Boulder, found that ribonucleicacid (RNA), traditionally considered to beonly a passive intracelktkw carrier of geneticinformation, can rdso act as an enzyme orbiocatalyst. This unexpected and unprece-dented discovery has been hailed as one ofthe most important scientific breakthroughsof the 1980s, and, in recognition of this,Altman and Cech were awarded the 1989Nobel Prize in chemistry by the RoyalSwedish Academy of Sciences.

Cech showed that the RNA moleculecould cut and rejoin itself and thereby alterthe material it produces, an operation thathad previously been thought to be the soleproperty of proteins. Altmatr showed thatRNA could have an independent catalyticfunction-that is, it could alter the makeupof RNAs other than itself, proving that RNAwas indeed as much a catalyst as proteins.The work of Altrnart and Cech has led to anew scientific field called RNA enzymol-ogy, and old theories of the origin of life

and cell timction have had to be rethought.In the words of the Swedish Academy,“Many chapters in our textbooks have to berevised. ” 1

Future applications of catalytic RNAcould include its use as gene shears todestroy RNA molecules involved in viralinfections such as the common cold orAIDS, in producing genetically engineered,disease-resistant agricultural plants, and, atsome date, correcting certain hereditag dis-eases in both animals and humans.

The chemistry prize carries an award ofthree million Swedish kronor, or about470,000 US dollars. The 1989 award to thetwo Americans continued a trend of thechemistry prize throughout the 1980s, where12 out of the 19 awards were given to USscientists. Altman and Cech became the 35thand 36th Americans to win the Nobel Prizein chemistry since it was first instituted in1901. There have been 114 chemistry prize-wimers through 1989.

Bbgraphical Data

Sidney Akman

Sidney AJtrnan was born May 8, 1939, inMontreal, Quebec, Canada. He spent hischildhood in the west end Montreal districtof Notre-Dame-de-Grace, where his fatherran a grocery store. Altman left Montrealin 1956 to attend the Massachusetts Institute

266

Sidney Ahnan Thomas R. Cech

of Technology (MIT), Cambridge, where hereceived a BS in physics in 1960. Sevenyears later, Altman earned a PhD in bio-physics from the University of Colorado.Subsequently, he obtained research fellow-ships from Harvard University (1%7-1969),Cambridge, and the Medical ResearchCouncil Laboratory of Molecular Biology(1969-1971), University of Cambridge, UK,where he worked with Sydney Bremer andNobelist Francis H.C. Crick. Ahrnan joinedthe faculty of Yrde University as an assistantprofessor in 1971 and has been a professorof biology there from 1980 onwards. Herdso acted as a dean at Yale College from1985 through last year. In 1984 Altmanbecame a US citizen while retaining hisCanadian citizenship.

Altman’s honors and awards includemembership in the US Academy of Arts andSciences (1988), the US National Academyof Sciences (NAS) (1990), and the AmericanPhilosophical Society (1990); the Lewis S.Rosenstiel Award for Distinguished Workin Basic Medical Research (given by Bran-deis University, Walthasn, Massachusetts)1989; the Yale Science and EngineeringAssociation Medal (1990); the DistinguishedService Medal, Teachers College, Columbia

University, New York (1990); and honorarydegrees from the University of Montreal andYork University, Toronto, Ontario, Canada(1990). He has also been an associate editorof Cell, 1983-1987, and a consulting editorof Amen’can Scientist, 1972-1990. Throughearly 1990, Altman has authored 89publications.

Zhomas R. Cech

Thomas Robert Cech was born in Chi-cago, Illinois, on December 8, 1947. Hereceived a BA in chemistry from GrimellCollege, Iowa, in 1970 and a PhD in chem-istry from the University of California,Berkeley, in 1975. After a stint at MIT asa National Cancer Institute fellow in molec-ular biology (1975-1977), he joined thefaculty of the University of Colorado in1978. Cech became a full professor therein 1983. From 1988 onwards he has beenan investigator of tie Howard Hughes Medi-crd Institute at the university.

Cech received the 1985 Pfizer Award inEnzyme Chemistry and was elected tomembership in NAS in 1987. In 1988 hereceived the American Association for theAdvancement of Science Newcombe-Cleve-

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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 indicatedwith an asterisk (*); research-front numbers appear in parentheses after the bibliographic &ta.

A

273

252

208

191

172

172

142125116

116

109

888176

72

65

61

58

Bfblfographic Data

●Kruger K, Grabowsfd P J, Zaug A J, Smrds J, G@tacbfing D E & Ceeh T R. Self-splicingRNA: autoexcision and autmycfization of the ritrosonud RNA intervening sequence ofTetrahymena. Cell 31:147-57, 1982. (84-1651, 85-3838, 86-1923, 87-7297)

Aftman S & Smith J D. Tyrosine tRNA precursor molecule polynucleotide sequence. Narure NewBiol. 233:35-9, 1971.

●Guerrier-Takada C, Gardiner K, Marsh T, Pace N & AttrtmrsS. The RNA moiety ofribonuclease P is the catalytic subunit of the enzyme. Cell 35:849-57, 1983. (84-1651, 85-3838,86-1923, 87-6317, 88-3760)

*Ceeh T R, Tamrer N K, Tmoco 1, Weir B R, Zuker M & Perhoan P S. Secondary structure ofthe Tetmbymena rihosormd-RNA intervening sequence: structursf homology with fungalmitochondrial intervening sequences. Proc. Na[. Acad. Sci. USA SO:3903-7, 1983.(85-3838, 86-1923)

*Ceeh T R & Boas B L. Biological cata.fysis by RNA, Annu. Rev. Biorhem. 55 :59S-629, 1986.(87-1678, 88-3760)

Cech T R, Zaug A J & Grafmwdd P J. /n vitro splicin8 of the ritroaomsl RNA precursor ofTetrahymena: involvement of a guanosine rurcleotide in the excision of the intervening sequence.Cell 27:487-%, 1981.

*Ceeh T R. RNA splicing. three themes with variations. Ce[[ 34:713-6, 1983. (85-3838)Akrnasr S. Isolatinn of tyrosine tRNA precursor molecules. Nafure New Bio/. 229:19-20, 1971.Ceeh T R. The generality of self-splicing RNA: relationship to nuclear messenger RNA splicing,

Celf 44:207-10, 1986,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,Ceeh T & Pardue M L. Cross-linking of DNA with trimethylpsorafen is a probe for chromatin

structure, Cef/ 11:631-40, 1977,Aftmars S. Biosynthesis of trsnsfer RNA in Escherichia coli. Cell 4:21-9, 1975.Ceeh T R & Hearst J E. Electron microscopic study of mouse foldback DNA. Cell 5:42946, 1975.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.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. USA73:2644-8, 1976,

Aftman S & Lerman L S. Kinetics and intermediates in intracellular synthesis of bacteriophage T4deoxyribonucleic acid. J. .Mol, Bid. 50:235-61, 1970.

Ceeh T R, Roaerrfeld A & Hearst J E. Characterization of the most rapidly denaturing squencesin mouse main-band DNA. J, Mol. Biol, 81:299325, 1973.

Ceeh T R & Rio D C. Localization of transcribed rezions on exrrachromcwmnaf ribmomal RNA8enes of Tetmhymena therrnophila by R-lwp ma.ppi;g, Proc. Nat. Acad. Sci. USA76:5051-5, 1979,

Iland Award, the Royal NetherlandsAcademy of Sciences Heineken Prize, theGairdner International Award for Outstand-ing Achievement in Medical Science (givenby the Gairdner Foundation, Willowdale,Ontario), and the Albert Lasker Basic Medi-cal Research Award (given by the Akrt andMary Lasker Foundation, New York). In1989 Cech received the Lewis S. RosenstielAward, which he shared with Altman. Cechrdso ha8 been an associate editor for Ceif(1986-1987) and serves on the editorialboard of Genes and Development ( 198’7on-wards). Cech has authored 105 publicationsthrough early 1990.

Sidney Akman: RNase-P and theOverturning of a Dictum

Altman’s early research focused on RNAprocessing, in particular that of transferRNA (tRNA). The tRNAs are the nucleicacid molecules responsible for delivering in-dividual amino acids to the protein-synthe-sizing machinery of a cell. In 1970 Altmanand his colleagues isolated a tRNA-pro-cessing enzyme found both in bacteria andhigher cells and named it RNw-P.Z Thecharacteristics of the enzyme are unusual inthat it contains RNA and protein in a single

268

package. This enzyme is responsible for oneof the reactions that produces tRNA.

RNase-P is found in human intestinalbacteria, Escherichia coli, and participatesin the activation of tRNA by removing a por-tion of the precursor to tRNA, which is un-necessary for the function of the maturetRNA molecule. Altman’s eady work inobserving and characterizing enzymaticactivity affecting tRNA resulted in a seriesof papers in the early 1970s. Several of theseappear in Table 1, a listing of the two Nobellaureates’ highly cited works. Altman’smost-cited work, ‘‘l$osine tRNA precur-sor molecule polynucleotide sequence,”3coauthored with J.D, Smith, Medical Re-search Council Laboratory of MolecularBiology, and published in Nature NewBi@ogy in 1971, dates from this period.

In 1976 one of Altman’s graduate studentsat Yale University, Ben C. Stark, showedthat both the RNA and protein were essen-tial components of the active enzyme.z

Akrnan and his colleagues at Yale alsofound that RNase-P seemed to lose itsenzymatic abilities when the RNA portionwas destroyed or removed. Originally,Altman thought that the RNA component ofthe enzyme RNase-P did not serve anyactive enzymatic function because of thewell-established “fact” that only proteinscould serve as catalysts.

The finding that RNA plays an essentialrole in an enzyme directly challenged thelong-held biochemical dogma of’ ‘proteinsonly as catalysts. ” Not surprisingly,Altman’s work met with some resistance inthe biochemical community.z.d,s Altman’scolleague, Yale biologist Joel Rosenbaum,observed to a Washington Post reporter: “Itwas heresy . . . . What Sidney was saying wasso revolutionary he had trouble getting hispapers published.”~

Indeed, it was two years after the Yalegroup’s experiments were completed that thepaper’ ‘Ribonuclease P: an enzyme with anessential RNA component” was publishedin the Proceedings of the National Academyof Sciences of the USA (PNAS’) in 1978. cAltman himself acknowledged personalreluctance to accept the revolutionary find-

ing that RNA was a necessary, if not suffi-cient, component of RNase-P. In his Nobellecture, he recalled: “When Stark’s experi-ments were published we did not have thetemerity to suggest, nor did we suspett, thatthe RNA component alone of RNase-P couldbe responsible for its catrdytic activity. Thefact that a simple enzyme had an essentialRNA subunit, in itself, seemed hereticalenough.”1

The paper reported the authors’ findingsin cautious and circumspect language. Westating that’ ‘the presence of a dkcrete RNAcomponent in RNase-P appears to be essen-tial for enzymatic function, ” the authorsacknowledged that “we do not know if theRNA component... is needed for stabiliza-tion of the protein moiety [of RNase-P] orif it plays a more active role in substraterecognition.’ ‘c Although the authors of-fered a “model.. for enzyme-substrate r&-ognition in which this RNA component

plays an important role, ” they also stressedthat “this scheme is hypothetical: wc haveno direct evidence that the critical position-ing of RNase-P on precursor tRNA sub-strates is determined by RNA-RNA ratherthan by protein-RNA interactions.’ ‘c

According to Science Citation Indexa(SCl@ ) data, the paper excited significant–but not extraordinary-initial interest asjudged by citations. It was cited once in 1978and 13 times in 19’79. Citations nearlydoubled to 23 in 1980 and 21 in 1981. Theydeclined sharply to six in 1982, and thepaper continued to be cited at about this levelthrough 1988. Clearly, the paper reportedan intriguing and even sensational hypoth-esis. But the impact of this hypothesis wasperhaps limited by the lack of hard experi-mental evidence to convince researchers ofRNA’s enzymatic properties. Experimentalverification was indeed reported four yearslater in 1982, and it came about serendipi-tously from the work of Cech.

Thomas R. Cech: Ribosomal RNA andSerendipitous Discovery

Cech’s research concentrated on under-standing how introns (or intervening se-

269

quences) are removed from RNA. Intronswere first seen in the mid- 1970s, whenmethods for determining nucleotide se-quences were introduced. g Researchersfound that genetic materird in some animalviruses as well as genes encoding proteinsin hemoglobin and antibody molecules haveextra stretches of DNA in their interiors.

In 1977 Cech became interested in theregulation of gene expression. He initiatedefforts to isolate the protein enzyme that waspresumed to control a splicing reaction inwhich certain extraneous segments are selec-tively removed from RNA molecules.Cech and his group of researchers at theUniversity of Colorado developed an in vitrotranscription system using a ribosomal RNA(rRNA) gene from the single-celled micro-organism Tetrohymena thermophila. Thegene from the microorganism happens tocontain a 400 base pair intron sequence.2

Some of Cech’s early results within vitrosplicing of Te~rahymena rRNA appared ina paper entitled “In vitro splicing of theribosomal RNA precursor of Tetrahymena:involvement of a guanosine nucleotide in theexcision of the intervening sequence.’ ‘gThis 1981 Cell paper, Cech’s third most-cited publication, can be seen as the penulti-mate work—’ ‘on the doorstep” as it were—to the experimental verification of RNA asa catalyst that was to be published thefollowing year by Cech and colleagues.’0

During development of the in viiro tran-scription system, the Boulder group made thesurprising discovery that RNA splicing alsowas occurring.2 This unexpected result ledCech to attempt the isolation and definitionof the enzyme responsible for the splicingreaction. An experiment was set up whereprecursor rRNA was to be used with cell-nuclei extract-containing enzymes, as wasan enzyme-free control without the extract.After incubation, the enzyme with cell-nuclei extract experiment worked. But so didthe control experiment-it, too, showed thatsplicing was occurring. Cech’s initial reac-tion was to suspect that the result was dueto a methodological error that may have con-taminated the control experiment. He re-called the comment he made to his colleague

Art Zaug and the painstaking follow-up ex-periments: ‘‘ ‘Well Art, this looks veryencouraging, except you must have madesome mistake rnizing up the control sample.’Yet several careful repetitions [five times]of the experiment gave the same restdt:[RNA splicing had] occurred independentof the addition of nuclear extract, and there-fore apparently independent of anyenzyme. ”11

The experimental data made Cech realizethat RNA was itself closely associated withRNA splicing activity. Further experimentsestablished that no protein was required forthe splicing reaction to take place. When anonsense segment is removed from the RNAmolecule, the loose ends thus formed arethen joined together. Still, the self-splicingRNA molecule could not be considered atrue enzyme because it could perform theoperation only once and was itself changedin the process. (True enzymes can catalyzea reaction repeatedly while emerging fromthe activity unchanged. ) Later, in 1986,Cech and his colleagues reported that theportion of the RNA molecule that had beenremoved then modifies itself subsequentlyso that it can function as an RNA-synthe-sizing enzyme-meaning that catalytic RNAcould also make new RNA.2

In 1982, when Cech and his group of re-searchers published their findings in Cellthat RNA indeed was a biochemical catalyst,it created a sensation, as well as instantacclaim for Cech from his peers. Accordingto SCI data, “Self-splicing RNA: autoexci-sion and autocyclization of the ribosomalRNA intervening sequence of Tetrahy -mena” has garnered over 270 citationsthrough 1989, making the paper Cech’smost-cited work. 10

The following year Altman and ~s groupat Yrde, in collaboration with Norman Paceand his group at the University of ColoradoMedical Center, Denver, published a pa~rthat amounted the discovery of the enzy-matic activity of the RNA moleculeassociated with RNase-P, The 1983 Cellpaper, entitled “The RNA moiety of riho-nuclease P is the catalytic subunit of theenzyme, ,, 1z demonsmat~ that under non-

270

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

60-

50-

lo-

0-

— Krug@r et M, 1982--- s~afk et~/c, 1978

. . . . . . .Guerrier-Takada et 8/., 1983

,4 -9-*\

#0

\94 \

#’ \,4 ---------- 4 \

@ ●

,1978 1979 1980 1981 198; 1983 1984 1985 1986 1987 1988 198

physiological conditions, the RNA subunitof RNase-P alone cleaves tRNA precursors.It received 14 citations in 1984,26 in 1985,and 51 in 1986. Through 1989, the paperaccumulated over 200 explicit citations,making it Altman’s second most-cited paper.

The graph in FiWre 1 shows the citationrecords of these two landmark papers, whichprovide convincing experimental verifica-tion of RNA’s catalytic properties. Alsoshown are annual citations to Altman’s 1978.PNAS paper, discussed earlier, that firsthypothesized an essential role for RNA inan enzymatic reaction.

Catalytic RNA segments quickly came tobe known as’ ‘ribozymes, ” and researchersall over the world turned to finding out howwidespread these ribozymes really are,Today, nearly 100 ribozymes have beenidentified. Work condrmm on identitjing thestructure of enzymatic RNA, where thereare clear indications that catalytic RNApossesses a specific three-dimensionalframework in the same manner as proteins.According to Altman, efforts are under wayto crystallize RNase-P for X-ray crystallo-graphic studies.z

Year

Historiographic Data Reveal Centrality ofLaureates’ Research

Figure 2 is a historiograph of ISI@ re-search fronts on RNA catalysis from 1984through 1988. A research front (SI cialty)is formed by the comections made by scien-tists in their referencing patterns. Using amethod called co-citation clustering, it ispossible to order automatically the scientificliterature into bibliographically distinct andintellectually coherent units. Articles that arefrequently cited together by current papersconstitute the “core” of the specialty. Theresearch front, in part, is composed of citingarticles and is named from phrases co-occur-ring in these citing titles.

The historiograph in Figure 2 is a graphicdisplay of citation data that can be used toshow key scientific events: their chronology,their interrelationships, and their relative im-portance. 13 Of special interest in the his-toriograph is the linkages between fronts,showing antecedent and follow-up research.These linkages are based on the percentageof similarity of the core documents that eachfront cites.

271

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

I

Seven of the nine research fronts in Fig-ure 2 contain core papers by Cech andAhmars-specifically, the pathbreaking Cellpapers that experimentally demonstrated thatRNA is an enzyme, which are discussedearlier. Both appear in research fronts#84-165 1, “Biological catalysis by RNA,”#85-3838, “Self-splicing RNA, ” and#86- 1923, “RNA catalyzed replication ofRNA. ” In addition, Cech’s 1982 Cell paperis a core document in research front#87-7297, “Self-splicing RNA and RNAenzyme, ” while Akman’s 1983 Cell paperalso appears in research front #88-3760,‘‘Mitoehondfid RNA and evolution ofcatalytic function:’ and #87-6317,“Chemistry of self-splicing RNA.” Otherpapers by the authors are also coredoetsments in research fronts #85-3838,#86- 1923, and #88-3760 as well as#87- 1678, ‘‘Rlxrnucleoprotein particles inpre-messenger RNA splicing. ”

It is interesting to note that two of thelinked research fronts, #86- 1923 and#87- 1678, indicate an explosion of researchon RNA enzymology. The number of corepapers increased fivefold, from 10 in 1986to 50 in 1987. The number of citing worksincreased 12-fold during this two-yearperiod, from 150 to more than 1,800.

I

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L“87-7s97SSIf-SpllCIng RNAandRNA enzyme

i-----J“8W7S0klttechO!Wtal RNAand evoturlonOf cafatytkWwtkn

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It should also be pointed out that bothlaureates appear as core authors in researchfronts prior to tlrose shown in Figure 2. Twoof Altman’s most-cited worlm—” TyrosinetRNA precursor molecule polynucleotide se-quence,”3 and “Biosynthesis of transferRNA in Eseherichia coli” 14—appeared inresearch fronts during the years 1975-1977and 1980; one of Cech’s papers, entitled“Characterization of the most rapidly de-naturing sequences in mouse main-bandDNA, ” is a core paper in a 1976 researchfront. 15

Conclusion: Catalytic RNA and theOrigins of Life

The scientific community has long beenfascinated by the origins of life and hasattempted to elucidate the conditions andmechanisms that gave rise to the incrdlblediversity of living organisms. For some timethere had been a dilemma-how could lifehave arisen if the DNA molecules of thegenetic material could only multiply and bedeciphered with the help of proteins,whereas proteins can ordy be built up withgenetic information from DNA? In thewords of Cech, “Which came first, thenucleic acid or the protein, the informationor the function?”11

272

With the discovery that RNA can be bothgenetic material and enzyme at the sametime, the dilemma appears to have beensolved-neither DNA nor proteins camefirst. RNA is the “grandfather” moleculefrom which DNA and proteins descended. 1

There appears to be some corroboratingevidence for this. Ribozymes are excellentcatalysts, although at present scientists findthat enzymatic RNA remains limited in ver-satility. There are also indications thatribozymes can recognize and manipulateamino acids directly (RNA can form a

specific binding site for at least one ainoacid 16,. More rewarch needs to be donebefore a consensus can be reached, but allthese findings point to a prebiotic, primor-dial soup on Earth four bfion years ago thatwas ruled by RNA.

But a big question remains: From whatdid RNA itself evolve? RNA is not a simple

molecule, and some life scientists feel thatit is too elaborate to come into being justby random reactions, so researchers arelooking for more likely candidates. Therehave been discussions recently in the scien-tific press as to what chemical processes oralternative molecules to RNA might have“kick-started” life on Earth. 17

This concludes our examination of the1989 Nobel Prize in chemistry. In a forth-coming essay, as is our custom, we will lookat 1989 Nobelists in physics from a cita-tionist perspective.

*****

My thanks to C.J. Fiscas and PeterPesavento for their help in the preparationof 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. News67(49):3 1-4, 4 December 1989.

3. Altmarr S & Sasdth J D. Tyrosine rRNA precursor molecule polynucleotide sequence. Nafure Nw BioL233:35-9, 1971.

4. Gfadweff M. Genetic breakthrough could affect the undcrstarrding of life, viruses. Wa.rhingion Posf13 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 essentiafRNAcomprent. 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 EncyclopediaBritarmics,1988.P, 357-9.

9. Ceeh T R, Zaug A J & GrabowsfdP J. In vitrosplicing of the ribusonud RNA precursur ofTetmhymenz involvement of a guanosirre nuclwtide in tk excision of the intervening sequence. Cell27:487-%, 1981.

10. Kruger K, GrabowsfdP J, Zaug A J, SnndsJ, GottschMrtgD E & Cech T R. self-splicing RNA:autuexcision and autocyctization of the ribosorusl RNA intervening sequence of Tetmhymem. Cell31:147-57, 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 Pis 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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