From Controlling Elements to Transposons: Barbara Mcclintock and the Nobel Prize Nathaniel C

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454 Forum TRENDS in Biochemical Sciences Vol.26 No.7 July 2001

Historical Perspective From controlling elements to transposons: Barbara McClintock and the Nobel Prize Nathaniel C. Comfort

Why did it take so long for Barbara correspondence. From these and other to prevent her controlling elements from McClintock (Fig. 1) to win the Nobel Prize? materials, we can reconstruct the events moving because their effects were difficult In the mid-1940s, McClintock discovered leading up to the 1983 prize*. to study when they jumped around. She genetic transposition in maize. She What today are known as transposable never had any inclination to pursue the published her results over several years elements, McClintock called ‘controlling biochemistry of transposition. and, in 1951, gave a famous presentation elements’. During the years 1945–1946, at Current understanding of how gene at the Cold Spring Harbor Symposium, the Carnegie Dept of Genetics, Cold activity is regulated, of course, springs yet it took until 1983 for her to win a Nobel Spring Harbor, McClintock discovered a from the operon, François Jacob and Prize. The delay is widely attributed to a pair of genetic loci in maize that seemed to Jacques Monod’s 1960 model of a block of combination of gender bias and gendered trigger spontaneous and reversible structural genes under the control of an science. McClintock’s results were not mutations in what had been ordinary, adjacent set of regulatory genes (Fig. 2). accepted, the story goes, because women stable alleles. In the term of the day, they Though subsequent studies revealed in science are marginalized, because the made stable alleles into ‘mutable’ ones. sometimes baroque variations in idea of transposition was too far-fetched The resulting patterns in the maize leaves eukaryotes, the operon remains the core and because her scientific style was too and kernels indicated systematic theme (also the textbooks’ historical intuitive, too holistic and too feminine to alterations in mutation rates. McClintock starting point) of gene regulation. be believed. concluded that she had disrupted the cell’s Immediately following publication of the Most scientists I know detest this story. mechanism for regulating gene activity. operon model, McClintock wrote an article McClintock loathed it too. She was not By early 1948, difficulties in mapping the for The American Naturalist on the scorned, biologists insist, because she was loci led her to conclude that they were not, parallels between the control systems in a woman nor for any other reason, she was in fact, loci – sites on the chromosomes – maize and bacteria1. For a time, she even misunderstood. The intricacy of her but chromosomal elements that moved referred to her elements as ‘operators’ and experiments, combined with her eccentric, from place to place. It soon became clear ‘regulators’. Jacob and Monod were not elliptical style of presentation, made her that these new elements were capable of influenced by McClintock; when they science difficult to understand. exerting a wide range of actions upon the referred to controlling elements, it was as Furthermore, her findings were limited to genes. By 1950, McClintock had developed extrachromosomal episomes, perhaps maize, and their significance was not clear a theory about how controlling elements viral in origin, not as components of the until the late 1960s and early 1970s, when orchestrated development and transposition was found to be widespread differentiation of the organism. The key to in nature and of general importance. The her theory was coordinated transposition. Nobel Prize was not late; McClintock was In each nucleus, platoons of controlling early, ahead of her time. elements would transpose in concert, Both of these stories are myths. Both inhibiting and modulating the effects of assume that, had things gone differently, the genes to execute the developmental the transposition prize might have been program of the cell. awarded sooner. Both neglect the fact that Transposition was never in doubt. McClintock did not win the prize for what Other geneticists quickly confirmed she thought was most important in her transposition in maize, but, at the time, work. A closer look at how McClintock came few considered it McClintock’s major to receive the Nobel Prize reveals that she claim. Although McClintock was indeed won only after her major contribution to difficult to understand, all her peers science was recast to fit better with current grasped that her real point was control. understanding of the genome. Yet, to most scientists, transposition The McClintock papers within the seemed a random process. By ~1953, once Nobel archive will not be accessible until McClintock had thoroughly documented 2033. However, nominators retain the the fact of transposition, she worked hard rights to their nominations. Several scientists have generously made their *A fuller account of McClintock's life and science, including the events leading up to the Nobel Prize, can nominations and stories of McClintock be found in The Tangled Field: Barbara McClintock's Barbara McClintock in the late 1940s. Photo kindly available to me; other evidence I have Search for the Patterns of Genetic Control by Nathaniel provided by the American Philosophical Society Library, gleaned from interviews and archived Comfort (Harvard University Press, 2001). Philadelphia, USA.

Karolinska Institute in Stockholm. He (a)Controlling elements (b) Operon nominated McClintock jointly with Repressor Seymour Benzer, best known for his work on the fine structure of the gene. The nomination form contained a box labelled Structural gene Ds P O Structural gene ‘The nomination is based on the discovery

Control sites of’. In this box, van Wyk entered ‘the Ac organization and function of the genome 4 Repressor in higher organisms’ . The summary Structural gene Ds P O Structural gene description of McClintock’s work stated that it had led to ‘the first clear

Ds Structural gene enunciation of the distinction between structural and regulatory genetic Ti BS elements.’An accompanying detailed Fig. 2. Both McClintock’s controlling elements (a) and Jacob and Monod’s operon (b) modeled gene regulation by the description informed by Gordon Sato, a action of elements outside the structural gene. In McClintock’s model, the regulatory elements (Ac and Ds) acted by friend of van Wyk’s and a regular summer transposing into or next to the structural gene. Here, Ac activates Ds, causing Ds to transpose to a new position next to a structural gene. Insertion of the Ds element inhibits transcription of the gene. In the operon model, each set visitor at Cold Spring Harbor, stated that of structural genes had its own adjacent regulatory elements [O (operator) and P (promoter)]. In this diagram, McClintock had ‘laid the conceptual binding of repressor to an operator sequence silences gene transcription. Figure is adapted, with permission, from groundwork for the historic achievements The Tangled Field: Barbara McClintock’s Search for the Patterns of Genetic Control, Harvard University Press. recognized in the honour conferred on Jacob and Monod by the Nobel Committee cell’s normal regulatory machinery. features that enabled these elements to in Physiology or Medicine.’After initial McClintock, however, saw her work as the remove themselves from and reinsert into review, the nomination was turned down. conceptual predecessor of the operon. the bacterial chromosome. Sequencing Meanwhile, transposable elements Seven years later, in 1967, McClintock and biochemistry revealed the existence of were being reframed in molecular terms. In won the Kimber Medal, which was unique base sequences – inverted repeats June 1976, at a Cold Spring Harbor awarded to distinguished geneticists by the – at either end of the elements that would meeting organized by Shapiro, Adhya and National Academy of Sciences each year be recognized by the enzymes responsible muologist Ahmad Bukhari, transposable from 1955 until 1970. McClintock had been for excision and ligation. By the mid- genetic elements or ‘transposons’ were on the short list since at least 1964. When 1970s, IS elements had been found in defined simply as pieces of DNA that can the award was to be announced, Marcus other organisms and shown to function in move from one place to another. By 1980, Rhoades, the award committee chairman a wide variety of ways. The rapid spread of transposons had become so important that and McClintock’s closest colleague, asked antibiotic-resistance genes was perhaps James Watson introduced the proceedings geneticist Tracy Sonneborn, the geneticist the most spectacular, but work on phage of a Cold Spring Harbor Symposium on best known for his investigations of mu, yeast mating-type switching and mobile genetic elements by writing that cytoplasmic inheritance in Paramecium, to other areas of research, all contributed to such a meeting had become ‘virtually review the proposed press release. a rapidly developing view that the unavoidable’5. By now, mobile elements Sonneborn, familiar with McClintock’s movement of genetic elements within and were mainly a subfield of microbial interpretation, suggested inserting ‘that between genomes was a mechanism of genetics. Papers at Cold Spring Harbor her studies of controlling systems of gene major importance. that summer touched on infectious disease, action were the precursors of the famous The new transposition workers quickly cancer, immunology and recombinant regulator–operon theory that won the discovered that McClintock had described DNA. However, one area that was not Nobel Prize for its promulgators and that transposition, in strictly genetic terms, discussed, was developmental regulation. their thinking was probably much long before. McClintock, then in her late In 1981, McClintock was besieged with influenced by Barbara’s notion of two- sixties, experienced a scientific awards. In June, she was made an member interacting controls’2. Although renaissance. In 1973, her friend Ernst honorary member of the Society for Rhoades had reservations about Caspari, a geneticist at the University of Developmental Biology and was awarded McClintock’s developmental-regulation Rochester, wrote to her, ‘I completely see the Thomas Hunt Morgan Medal of the theory (though not about transposition), he that you are going to have many calls for Genetics Society of America. In October, inserted the text. many activities. You are, after all, very the Wolf Foundation, based in Israel, That same year, James Shapiro, then famous now, and it is rather astonishing awarded McClintock their $50 000 prize in at the Postgraduate Medical School of how your fame has increased at a time of medicine. On 17 November, the John D. London, and Sankhar Adhya, at the life where in most people it is on the way and Catherine T. MacArthur Foundation University of Wisconsin, published their down’3. named her their first MacArthur discovery of insertion sequence (IS) In 1976, McClintock was nominated for Laureate, a now discontinued plum for elements in bacteria. IS elements were the Nobel Prize by Judson van Wyk, an senior scholars, which awarded discovered because of their ability to endocrinologist at the University of North McClintock $60 000 a year for life, tax produce strongly polar mutations in Carolina, Chapel Hill. Van Wyk had free. The next day, she won the Albert known operons. Subsequent nominating privileges as a result of having Lasker Basic Medical Research Award. characterization showed distinctive spent the previous four years at the She had also been nominated for the Nobel

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Prize once again, this time by Stanley N. completely new concept. In it, significance to medicine. It had little in Cohen and Howard Temin. But the 1981 chromosomes are no longer rigid common with what McClintock herself prize went to neurobiologists Roger structures, but show flexibility and had been arguing for decades. Sperry, David Hubel and Torsten Wiesel. thereby allow reorganization of the McClintock won the prize that year, Meanwhile, Nobel nominations genetic material.’ Shifting to a discussion solo, although the suggestion had been continued to roll in. Herschel Roman, a of transposable elements in bacteria, floated that she share it with Susumu geneticist who had been a graduate Saedler continued, ‘Such a reorganization Tonegawa, whose brilliant experiments student at the University of Missouri in of the ordering of genes is catalyzed by demonstrated genomic rearrangement the late 1930s when McClintock was transposable elements. These genetic among antibody genes. (Tonegawa won in there, and who was now at the University units not only jump from one place in the 1987.) One cannot yet know whether the of Washington, began to canvass for her. genome to another, they can also influence Herskowitz–Alberts letter was the In September, apparently unaware of the the expression of other genes; because of determining document; Nobel campaigns previous nominations, he wrote to Marcus this McClintock at first named them often acquire a momentum and other, Rhoades that ‘It seems to several of us “controlling elements”. … The “controlling independent nominations might well have that Barbara McClintock should get or elements” were the first model system for been submitted. But, regardless of share in the Nobel Prize, now that the control of gene activity’10. whether it defined or represented the transposons have been shown to be so Saedler’s letter accomplished several shift, the Herskowitz–Alberts nomination widespread and so important. I just had a things. It represented McClintock’s work expressed beautifully McClintock’s letter from Beets [George Beadle] and he reasonably accurately, describing contribution to science as being the agrees and would support a nomination. transposition, influence on gene action discovery of genetic transposition, not He suggests that you lead the charge’6. and McClintock’s recent emphasis on genetic control. This is the standard view Rhoades suggested Jim Watson as a genomic rearrangements. It also placed of her work today. Nobel-laureate sponsor but Watson her work in the context of contemporary McClintock seems to have been demurred, saying he was already transposon research. Yet it retained the bittersweet about the Nobel Prize. supporting another candidate. Joshua idea of genetic control. McClintock’s year Though, of course, she was glad of the Lederberg, however, assented happily to was not to be 1982. scientific recognition, Jim Shapiro Roman’s request: ‘Barbara for Nobel is of The next year, a nomination by Ira believes she was pained by the fact that course an excellent idea although my own Herskowitz and Bruce Alberts reflected a the recognition missed her central point. view is that Ac/Ds [her first controlling change in strategy. Both Alberts and In 1999, he said, ‘She told me she wasn’t element] was not even her most important Herskowitz were in the biochemistry interested in transposition. She was work. I would be glad to endorse and faculty at the University of California at interested in regulation’12. McClintock forward a nomination that you and San Francisco. Herskowitz led the herself expressed this opinion in Marcus prepared.’ Other affirmations of laboratory group that developed the interviews throughout the 1970s and support came in from Nobelists Salvador cassette model of yeast mating-type early 1980s. Luria, Alfred Hershey and Francis Crick7. switching; Alberts, today head of the Barbara McClintock could not possibly Lederberg’s nomination, informed by National Academy of Sciences, used have won a Nobel Prize for transposition Rhoades’ summary appraisal of bacteriophage to study the mechanism of before 1976. Until then, her work since McClintock’s research, noted the DNA replication. In the box ‘The 1944 had been understood as she had importance of ‘her work on the nomination concerns the discovery of,’ wished: a theory about development and mechanism of control of gene action in Herskowitz and Alberts entered simply, differentiation. That theory had never maize, involving the action and ‘Transposable genetic elements’11. The been widely accepted and, by the 1970s, it interaction of two independent loci. This nomination began, ‘Barbara McClintock had been solidly refuted. Between 1976 system was the forerunner of the discovered that discrete pieces of genetic and 1983, her Nobel nominations regulator–operon theory of Jacob and information are capable of movement from gradually overcame a quarter-century of Monod’8. When I interviewed Lederberg one chromosomal location to another and inertia, as transposition was reframed as in the late 1990s, he did not believe that this movement of ‘transposable the basis of genomic rearrangement. Any McClintock’s work was the forerunner of elements’ can lead to major alterations in delay for her award seems to be a the operon theory and did not recall gene expression.’ Nothing is mentioned consequence of residues of that old having said it was9. Indeed, it is doubtful about gene regulation or the control of developmental framework clinging to her that he believed it in 1981; the words are development and differentiation. nominations. probably those of Marcus Rhoades. A busy Herskowitz and Alberts mentioned recent This view in no way diminishes either man, Lederberg probably wrote a gloss on work that had ‘shown that transposable McClintock or the prize. The reframing of Rhoades’ evaluation and sent it off, genetic elements are present in essentially great discoveries could, in fact, be rightfully pleased to have done a good all organisms.’ In addition, they went on to common, and might sometimes happen deed for a deserving colleague. note the implications of transposition for repeatedly: recently, in some circles, In another nomination letter for 1982, cancer research, yeast genetics and dogmatic opposition to transposon- Heinz Saedler, a bacterial geneticist in genome evolution. In short, the mediated gene regulation has softened(for Cologne working on insertion elements, nomination presented genetic examples, see Refs 13–17). What does wrote perceptively that McClintock’s transposition as universal, of basic seem rare is for a scientist to live to see her contribution ‘lay in the introduction of a importance to organisms and of great work take on new meaning. McClintock

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was fortunate in earning the admiration of 6 Herschel Roman to Marcus Rhoades, 2 September reading promoter near the end of Mu1. Proc. Natl. junior colleagues who, within her lifetime, 1981, folder 16, box 12, Marcus Rhoades Acad. Sci. U. S. A. 88, 3502–3506 rewrote history to ensure due, but not collection, Lilly Library, Indiana University 14 Martienssen, R.A. (1996) Epigenetic phenomena: 7 Herschel Roman to Joshua Lederberg, 1 October paramutation and gene silencing in plants. Curr. overdue, credit to one of the century’s 1981; Lederberg to Roman, 5 October 1981; Biol. 6, 810–813 great geneticists. Roman to Lederberg, 30 December 1981; folder 15 The Cold Spring Harbor Laboratory, Washington 16, box 12, Marcus Rhoades collection, Lilly University Genome Sequencing Center and PE References Library, Indiana University Biosystems Arabidopsis Sequencing Consortium 1 McClintock, B. (1961) Some parallels between 8 Joshua Lederberg, undated nomination letter, box (2000) The complete sequence of a gene control systems in maize and in bacteria. 12, folder 16, Marcus Rhoades collection, Lilly heterochromatic island from a higher eukaryote. Am. Nat. 95, 265–277 Library, Indiana University Cell 100, 377–386 2 Tracy Sonneborn to Marcus Rhoades, 21 March 9 Joshua Lederberg, interviews with the author, 12 16 Dimitri, P. and Junakovic, N. (1999) Revising the 1967; press release dated 17 April 1967, box 12, March 1996, 1 July 1999 selfish DNA hypothesis: new evidence on folder 12, Marcus Rhoades collection, Lilly 10 Heinz Saedler, ‘Nomination: Nobel Prize for the accumulation of transposable elements in Library, Indiana University year 1982: Dr. Barbara McClintock,’ 25 January heterochromatin. Trends Genet. 15, 123–124 3 Ernst Caspari to McClintock, 27 June 1973, folder 1982, from Saedler’s private collection 17 Buckler, E.S. et al. (1999) Meiotic drive of ‘McClintock, Barbara,’ Ernst Caspari Collection, 11 Ira Herskowitz and Bruce Alberts, ‘Nomination chromosomal knobs reshaped the maize genome. American Philosophical Society Library, for the 1983 Nobel Prize in Physiology or Genetics 153, 415–426 Philadelphia, PA, USA Medicine,’ 26 January 1983, from Herskowitz’s 4 Judson Van Wyk, nomination for 1976 Nobel Prize private collection Nathaniel C. Comfort in Physiology or Medicine, 28 January 1976, from 12 James Shapiro, interview with the author, 14 July Center for History of Recent Science, Dept of van Wyk’s private collection 1999 History, The George Washington University, 5 Watson, J.D. (1980) Foreword. Cold Spring 13 Barkan, A. and Martienssen, R.A. (1991) Harbor Symposia on Quantitative Biology Inactivation of maize transposon Mu suppresses a Washington, DC, USA. 45, xiii mutant phenotype by activating an outward- e-mail: [email protected]

Book Review

Going everywhere: function, the components have to be Because it is a multiauthor text, there targeted and transported to their is inevitably some variation in the way molecular transport in destinations. This text describes some of material is presented and its readability, eukaryotic cells the targeting signals and transport but this is only a minor limitation. mechanisms that account for this Overall the reviews are concise, well- relocation. In the case of proteins (the illustrated accounts that present the Essays in Biochemistry: Molecular major focus of these reviews) relocation major findings without submerging the Trafficking (Essays in Biochemistry, Vol. 36) often entails transport into or across reader in the considerable and often very edited by Philip Bernstein cellular membranes. Since Blobel, many complicated experimental details that Portland Press, 2000. $24.00 (131 pages) scientists have tackled this area and, that support them. Although this text ISBN 185578131X although much remains to be discovered, a might not suit specialists working in the general understanding of the mechanisms areas under review, it is very appropriate This book contains ten short reviews on a of molecular trafficking has begun to for the target readers: students and variety of topics within the area of emerge. This book reviews a selection of teachers of biochemistry and molecular molecular trafficking in eukaryotic cells. the relevant developments. cell biology. Each chapter ends with a take In the preface, the editor emphasizes the The first two chapters discuss protein home message derived from a summary of timeliness of this volume, particularly as targeting and translocation to the the state-of-the-art literature about the the 1999 Nobel Prize for Physiology and membrane of the endoplasmic reticulum particular topic. Furthermore, the Medicine was awarded to Günter Blobel (ER), and the immunological properties authors themselves are active for the discovery that ‘proteins have of ER chaperones. Emphasis then shifts contributors to the areas they review, and intrinsic signals that govern their to the role of glycosylation in protein generally write with the confidence and transport and localization in the cell’. transport, and to the mechanisms of authority this allows. Blobel’s contribution to the field is also vesicular transport itself. Next come Molecular trafficking in eukaryotic cells highlighted throughout the rest of the accounts of protein import into is a huge topic maintained by a massive text: many of the authors have either a chloroplasts and mitochondria, and the research effort that has generated a direct or an indirect connection with the last four chapters discuss transport of correspondingly impressive bulk of Blobel laboratory. proteins and nucleic acids into and out of literature. In such a rapidly moving field, Eukaryotic cells are structurally the nucleus. More specifically, these the production time for a book of this type complex and require the specific describe the nuclear pore complex, the (or the failure of contributors to meet the localization of many factors to function signals and mechanisms that mediate editor’s deadlines!) means that the correctly. The molecular components bi-directional transport, how regulated material it contains is out of date by the involved are diverse and numerous, and nuclear transport controls gene time it appears in print. This limitation are constantly being degraded and expression, and how viral models are applies here to a certain extent – most of renewed. Because the site of synthesis is being used study RNA export from the the references cited in the bibliographies frequently different from the site of nucleus. appeared in 1998 or earlier – but this is not