NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA VOLUh,IE 54 NATIONAL ACADEMY PRESS \\`ASHINGTOS, D.C. 1983 COLIN MUNRO MA~LEOD January 28, 1 SW-February 11, 1972 BY M'ALSH hlcDERMOTT S A BEGISSER in science, Cohn ibfunro %lcLeod ~2s A granted the most tvonderful of gifts, a key role in a major disco\,ery that greatly changed the course of biology. Great as this gift \\.as, it came not as unalloyed treasure. On the contrary, for reasons that are not wholly clear even toclay, the demonstration by ,4very, RIacLeod, and McCarty that deoxyribonucleic acid is the stuf-f that genes are made of \\sas slolv to recei1.e general acceptance and has never really been saluted in appropriately formal fashion. The event was origi- nally recorded in the noiv famous paper of 1944 in the JozcwaI of Ex~rritnrntal Rfrriicinr, ' entitled: "Studies on the Chemical Nature of the Substance Inducing Transformation of Pneumococcal T),pes. Induction of Transformation by a Desosyribonucleic Acid Fraction Isolated from Pneumococ- cus Type I I I ." The title tells the story; clearly this \vas an historic Lvatershed. Sir hlacFarland Burnett states that "the discover), that 11s~ could transfer genetic information from one pneu- mococcus to another heralded the opening of the field of molecular biology." ' \Yriting in hTntrtrr in the month before MacLeod died, H. V. \t'yatt" reports it as "generally ac- cepted" that the field of molecular biology began with the 183 184 RIOGR.APHIC:AL \IFIlOIRS appearance of this paper. Lederberg terms the work "the most seminal discovery of tv\.entieth-century, biology." To make an important individual contribution to one of history's great scientific achievements was an act of creation of a special sort. It took place in the decade between Macleod's twent),-fourth and thirty-fourth years. He could have rested on this achievement; he could have continued with it, thus emphasizing his role; or he could have gone on to something else. As things worked out, he folIoI\-ed the last-named road, influenced to an undeterminable extent by M'orld M'ar II. But there are other forms of creation in science, and, in some of these, hIacLeod also excelled. Before looking at these aspects of his life, it is worthwhile to pause a moment over the question of hovv he had been prepared so that he might make such great contributions. (Dr. Robert Austrian, in a sensitive and perceptive piece, has described i\lacLeod's earl?, years .") One of eight children of the union of a schoolteacher and a Scottish Presbyterian minister, the young MacLeod skipped so many grades in school that after being accepted at hfcGil1 Universitv he had to be "kept out" a year because he v\~as too young. EIis birth on January 28, 1909 took place in Port Hastings, Nova Scotia. In his early childhood, he movfecl vvith his family back and forth across Canada from Nova Scotia to Saskatchewan to Quebec. He obviously was a splenclid stu- dent, for, as related by his sister, hIiss Margaret MacLeod, he skipped the third, fifth, and seventh grades and graduated from secondary school (St. Francis College, Richmoncl, Quebec) vzhen only fifteen years of age. His career as an educator started almost immediately. Ft'hile being "kept out" of school to become old enough for !v1cGill, he vvas induced to leave an office job to serve at the age of sixteen as a substitute teacher of the sixth grade in a Richmond school. He held this job wholly on his ovvn for the entire year. These COLIN hiUSRO hf \c.I.EOD 18.5 early signs of superior intellectual capacity were not a part of the stereotype "infant prodigy." Incleecl a clear sign to the contrary \vas the fact that within only a few years he was on the hlcGil1 varsity hockey team-then, as no\v, a most im- pressi\,e athletic achievement. After two years of premedical education at McGill, he entered the Medical School and received his degree in med- icine in 1932. In 1934, at the age of t\\`enty-four, after two years of residency training at the Montreal General Hospital, he came to New York. Less than ten years later, he woulcl make his o\vn highly important individual contribution to the Avery- MacLeod- McCarty study. The nature of the reception of this bvork was to test the remaining thirty years of his life, for its significance did not receive the early attention it might be thought to have merited. Shortly before MacLeod died, this aspect of the story formed the basis of several articles in scientific and popular periodicals.;' He had the chance to see these, but sadly enough, he did not live to see the most extensive and authoritative account, published in 1976 by R. J. Dubos in his book, The Proftmor, the Institute and Dh'A." There is no intent here to attempt to add to this literature. The chance of painting a distorted picture is too great for one who was not close to the situation at the time. Moreover, the endpoint of "acceptance " is hard to measure, for in science it does not occur all at once like a directed plebiscite in a totali- tarian state. Some highly knowledgeable scientists perceive the full significance of a particular discovery right away; others require longer. It is necessary, however, to cite the major events in the research itself in order to describe Macleod's clearly definable and individual contribution. And, given that contribution, some mention of what hap- pened to the recognition of the work is inescapable in telling the story of MacLeod's career in science. For it is the way the 186 BIOGRr\PHIC;AL. hlE\lOIRS whole story seemed to him that could have had a telling influence on his subsequent career. When he first arrived at the Rockefeller Institute, hiacLeod fell under the influence-or spell-of 0. T. Avery, or "Fess" as he was called, fvho was the inspiring teacher of so many others, including Rene Dubos, Maclyn McCarty, and the late Frank Horsfall and Martin Henr) DaFj.son. Some years before, as related by Dubos, an old school friend of MacLeod's, Henry Dawson, had been asked by Avery to investigate the variations in pneumococcal colonial morphology from "rough" to "smooth" (R/S) then being studied by Griffith in England. Several years later, when Griffith' demonstrated that one pneumococcus type could be transformed irz uizlo into another, in effect a directed and heritable alteration, Dan.son \vas captivated by the feat. il'orking Lvith R. H. P. Sia, he \vas able to repeat the experi- ment and to produce the change." Dawson had to abandon the project, \\.hich xvas taken up by J, S. Alloway," who was able to show that the substance responsible resided in a thick, syrupy preparation. The techniques used by Dalvson, Sia, and Alloway were not at all reliable. Neither the phenomenon of transforma- tion nor the harvesting of transforming principle could be reproduced lvith a high degree of predictability. A phenome- non of potentially great biologic significance had been clearly identified. Yet Ivithout methods to produce it with predict- ability and to extract its active principle in \+.ays permitting precise characterization, any attempts to study the matter further lvere bound to be marked by frustration. Neverthe- less, because of the potential significance of the phenome- non, Avery decided that the lvork must go on. He continued to see the first essential task to be the chemical characteriza- COLlS hlUSKO Xl \(.l.EOD 187 tion of the active material, but the a\,ailable techniques Lvere obviously not sufficiently reliable to permit such chemical studies. It was at this point that MacLeocl entered the picture in 1935. By improving the medium and isolating a consist- ently reproducible rough strain of pneumococci, MacLeod made it possible (with Aver)f's encouragement and counsel) to move the project from what \\.as the stud). of a fascinating phenomenon, but one of irregular occurrence and not pos- sible to assay, to a predictable one. The critical substance could then be fully characterized in chemical terms. The subsequent phase of the study, the actual conduct of these chemical studies, became the responsibility of McCarty. Each of the six investigators Lvho Fvorked with Avery thus made a contribution to the solution of Griffith's mystery, but it is now fully conceded that the critical contributions were those made by MacLeod and hfcCarty under the continuing, brilliant intellectual stimulation, advice, and counsel of Avery himself. Oddly enough, as Dubos has described, although MacLeod and McCarty worked closely together on the proj- ect, they were not officially at the Institute at the same time, for in 194 1, at age thirty-two, MacLeod became chairman of the Department of Microbiology at the New York University School of bledicine. He left the Institute as McCarty arrived. As the Medical School of ~1.u and the Rockefeller labora- tories are both in the mid-East Side of Manhattan, it \vas easy for MacLeod to travel back and forth, and he maintained a continued and wholly recognized association with the project. In large measure, ho\vever, whether it \\-as realized or not at the time, he had made his contribution. He had taken an almost formless, erratic phenomenon and made it into some- thing predictable and measurable. This had to be done, and he did it. Thus, the problem had been brought to the ver) stage at rvhich McCarty's own considerable biochemical ex- 188 BIOGRAPHICAL hIEllOIRS pertise was exactly what the situation called for. Two years later (November 1943), the paper was submitted to the Journal of Experimental Medicim. I" In subsequent years, MacLeod continued to work on this problem in his laboratory at New York University, first with M. R. Krauss" and R. Austrian,`" and at a later period with E. Ottolenghi. I3 It is approp riate to postpone discussion of these subsequent phases of his scientific career in universities and government and to dwell for a moment on the story of how the finding presented by Avery and his two younger colleagues in the 1944 paper was received. A revolutionary concept, as pointed out by Kuhn," does not usually increase knowledge by adding on to it; it is more apt to replace it. A problem in 1944, and a far greater one today, is hobv one can evaluate nelv research with implied revolutionary findings when, as a practical matter, one can- not employ the techniques necessary to repeat it. The scientists who read the 1944 paper by Avery, MacLeod, and McCarty had, in theory, two choices: they could accept or deny the validity of the demonstration on the basis of comprehension, or they could repeat the experi- ments. To do the former requires an intimate knolvledge of the reliability of the techniques. At first glance that is a state- ment of the obvious-something that occurs on the reading of any scientific paper. But such is really not the case. Most of the time, in biomedicine at least, published experiments represent logical sequences in a series of experiments on the same subject. The degree of reliability of the key methods is knotvn to be understood by those intimately engaged in the field, and the rest take it on faith. 1Yhen this is not the case- w-hen the results depend on a new method-if the field is reasonably in the scientific fashion of the day, it contains other lvorkers. These other ivorkers soon define the limits of the technique. Ob\riously, this system depends on the judg- co1.1iK hlUNK0 51 \(.L.EOL) 189 mental decisions of presumed experts, but the scientific cotn- tnunity and the public are protected against prolonged error by the competitive nature of the studies in a particular field. It is one part of the familiar "marketplace of ideas." The trouble L\.ith the A\,er~-hIacLeocl-McCarl~ studies was that the approaches they used did not happen to be f~ashionable. They \\`ere not part of a race to glory, such as that described bj, \Yatson in the Double Hdix. `,' Or-, more accurately, the successful approaches that were used by the Rockefeller group cvere far out of the ken of most of those ~.ho r\`ere lvorking actively to solve the question. Moreover, the nucleic acids \vere not believed to have an)' biologic activ- it\, not- \vas their structure j\.ell defined. There really was no community of competing investigators fully armed \\rith the requisite techniques ready to jump in and repeat the experi- ments. Indeed, to do this ~vould require assembling a team \\ith the talents, experience, ancl expertise of Avery, >IacLeod, a11d hIcCarty. 1l'hat is more, it ~vould ha\,e to be assembled from a markedly constricted biomedical research community, for by this time the U.S. involvement in 1Vorld \Car II had begun. Acceptance of the chemical basis of transformation might seem to have been slolv, although clearly there was no set period ltithin \\-hich it should have occurred. There is now a small body of published material on this question of accept- ance by some of' the people \\,ho \j.et-e close to the field at the time. Some of these comments Lvet-e recorded during the period in question or a little later; others are present-da) recollections of lvhat \vas thought at the time. As might be expected, these reports ranged from outright acceptance of the role of ns.4 to a definite interest short of conviction, to, at the other extreme, a belief that the phenomenon was not mediated by nucleic acid at all, but by minute amounts of contaminating protein. Stent believed the \vork had little itn- 190 BIOGRAPHICAL I\IERlOIRS pact on genetics. *Ii Lederberg strongly dissents from this point of view and presents important contemporary citations in support of that position.17 Indeed, in the year following the original report, J. Ho\\.ard Mueller I' appears to hai,e cor- rectly perceived the whole stoq', as may be seen in his article in the A~rwal Revieu of Biochemistry. Dubos,`!' in his 1976 analysis of the entire record, suggests that one of the factors in the slot\- acceptance \vas the starkly noncommittal lvay the results Iver-e presented, Lvhich was notable even in a scientific report. In those days at the Rockefeller Institute, there was a philosophy concerning the style in which experimental re- sults should be presented. This style was largely initiatecl by Avery but was also adhered to with comiction by most of his younger associates, especially hlacleod. In this style, the I;el words r\-ere carefully chosen to convey only that lvhich had been clearly proved and nothing more; any suggested impli- cations \\`ere rigorously excluded. Leclerberg also credits this attribute, which he terms "Avery's own a-theoreticism," lvith helping to postpone "the conceptual synthesis that now iclen- tifies `gene' \vith DNA fragment.""' M%ether or not acceptance was slolv, it evolved steadily. For Lederberg also mentions: "In 1936, at the Cold Spring Harbor Symposium, where Tatum and I first reported on recombination in Escherichia coli, we Lvere incessantly chal- lenged lvith the possibility that this was another example of transformation, a la Griffith and Avery.""' Dubos cites a summary by Andre L\voff of a 1948 con- ference in Paris in lvhich the genetic role of the nucleic acids is obviously accepted. But as Dubos also states: It tool, an experiment, outside of the Institute, with a biological system completely different from that used by Avery to liin universal acceptance for the genetic role of m.4. Using coliphage marked with 32P (restricted to the ~1.4 component of the virus) and tvith 35s (restricted to the protein component), Hershey and Chase at the Cold Spring Harbor Laborator) COLIS .\lUSKO Sl\(.LEOLI 191 show& in 1932 that most of the viral I)\-\ penetrates the infected bac- terium, whereas most of the protein remains outside. This fincling sug- gested that I)SA, ant! not protein, x\.as responsible for the directed specific sythesis of bacteriophage in infectecl bacteria. In reality, the intel-preta- tion of this wx~cierful experiment was just as questionable on technical grountls as was the chemical interpretation of pneumococcal transforma- tion, but those obtained by Aver! 10 years before, that the fwv remaining skeptics Lvere convincecl. The case for the view that 1)x4 is the essential and sufficient substance capable of inducing genetic trarlsf'or-rnatitrns in bacte- ria ~vas not \von b) a single. absolute demonstration. but by tWo inciepen- dent lines of evidence.YY In his Nobel Prize lecture,23 Leclerberg puts it in essen- tially the same \vay. He attributes to Aver), and his colleagues the demonstration that the interpneumococcus transference of an inherited trait \vas through ~x.4, the broadening of the evidence to Hotchkiss,`? and the reinforcement of this con- clusion to Hershe). and Chase,2" tvith their proof that the genetic element of a virus is also DSA. Eventually such situa- tions right themselves. Toda)- if one looks in elementary texts 011 human genetics, the Avery-hlacLeod- McCarty 1944 paper is cited, in effect, as the historic watershed.2" Little imagination is required for anyone who has ever been engaged in science to envision \\.hat a deep-seated disap- pointment the relative lack of formal recognition of his key contribution to the DSA r\,ork could be to a scientist, especially to one \l.ho \vas just starting out in his career. A sense of having in some tvay suffered an injustice I\-ould not be at all unusual. This could ib,ell lead to bitterness, particularly as the years Lvent on and others reaped i\ide professional and pub- lic recognition for studies on ~s.4. But MacLeod Lvould have none of this. Not for him ~$7oulcl be the stereotype of the unhappy investigator living off scientific "might have beens." Indeed, as far as I have been able to ascertain, at no time did he ever publicly express, even by indirection, the thought that, in the ~x.4 story. he had been slighted in an)- lvay. 192 BIOGRAPHICAL l\IkaklOIRS MacLcod's seven J'ears in Avery's "department" at the Institute Fvere Ilot all occupied by the work on the pneumo- coccal transforming factor. On the contrary, he \\.as engaged in a number of other studies, as may be seen from his sixteen publications of this period, eleven of tvhich list him as senior author. T\vo things are striking in looking over this list today. First, although a number of different topics appear to be involved, they almost all deal lvith host-parasite relations at the very time antimicrobial therapy was coming on stage, so that the influence of this intervention in the disease mecha- nism could also be embraced b), the studies. Second, virtuall) all lucre concerned with pneumonia, notably pneumococcal pneumonia; there w'as one stud!. on the so-called prirnar), atypical pneumonia"7 just then coming into medical recogni- tion. Given Avery's preoccupation ivith pneumococcus, the fact that MacLeod, \\.orking in his laboratory, published a number of studies on pneumonia may not seem too surpris- ing. \Vhat is important, however, is that this interest led hIacLeod to highly productive studies in his subsequent career. MacLeod's start as a university professor coincided roughly jvith the entrance of the United States into World M'ar II. VieM.ed in retrospect, the impact of so pervasive a force as \Vorlcl War II was bound to have deep and enduring effects on a young man-just emerging as a leader in science. From this time on, three characteristics were prominent. He \\.as forever conscious that the university department he headed \vas in a school for the training and education of physicians, he r\-as deeply- convinced of the social value of unfettered basic scientific research, and he felt a responsibil- ity to contribute w,hat he could to the shaping of public policy in that interface of government and the universities that developed so rapidly in importance dating from that time. To a considerable extent, all three characteristics tended tobvard