14—
The Caltech Years—
The Fulfillment
My first decade at Caltech was exhilarating. Not only was the f X research progressing steadily and successfully into previously inaccessible processes, but more broadly it was a breakthrough era for molecular biology. The many ideas, hypotheses, and concepts immanent in the double helix model for DNA were about to receive experimental test and verification or refutation. Within the next half dozen years the "semiconservative" model for DNA replication was demonstrated, the genetic code was deciphered, and "messenger RNA" was discovered as the intermediate in the conversion of genetic information into protein structure. Caltech played a significant role in each of these advances.
The invention and theoretical analysis of the technique of density gradient centrifugation by Jerome Vinograd at Caltech provided the methodology for the brilliant experiments of Meselson and Stahl, as well as some of our own crucial f X experiments. Using the stable heavy isotope of nitrogen (N15 ) as a density label, Meselson and Stahl were able to demonstrate that, on replication of DNA, the two strands of the double helix dissociate; each strand, remaining intact, becomes one of the two strands of each daughter double helix. This process had been postulated but, as we were completely ignorant of the complex enzymatic processes involved, had remained in doubt until experimentally verified.
The correspondence between DNA as gene and protein as "gene product" was clear, but the nature of the biochemical connection was unknown. Progress in the achievement of protein synthesis with extracts
from disrupted cells had indicated that proteins were actually synthesized on subcellular particles called ribosomes. But the ribosomes of a cell were chemically uniform. How then were individual ribosomes programmed to produce different proteins? Experiments in several laboratories, including particularly research conducted by Brenner, Jacob, and Meselson at Caltech, demonstrated that a minor, ephemeral variety of ribonucleic acid, called "messenger RNA," was made by copying into RNA portions of the cellular DNA. Such messenger RNA molecules then served for a short time as programs to be read by the ribosomes, to produce the specific proteins.
Clearly, the genetic information contained in the sequences of nucleotides in DNA, as copied into the messenger RNA, somehow specified the sequences of amino acids in the various proteins. But the code connecting these two sequences was unknown. Delbrück was very interested in this and there was much speculation and many seminars about the nature of the genetic code: overlapping codes, self-correcting codes, commaless codes, two-letter, three-letter, four-letter, six-letter codes were discussed with proponents of one or another variety proclaiming its apparent or unique merit. It was a game many could play and it seemed that if one were really clever, one could deduce the one clearly optimal code that nature had chosen to use.
But the experimental problem of deciphering the actual code seemed difficult to resolve until the highly fortuitous discovery by Marshall Nirenberg that, under nonphysiological conditions, protein-synthesizing systems extracted from cells could use artificial polynucleotides as messenger RNAs. Such synthetic polynucleotides could be made of very simple sequences, resulting in the synthesis of correspondingly simple chains of amino acids. The obvious correlation then of amino acid sequence with nucleotide sequence permitted the complete decipherment of the genetic code within a few years.
At this same time, the discovery of the "amber" viral mutants by Richard Epstein at Caltech—mutants that could only grow in cells that used a slightly altered genetic code—opened a wide new path to the identification of specific genes and the analysis of their function.
It was an heroic time in which individual scientists working with small groups could, with great effort and ingenuity, make momentous discoveries. These discoveries, over a space of little more than a dozen years, transformed biology by exposing the molecular machinery of genetic reference, thereby prying open and making accessible the basic processes of life.
It is hard now to realize how difficult those first steps were, how months of step-by-step experimentation and correction were required to obtain a result that could now be achieved in a few hours. But it was accomplished, and those involved will always remember the thrill as each new discovery, was announced, each new perspective revealed.
After these discoveries of the 1950 to 1965 period had been consolidated, some, notably Gunther Stent, proclaimed the end of the "golden age" of molecular biology: all of the great discoveries had been made and, most disappointingly, no new fundamental principles had been discovered; biology turned out to be a specialized form of chemistry. But Stent's vision was woefully short. He did not foresee the second and even greater flowering of molecular biology.
The invention of recombinant DNA in the early 1970s by Herbert Boyer and Seymour Cohen enabled the extension of molecular biology techniques far beyond the small discrete nucleic acids of viruses to the much more complex nucleic acids of higher organisms. And the development of methods for the determination of nucleotide sequence in DNA by Walter Gilbert and Fred Sanger improved the effective discrimination of these techniques by orders of magnitude, permitting the detailed analysis of mutation and genetic control mechanisms. The previously inaccessible fields of developmental biology and physiological control then became open to analysis at the molecular level.
In this same period, Caltech was the scene of other remarkable scientific accomplishments. Martin Schmidt and Jesse Greenstein discovered quasars, the intensely powerful and distant sources of radiation that are believed to be at the centers of galaxies. Murray Gell-Mann developed his quark model of the structure of subatomic particles, which brought coherence to an increasingly diverse and puzzling set of data. The Jet Propulsion Laboratory, after some difficulties, successfully sent the first probes to land on the moon.
Richard Feynman in physics was the one true genius I have known. He had an unsurpassed depth of understanding of the fundamental principles of physics in the broadest sense. I watched in astonishment as, on more than occasion, he swiftly applied this deep insight to completely novel situations and assorted and arrayed the important variables in some comprehensible manner.
What special qualities made Caltech the locus of many of these advances? Caltech is unique. A small, private school, focused on basic science and advanced engineering, it attracts the brightest freshmen (as
measured by SAT scores) of any college or university in the country. Because the freshmen class is limited to 220 students, now about 25 percent female, the resultant undergraduate student/faculty ratio of three to one permits individual student recognition. At the same time, the faculty is able to devote most of its time to research, working with some one thousand graduate students and four hundred postdoctoral fellows. The concentration of interests, the uniformly high level of talent, and the opportunity for close interaction with faculty make it an excellent school for students who know their educational goals. For others its opportunities can be too limited.
The renown of the institute has enabled it to develop extensive funding and it has the highest ratio of endowment per faculty member of any private university. Its ability to provide substantial research resources in support of individual faculty members has in turn enhanced the ability of those members to achieve and maintain eminence in their fields.
Absent the elemental necessity to hire enough faculty to provide the needed classes for large numbers of students, Caltech can be highly selective in its choice of permanent faculty. Similarly, as a private institution, Caltech can be highly selective about the fields of education and research it chooses to undertake. This freedom to focus energy and resources on a few specific areas within, for instance, biology or physics has permitted the institute to deliberately select the most promising frontiers in any period and to develop a "critical mass" of interactive faculty to explore and exploit those fields.
Careful screening among the most promising applicants for junior faculty positions is followed later by application of rigorous criteria for the award of tenure. As a general principle, tenure is awarded only to junior faculty members who have demonstrated accomplishments sufficient to place them among the "top five" in their field and age group in the country. In consequence, 21 percent of its faculty are members of the National Academy of Sciences and 8.5 percent are members of the National Academy of Engineering.
The small size of the Caltech faculty facilitates scientific contacts among the several disciplines. Such interaction is fostered by the presence of a rather elegant faculty club, the Athenaeum.
The Athenaeum often provided an intellectual feast, a gourmet meal for a science junkie. One might talk with a geologist about the age of the earth, the composition of moon rocks, or the temperatures of ancient seas. Or with an astronomer about the precise periods of pulsars
or the mysteries of quasars. Or with a physicist about the zoo of fundamental particles, the oddities of quarks, or the postulated gravity waves. Or with an engineer about supersonic planes and rockets, space exploration, lasers, or the potential of computers. Or with economists or historians to hear their perspectives on the events of the time. To be sure, there was always politics or the fortunes of the Dodgers to fill in the gaps.
As compared to MIT, Caltech is much smaller, with about one fourth the student body. Caltech is much less diverse, with a narrower spectrum of engineering disciplines and no schools of architecture or management. Basic science plays a larger and engineering a lesser role. The breadth and scale of MIT permit it to consider problems of large societal concern and impact such as transportation, urban planning, and industrial competitiveness.
Caltech is quite comfortable with itself in its present format. It has a self-image of singular superiority, which is not quite as readily justified as formerly. For, while still unique, Caltech is not as peculiarly distinctive as it was in an earlier era. Other, much larger institutions have developed concentrations of faculty in one or more specific areas able to rival those at Caltech. But it remains an extraordinary constellation of excellence in its chosen sectors. While the limitations imposed by its narrow focus on curricular offerings and research opportunities are recognized by some, the institute has not been sufficiently motivated to undertake the major effort required to transcend them.
When we arrived in the 1950s, Pasadena still had a small-town atmosphere. Unfortunately, at times it also had intense smog generated in downtown Los Angeles and blown up against the mountains. Nevertheless, there were pleasant residential neighborhoods within walking distance of the Caltech campus in which many faculty lived. We found a residence nearby that greatly facilitated return in the evening to initiate or complete laboratory experiments. This ease of access also facilitated attendance at evening lectures and engendered a sense of campus community.
The biology faculty at Caltech was, and continues to be, extraordinary. When I arrived, George Beadle was the chairman and unquestioned leader, a "father figure" to most of the department. When he subsequently became dean of the faculty and proposed that the department find another chairman because of the demands on his time,
we would have none of it. Only when he left to become president of the University. of Chicago did we replace him.
Henry Sturtevant was the "grand old man" of the genetic community. A student with Morgan in the early days of drosophila research at Columbia, Sturtevant had known personally almost all of the great figures in the history of genetics. He had an encyclopedic knowledge of genetics and entomology as well. As I saw him age and reach retirement, I realized how much is lost when such an irreplaceable mind is gone. Ed Lewis, his protégé, would later carry on the drosophila tradition with great distinction. Sterling Emerson, Ray Owen, Norman Horowitz, and Herschel Mitchell completed an extraordinary grouping of geneticists.
Frits Went with his Phytotron, in which he could grow plants under precisely controlled conditions, and James Bonner and Arie Haagen-Smit, who were more biochemically oriented, constituted an impressive program in basic plant biology. Haagen-Smit had already become interested in the chemistry of the pervasive smog problem and had deduced the primary chemical reactions involved. Meanwhile, Henry Borsook, a long-time biochemist, was pursuing protein biosynthesis, a problem that would only be solved subsequently by much newer approaches. "Case" Wiersma and Anthonie Van Harreveld were classical neurophysiologists engrossed in electrophysiology, while Roger Sperry undertook his brilliant and idiosyncratic studies of the factors determining neuronal connections.
All told, a remarkable group.
Caltech provided me the opportunity for research at a higher level of intensity and on a different scale than had Ames. Very able graduate students wanted to work in my laboratory; postdoctoral fellows from Europe, Asia, and the U.S., eager to have experience at Caltech, soon applied to study with me. Over the years, I had scholars from Great Britain, the Netherlands, Belgium, France, Sweden, Germany, Czechoslovakia, Hungary, the Soviet Union, Japan, and Chile work in my laboratory. It is a great pleasure now to be able to travel about the world and visit former students and see how well they are doing in their science in their native countries. Science is truly an international activity. I can walk into a lecture room in Kyoto, Santiago, or Stockholm and see the periodic table on the wall and feel at home.
And, as well, I regularly had one or two bright undergraduates, eager to begin research, working part-time with me.
Teaching responsibilities at Caltech were much different than at Iowa State. Formal classroom obligations were much reduced and there were no large "service" courses akin to engineering physics. However, there was much greater informal educational activity, of a more personal and focused nature, with graduate students and postdoctoral fellows in my laboratory and in small seminar-type courses in which a current topic of interest would be discussed in depth.
"Introduction to Biology," which was taken for one academic quarter by about half of the freshman class, was the largest biology course; I taught this for six years and enjoyed presenting the essence of the new biology to such bright students. In later years, I introduced a course in "Biology and Society" that considered the social and ethical issues arising from the advances in biology and medicine.
In addition to attendance at seminars and lectures, I sought to broaden my knowledge of biology by sitting in on various classes. Max Delbrück presented a one-quarter lecture course each year on a topic of his choice He took this quite seriously and used the course as an opportunity to learn about a subject of interest. One year he talked about biological membrane structure and function, another about basic solid-state physics and order-disorder phenomena, a third about transducers in sensory processes with particular discussion of the optic retina.
Caltech undergraduates are remarkable. Uniformly bright, with high analytical skills, they have a focused intensity that certainly surpasses the students of my MIT days. Perhaps because they are more oriented toward science than engineering, the student culture is less attuned to the larger world and thus more susceptible to the fascination of research and scientific discovery.
As one result of the exclusively technical emphasis at Caltech, life for students and faculty can be remarkably insulated from even major external political or social concerns. In the 1960s we observed the upheavals, riots, and drug scenes on such campuses as Berkeley and Columbia from a distance with dismay and some disdain. Caltech students were too serious and committed for such folly. But our own children were not immune to the peer movements of the time and through them we achieved at least some understanding of the youthful idealism and depth of feelings involved. And Vietnam reached even Caltech, however peacefully, with large discussions and teach-ins.
From the beginning, this military action had made no sense to me. Vietnam seemed to be primarily a civil war with which the U.S had no great concern. And as the war dragged on, with growing casualties and
an evident unwillingness in Washington to do what would be necessary. to win it, our involvement seemed ever more a cruel and grotesque mistake—one from which the nation would long suffer.
A senior faculty position at Caltech conveys a prestige that launches one into larger spheres of activity. Soon after I joined the institute, John Kendrew came through to see me about starting a new journal. Publication in the field of molecular biology was still hindered by the too-specific interests of existing journals. Out of this came the Journal of Molecular Biology with Kendrew as editor-in-chief and myself as one of the associate editors. It is today still a leading journal in the field of macromolecular structure and function. I served as an associate editor for ten years. After this, I served for five years as a member of the editorial board of the Annual Reviews of Biochemistry, and, subsequent to that, for eight years as chairman of the board of editors of the Proceedings of the National Academy of Sciences . Such service is important to the maintenance of the scientific standards of publication in the field; it also enables, indeed requires, one to keep up with the advances across a much wider sector of biology than one's own research interest. However, it requires a considerable investment of time and intellectual energy that is not usually professionally rewarded.
I was asked to chair—which meant to draw up the program and select the participants—the 1960 Gordon Conference on Proteins and Nucleic Acids. This was the last year when both fields could be considered together. Organizing such a conference is a considerable logistic task and the supporting services of an institution such as Caltech are essential. It is a pity that video records could not have been made of these Gordon Research Conferences in the 1950s and 1960s. It was here that great advances and important smaller steps were often first presented, discussed, and critiqued in the informal, open, and friendly fashion of science.
Here Fred Sanger presented the first amino acid sequence of a protein, insulin. The sequence of the adrenocorticotropic hormone was not far behind.
In the early days, DNA or RNA preparations from cells were, of necessity, treated as homogeneous substances. While, for many purposes, this proved to be valid for DNA, many years and many conferences were required to sort RNA into its various classes of messenger, transfer, and ribosomal RNA with different structures, functions, cellular locations, and turnover rates.
At the Gordon Conference, Julius Marmur and Paul Doty described
their remarkable discovery of DNA renaturation, the basis of much of today's molecular genetics. The enzymology of DNA and RNA was progressively elaborated. Here, too, Francis Crick presented his "adaptor" hypothesis to link nucleic acid sequence to amino acid sequence and his "commaless" code. Mahlon Hoagland and Paul Zamencik described their discover, of transfer RNA. Bacterial transforming factors and viral structures were analyzed and the relationship of DNA structures to genetic factors was progressively refined. Gradually, year by year, the modes of synthesis of DNA and RNA and protein were clarified.
I will never forget Seymour Benzer's succinct interruption of a prolonged presentation by Gunther Stent. Gunther, in a late evening session, was describing in endless detail a complex bacteriophage experiment that had yielded negative results. Finally Seymour spoke up, succinctly and loudly: "Big deal!" The meeting abruptly adjourned to the bar.
Each year the field progressed and expanded with new knowledge and new researchers. After 1960, the meetings had to become more numerous and more specialized.
In 1960 I received my first invitation to an international scientific conference at Chamonix in France and so had my first experience abroad. Europe was a daily surprise. We landed in Copenhagen and I was stunned to be in the midst of crowds of people all speaking a tongue that meant nothing to me. Of course —but it felt like a verbal assault. In Europe, at least I could read the signs in the streets and at terminals. A few years later in Japan, I realized what it must be like to be illiterate, to be totally unable to find one's way—or worse, even to ask.
We visited the major cities: Copenhagen, Brussels, Paris, Geneva, London. The wealth of architecture and art accumulated over the centuries was astonishing. London still bore the scars of the wartime bombing. Each distract sector of Paris entranced us. I was struck by the omnipresent weight and legacy, of the past, so different from the ever-changing life at home.
The remarkable progress in molecular biology received wide attention. Many universities had not kept up and now lacked faculty in this suddenly important area of biology. I began to receive unsolicited invitations from other universities, many quite distinguished, to join them, often accompanied by commitments to develop a major program in molecular biology that I would head.
At first I was flattered and considered a few of these sufficiently se-
riously to visit and learn of their plans and evaluate the opportunity. But I soon realized that I simply did not want to leave Caltech to be a professor elsewhere. I had all of the resources and opportunity I needed, I had first-rate colleagues, and I liked the living arrangements. Thereafter, although I continued to receive such invitations, I simply declined with thanks, not wanting either to waste their time or to stir up uncertainty in my own life. In a few instances, however, I fear my refusal even to look at their offer was misinterpreted as arrogance.
According to Watson-Crick double helix model for DNA, the two strands of the double helix should be separated upon replication of the DNA. In 1958, in an important test of the model, Matthew Meselson and Frank Stahl experimentally confirm this separation.
Following the discovery of the double helix structure, it was expected that all DNA would be double-stranded. It is thus a considerable surprise when, in 1959, the DNA of the virus f X is shown to be single-stranded.
In 1960, Paul Doty and Julius Marmur discover DNA renaturation, wherein if the two strands of a DNA double helix are separated in solution, they will subsequently pair up in precise register and reform the double helix. This process forms the basis for much of the technology of molecular genetics.
Also in 1960, Marshall Nirenberg produces, in vitro, a polypeptide translated by a cellular extract from a synthetic polyribonucleotide Further exploitation of this discovery by Nirenberg, Severo Ochoa, and Gobind Khorana leads to the complete elucidation of the genetic code linking nucleotide sequence to amino acid sequence.
DNA does not serve directly as the source of information for protein structures, but is first copied into another form of nucleic acid, called "messenger RNA." Messenger RNA then participates directly in protein synthesis. In 1961, Sydney Brenner, François Jacob, and Matthew Meselson first describe messenger RNA.
DNA was thought to exist exclusively as long linear strands. In 1962, the DNA of the virus f X is demonstrated instead to be a ring molecule, a structure since shown in many viral and other DNAs.
In 1967, Mehran Goulian, Arthur Kornberg, and Robert Sinsheimer reproduce an infective DNA (f X) in vitro This accomplishment confirms the validity of many of the biochemical procedures then in use and opens the door to the test-tube synthesis of infective DNAs.
Research upon DNA was long inhibited by the inability of scientists to break the long chains into defined, reproducible segments. In 1970, Hamilton Smith
first characterizes an enzyme, a "restriction endonuclease," that can cleave DNA only at specific nucleotide sequences and thereby reduce a chain to a specific set of fragments. Hundreds of such enzymes, with varied sequence specificities, are subsequently isolated and form an indispensable tool for molecular genetics.
In 1973, Stanley Cohen and Herbert Boyer invent plasmid cloning, thereby making recombinant DNA a practical tool Plasmid cloning permits the amplification a millionfold or more of any specific piece of DNA isolated by the use of restriction enzymes or synthesized in vitro.
In 1977, Fred Sanger describes the first complete sequence of a DNA virus, f X. Knowledge of this complete sequence, 5,386 nucleotides in all, permits the definitive determination of all of the genes of this small virus and provides the first instance of overlapping gene sequences.