Life's Greatest Secret (24 page)

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Crick’s 1957 ‘central dogma’ lecture emphasised that the race to decipher the genetic code was closely intertwined with the attempt to understand protein synthesis. Although neither Monod nor Szilárd was involved in the coding problem, they were both convinced that studies of bacteria would provide insights into how genes function, by understanding the main thing they do, which is to enable the cell to produce proteins. At a conference in 1952 Szilárd and his Chicago colleague, Aaron Novick, described their hypothesis for how protein synthesis was controlled, focusing on how the cell knew when to stop synthesising a particular amino acid: ‘somehow the increased concentration of each amino acid depresses the rate of the individual steps of synthesis leading to the formation of that amino acid.’
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As more of a particular amino acid is produced, the rate at which it is synthesised slows down. Novick and Szilárd thought that protein synthesis involved a negative feedback loop, just like those seen in the cybernetic devices studied by Wiener a few years earlier.

In 1954, Szilárd explained his idea to Monod. The Frenchman later admitted he found it ‘a rather startling assumption’ and did not agree. This was surprising, because a year earlier Monod had shown that the biosynthesis of some enzymes was suppressed by their respective end-products – a negative feedback loop – but had been unable to explain his finding.
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It took several years for Monod to realise the significance of what he had discovered. By the end of the decade, Szilárd’s idea loomed large in Monod’s thinking, eventually influencing how we understand gene function.

Other scientists were also looking at protein synthesis in bacteria and were groping towards the same kinds of interpretation as Novick and Szilárd. In 1954, Richard Yates and Arthur Pardee, from the University of California at Berkeley, described an investigation into the biosynthesis of the pyrimidines uracil and cytosine in
Escherichia coli
bacteria. They found that once pyrimidines began to appear in the cell, their presence led to a decrease in the levels of the enzymes involved in the biosynthetic chain. Their 1956 report of their experiment concluded:

Inhibition by an end-product of its own synthesis appears to be a common control mechanism in the cell.

Although Yates and Pardee had clearly described a negative feedback mechanism, they used the term ‘feedback’ only in the title of their article, and beyond asserting that such systems were widespread in the cell, they did not explore the matter any further. At about the same time, Edwin Umbarger of Harvard Medical School used negative feedback to interpret a study of
E. coli
in which the presence of the end-product of a biosynthetic pathway inhibited its own biosynthesis. In 1956, Umbarger published an article in
Science
that began with a statement that revealed the influence of Wiener’s cybernetics on the new generation of scientists known as molecular biologists:

Recent developments in automation have led to the use in industry of machines capable of performing operations that have been compared with certain types of human activity. In the internally regulated machine, as in the living organism, processes are controlled by one or more feedback loops that prevent any one phase of the process from being carried to a catastrophic extreme. The consequence of such feedback control can be observed at all levels of organization of a living animal’.

For Umbarger, the relatively simple system of bacterial biosynthesis provided an opportunity to explore the molecular mechanisms involved in negative feedback, and he was sure that the example he described – the biosynthesis of isoleucine – was just one case among many.

Although all these researchers contributed to the shift in thinking about how protein synthesis worked, the people who linked the ideas of feedback and genetic information, changing our view of life and of the genetic code, were Jacob and Monod, the new Paris team.

Monod had realised that for a full understanding of protein synthesis he needed the help of a geneticist, so he contacted a colleague at the Institut Pasteur in Paris, François Jacob. Jacob, a physician who had signed up with de Gaulle’s Free French in 1940 and had been severely wounded after D-Day, joined André Lwoff’s laboratory at the Institut Pasteur in 1950 to study the interactions between bacteriophages and their single-cell hosts.
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Jacob’s skills in bacterial genetics and his wide-ranging philosophical interests formed a perfect complement to Monod’s more biochemically centred approach and his interest in existentialism. The result was an intellectual partnership that rivalled that of Watson and Crick and which in some ways surpassed it in terms of providing a model to young researchers around the world – Crick himself called it ‘the great collaboration’.
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Jacob and Monod’s joint research, which began in 1957, took place in the heart of Paris, in an Institut Pasteur attic laboratory that was nicknamed the
grenier
(loft). Jacob and Élie Wollman had been studying how bacterial mating (‘conjugation’) affected the growth of bacteriophage viruses; with Monod, Jacob now used conjugation to explore the genetic basis of induction in bacteria. They carried out these experiments in late 1957 and early 1958 with one of many US visitors to the Institut, Arthur Pardee – the studies became widely known as the PaJaMo (Pardee, Jacob and Monod), or, more colloquially, Pajama (or even Pyjama), experiments.
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Pardee had arrived in Paris in September 1957, and began studying how one of Jacob and Monod’s bacterial mutants responded to induction.
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Normal bacteria could digest lactose by producing induced β-galactosidase. These cells were called lac+ –
lac
(short for lactose) referred to a region of the bacterial chromosome containing several genes involved in this complex phenomenon, and the ‘+’ indicated that this was the normal, or wild, type. Mutant bacteria – known as
lac
^–
– could not grow on lactose unless they acquired the relevant genes by mating with a lac+ individual. Pardee showed that when the z+ gene, which produced the β-galactosidase enzyme, was transferred into a
lac
^–
individual, it became active within minutes. This implied that there was an immediate chemical signal that passed directly from the introduced gene to the host cell’s protein synthesis system. Over the next year or so, the Paris group became focused on the nature of this mysterious messenger molecule, which they called X.
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The PaJaMo experiments also investigated bacteria that continually produced β-galactosidase in the absence of an external inducer molecule. These were known as constitutive strains because they produced the enzyme as part of their constitution. A single gene called
i
seemed to be involved: i+ bacteria were inducible, whereas
i
^–
individuals were constitutive (the system also involved another gene,
y,
which controlled the action of an enzyme, permease, that allowed lactose into the cell). Things got interesting when the group explored the interaction between the
i
gene and the
z
gene that allowed bacteria to produce β-galactosidase. When Pardee introduced both i+ and z+ genes into bacteria carrying the
i
^–
and
z
^–
forms, the bacteria initially produced high, constitutional, levels of β-galactosidase, showing the action of the
z
⁺
gene. But then something odd happened: β-galactosidase production declined rapidly. The i+ gene seemed to start repressing the activity of the z+ gene.
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To the untutored eye, this complex set of results seems either bewildering or boring, or both. But what Jacob and Monod did with these data – the way in which they interpreted them – altered our understanding of what genes do.

The first step forward in this new view of life came at the beginning of 1958, when Leo Szilárd was visiting Paris as part of a tour of laboratories during which he presented his negative feedback model of protein synthesis.
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Five years earlier, Szilárd had proposed to Monod that a form of negative feedback might explain induction; now he made a mind-twisting suggestion that expressed the idea at an even more complex level. Perhaps, he said, ‘induction could be effected by an anti-repressor rather than repression by an anti-inducer?’
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Szilárd was proposing that induction might work by stopping the action of a molecule (a ‘repressor’) inside the bacterial cell that normally repressed enzyme formation. The effect would be a bit like releasing a brake. This could be understood as an example of two negatives producing a positive or, as Monod put it in his 1965 Nobel Prize lecture, a ‘double bluff’.
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Szilárd had not come up with this idea himself. In April 1957, the bacteriologist Werner Maas had given a lecture in Chicago that Szilárd had attended; Maas, who was studying inducible enzymes, had hypothesised that:

inducers which enhance the formation of an enzyme when added to a growing bacterial culture may perhaps be capable of doing so only because there is a repressor present in the cell, and that the inducer might perhaps do no more than inhibit some enzymes.

Maas later stated that on hearing this, Szilárd ‘jumped on the hypothesis … and became quite excited’.
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Szilárd wanted to publish the idea straight away, but Maas refused, because he had no evidence to back it up. At the same time, Henry Vogel of Yale University submitted an article in which he suggested that a common theoretical framework could understand both induction and negative feedback inhibition of biosynthesis – they involved what Vogel called regulator molecules.
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Although Maas’s idea of a repressor and Vogel’s regulatory framework were both based on interactions between proteins, not genes, the ideas of regulation and repression were in the air.

By the time that Szilárd visited Paris less than a year later, he was clearly convinced that induction might not be a positive effect, but rather a ‘de-repression’. The Paris group were intrigued by this suggestion and they briefly outlined the concept alongside the first publication of the PaJaMo results (in a French journal in May 1958, where it was described as an ‘initially surprising hypothesis’) and again in a lecture by Jacob in June 1958. Even though the team were prepared to go public with the idea, they had made no experimental test of the hypothesis and it was still not certain that there were any general implications beyond the narrow world of bacterial genetics.
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Jacob later recalled that the decisive moment came on a Sunday afternoon late in July 1958. All his colleagues were on holiday, while he remained in Paris with his wife, Lise, preparing for a lecture he had to give in New York on how phage viruses hijack the genetic apparatuses of their host. Unable to work, Jacob went to the cinema with his wife. He could not concentrate on the film, so he closed his eyes and suddenly in ‘a flash’, he realised that the two experiments he had been thinking about – the PaJaMo experiment and his own work with Élie Wollman on phage reproduction – were in fact fundamentally identical. He now understood that they both involved the modulation of gene activity by directly affecting the DNA. Jacob later described his almost mystical experience as he realised the connection between his two problematic experiments:

Same situation. Same result. Same conclusion. In both cases, a gene governs the formation of a cytoplasmic product, of a repressor blocking the expression of other genes and so preventing either the synthesis of the galactosidase or the multiplication of the virus. … Where can the repressor act to stop everything at once? The only simple answer, the only one that does not involve a cascade of complicated hypotheses is: on the DNA itself! … These hypotheses, still rough, still vaguely outlined, poorly formulated, stir within me. Barely have they emerged than I feel invaded by an intense joy, a savage pleasure. A sense of strength as well, of power. As if I had climbed a mountain, attained a summit from which I saw in the distance a vast panorama. I no longer feel mediocre or even mortal. I need air. I need to walk.

It was over a month before Jacob could share his insight with Monod. When the two men finally met up, in September 1958, Monod was initially unconvinced. The link between the two experiments was tenuous, and the idea that the repressor acted directly on DNA seemed outlandish. Genes had previously been seen as pure and abstract entities, solidly placed in the background of the cell, passive repositories of information and nothing more – as Monod later put it, they were thought to be as inaccessible ‘as the material of the galaxies’.
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According to Jacob’s new view, genes were intimately involved in the messy reality of cellular processes. Over weeks and months of increasingly intense argument and repeated cycles of experimentation, Jacob and Monod explored their ideas, testing hypotheses by creating new mutants and predicting how they would behave if the model were correct. Jacob later recalled their discussions ‘moved at top speed, in bursts of brief retorts, like a ping-pong match’. As they scribbled feverishly on the blackboard, the model and its predictions became clearer.
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The language they were using changed, too.

Monod began to describe gene function in terms of information transfer, and he explicitly embraced cybernetic thinking by describing protein synthesis in terms of patterns of control. This shift took its clearest shape in a plan for a book entitled
Essays in Enzyme Cybernetics,
which he began writing with Melvin Cohn of Washington University towards the end of 1958.
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Even though the project was never completed, its very existence was extremely revealing. Ephrussi and Watson’s weak satirical joke that had fooled the editors of
Nature
six years earlier had become a reality: cybernetics was being used to understand biology. This was not a reflection of some fashionable trend; it was a powerful interpretative approach that had very real advantages for understanding biological processes. However, as with information theory, it was the general framework, rather than the precise mathematical detail, that was being employed. For biologists, cybernetics was becoming an analogy, a metaphor, a way of thinking about life in terms of flows of control and information, a way of thinking about how genes worked.