Life's Greatest Secret (8 page)

The genetic interpretation of this phenomenon is supported by the fact that once transformation is induced, … both capsule formation and the gene-like substance are reduplicated in the daughter cells.

Nevertheless, he wrote in typically cautious style that proof was still lacking and that all his conclusions were provisional:

If the present studies are confirmed and the biologically active substance isolated in highly purified form as the sodium salt of desoxyribosenucleic acid actually proves to be the transforming principle, as the available evidence now suggests, then nucleic acids of this type must be regarded not merely as structurally important but as functionally active in determining the biochemical activities and specific characteristics of pneumococcal cells.

Shortly afterwards, on 13 May 1943, Avery began writing a letter to his younger brother, Roy, who was professor of microbiology at Vanderbilt University in Nashville, Tennessee. While most people called Oswald Avery ‘Fess’, to Roy – 15 years younger – Avery was simply ‘Brother’.
¹⁶
In the letter, written in spidery handwriting, Fess told Roy about his plans to retire and join him in the South: ‘If this War wasn’t on I tell you frankly I would liquidate my affairs here and start for Nashville this fall’, he wrote. Two weeks later, on the night of 26 May, Avery finally got round to completing what he called ‘a rambling epistle’. In this second part, he broke with the personal and slightly weary tone of the opening section and explained to Roy exactly what he and his group had discovered. Avery first reminded his brother about Griffith’s discovery of transformation and the steps that his laboratory had taken in the 1930s to identify the chemical basis of the phenomenon. Then he allowed an element of triumph to creep into his description of the hard work that was involved, before his habitual caution reasserted itself:

Some job – and full of heartaches and heart breaks. But at last perhaps we have it … the substance is highly reactive and on elementary analysis conforms very closely to the theoretical values of pure desoxyribose nucleic acid (thymus type). Who could have guessed it? … We have isolated highly purified substance of which as little as 0.02 of a microgram is active in inducing transformation … this represents a dilution of 1 part in a hundred million – potent stuff that – and highly specific. This does not leave much room for impurities – but the evidence is not good enough yet.

Avery then explained the implications of his discovery, showing that he fully understood the importance of what he had found:

If we are right, and of course that’s not yet proven, then it means that nucleic acids are not merely structurally important but functionally active substances in determining the biochemical activities and specific characteristics of cells – and that by means of a known chemical substance it is possible to induce predictable and hereditary changes in cells. This is something that has long been the dream of geneticists … Sounds like a virus – may be a gene. … Of course, the problem bristles with implications. It touches the biochemistry of the thymus type of nucleic acids which are known to constitute the major part of the chromosomes but have been thought to be alike regardless of origin and species. It touches genetics, enzyme chemistry, cell metabolism and carbohydrate synthesis etc.

But Avery would not have been Avery had he let his excitement continue, and he concluded with his usual self-deprecation:

It’s lots of fun to blow bubbles – but it’s wiser to prick them yourself before someone else tries to. So there’s the story Roy – right or wrong it’s been good fun and lots of work. … Talk it over with [your colleague] Goodpasture but don’t shout it around – until we’re quite sure or at least as sure as present method permits. It’s hazardous to go off half cocked – and embarrassing to have to retract later. I’m so tired and sleepy I’m afraid I have not made this very clear. But I want you to know – and sure you will see that I cannot well leave this problem until we’ve got convincing evidence. Then I look forward and hope we may all be together – God and the war permitting – and live out our days in peace.

And after signing off ‘with heaps and heaps of love’, Avery added a final postscript:

Good night – it’s long after mid-night and I have a busy day ahead. God bless us, one and all. Sleepy, well and happy –

*

By the autumn of 1943, Avery and McCarty were as certain as they could be that the transforming principle was composed of DNA, but Avery remained concerned that there might be some protein contaminants that were producing the effects. So he asked for advice from the protein chemists John Northrop and Wendell Stanley, and the Rockefeller biochemists Van Slyke and Max Bergmann. They all gave the same, unhelpful answer. There was no magic solution: Avery wanted to prove a negative, to show that his extract was completely free of proteins, and this was impossible. The only thing he could do was get as much evidence as possible, using a variety of techniques, and then publish. Interestingly, McCarty’s notes of their meeting with Bergmann reveal that the senior biochemist had similar doubts about the allegedly uniform nature of DNA to those expressed by Jack Schultz in 1941. According to McCarty’s notes, Bergmann felt that the previous certainties about nucleic acids were beginning to weaken:

In the light of present knowledge, the statement that all nucleic acids are the same regardless of the source from which they are derived is nonsense. If they are large polymeric compounds, there is an endless number of possible combinations all of which would possess the same elementary composition but would differ in chemical structure none the less. Nucleic acids hold too prominent a place in biology to be completely non-specific substances. The lack of evidence of any specificity associated with nucleic acids is only due to the fact that they have not been investigated sufficiently.

For two months Avery and McCarty wrote up their findings for publication, with Avery weighing every word and often using material from his reports to the Rockefeller Institute as his starting point. From the opening sentences, the article showed the context in which the study was carried out, and gave a hint of the implications of the main findings: ‘Biologists have long attempted by chemical means to induce in higher organisms predictable and specific changes which thereafter could be transmitted in series as hereditary characters.’ The results they presented were detailed, and their identification of the transforming principle as DNA was based on several strands of evidence – chemical composition; inactivation of the extract by enzymes or temperatures that affected DNA; no effect of enzymes that digested proteins; absence of immune reactions typical of those produced by proteins; responses to centrifugation, electrophoresis and ultraviolet radiation were all identical to those of DNA. Every result converged on the same conclusion: the transforming principle was composed of DNA.

The discussion section of the article outlined the genetic context of their findings, using similar terms to their Rockefeller Institute report from earlier in the year:

The inducing substance has been likened to a gene, and the capsular antigen which is produced in response to it has been regarded as a gene product.

Furthermore, the potential links of transformation with ‘similar problems in the fields of genetics, virology, and cancer research’ were indicated, clearly outlining the implications of their discovery. And yet, despite the overwhelming evidence, all of which suggested that the transforming principle was made of DNA and that genes might be, too, the final paragraph of the article opened with a phrase that suggested the team were not quite as confident as they ought to have been:

It is, of course, possible that the biological activity of the substance described here is not an inherent property of the nucleic acid but is due to minute amounts of some other substance adsorbed to it or so intimately associated with it as to escape detection.

Despite the fact that this deflating phrase was immediately followed by a set of counter-arguments, the tone tended to undermine the reader’s confidence. Even the final bold sentence introduced doubt where none was needed:

If the results of the present study on the chemical nature of the transforming principle are confirmed, then nucleic acids must be regarded as possessing biological specificity the chemical basis of which is as yet undetermined.

On 1 November, Avery handed the manuscript to his colleague Peyton Rous, who was the editor of the
Journal of Experimental Medicine,
which was published by the Rockefeller Institute. Rous did not send the article out for other scientists to review before publication – that was not the general practice at the time – but instead made some editorial suggestions, including cutting what he considered to be a speculative passage about the role of nucleic acids.
²⁰
And with that the article was accepted.

On 10 December 1943, Avery presented the findings at the regular Friday afternoon Rockefeller Institute staff meeting – the first time in years that he had talked about the work of his group. After Avery spoke there was a warm round of applause, followed by a deafening silence. There were no questions. Eventually Dr Heidelberger of Columbia University rose and emphasised the many years that Avery had been working on the problem. Then he sat down again and another silence ensued. The meeting was then closed. Avery had described one of the most momentous discoveries in the history of science, and no one could think of anything to say.

*

The article appeared in the 1 February 1944 issue of the
Journal of Experimental Medicine,
but Avery’s work was not over. As the concluding part of the paper indicated, Avery and McCarty feared that the evidence would not convince the majority of scientists who thought that genes were made of proteins. Even if all the protein-removing procedures were working at their best, molecules are so tiny that even in the smallest amount of ‘pure’ extract that worked in their system – a mere 0.003 micrograms, or 0.00000003 grams – there could still be millions of protein molecules in the sample, each of which might correspond to a gene. The biochemical and analytical techniques available at the time meant that it was not possible to confidently remove that final portion of protein, or to prove that a sample was completely protein-free. So in 1944 Avery and McCarty tried to attack the problem from the other side by showing that even minute quantities of an enzyme that attacked DNA would stop transformation. Both men began to feel the strain. Avery became increasingly withdrawn and even depressed as he tried to find evidence that would demonstrate the proof of his discoveries, but McCarty was unsympathetic. McCarty later recalled that Avery often had a ‘gloomy outlook’ and an ‘apathetic expression’, concluding with an element of self-criticism: ‘I found it difficult to cope in this situation with the necessary restraint and good humour, and I’m afraid that I was not nearly as patient with Fess as I should have been.’
²¹

In October 1945, Avery and McCarty submitted two more papers to the
Journal of Experimental Medicine.
These contained yet more biochemical evidence showing that the transforming principle was composed of DNA, and extended the finding to other forms of transformation in pneumonia bacteria.
²²
The two articles appeared together in the January 1946 issue of the journal, and robustly addressed the potential criticism that minute amounts of protein were responsible for the effect: ‘There is no evidence in favour of such a hypothesis, and it is supported chiefly by the traditional view that nucleic acids are devoid of biological specificity.’
²³
Avery and McCarty made no claim for how specificity was represented in DNA. They did not use the word ‘code’ or anything like the concept of a code, but they clearly stated that there must be something in DNA that enabled genes to be so varied:

It remains one of the challenging problems for future research to determine what sort of configurational or structural differences can be demonstrated between desoxyribonucleates of separate specificities.

They explained that it was probable that only a small proportion of the DNA they extracted was involved in transforming the bacteria from rough to smooth. There could also be a large number of other DNA molecules that would ‘determine the structure and metabolic activities’ of both forms of pneumococcus. This was a conceptual advance: they were arguing that all the genes possessed by these bacteria were made of DNA.
²⁴

Despite the dislocation in scientific communication caused by the war, the response to Avery’s publications was immediate and positive. In 1944,
Nature
described Avery’s work in glowing terms: ‘The genetic implications of this work are considerable’, wrote one scientist, and three months later another astutely suggested that ‘slight differences in molecular configuration’ of different forms of DNA might explain differences in biological activity, concluding, ‘this in itself must represent an entirely new and highly promising field’.
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In October 1944, the New York Academy of Medicine awarded Avery its Gold Medal. Although this was primarily for his decades of work on pneumococci, the citation, which was printed in the widely read US journal
Science,
referred to his isolation of the transforming principle and concluded, ‘this discovery has very far-reaching implications for the general science of biology’.
²⁶
In 1945, the Royal Society of London followed suit and awarded Avery the Copley Medal, again primarily for his microbiological work but with a powerful recognition of the importance of the 1944 paper: ‘the interest and importance of this work, to chemists and biologists (and perhaps most of all to geneticists) is outstanding’.
²⁷
In November 1945, Hermann Muller gave the prestigious Pilgrim Trust lecture to the Royal Society. His subject was ‘The gene’ and he focused on recent discoveries about its physical nature. One section dealt with ‘possible roles of nucleic acids’ and described the ‘remarkable experimental evidence’ from Avery’s group. ‘If this conclusion is accepted’, said Muller, who was highly sceptical, ‘their finding is revolutionary’.
²⁸
At around the same time, the biochemist Howard Mueller wrote a review in which his enthusiasm was evident. He began by summarising Avery’s findings: