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A path to discovery: the career of Maclyn McCarty

By Joshua Lederberg and Emil C. Gotschlich The Rockefeller University.

Maclyn McCarty, who devoted his life as a physician-scientist to studying infectious disease
organisms, was best known for his part in the monumental discovery that DNA, rather than
protein, made up the chemical nature of a gene. Uncovering the molecular secret of the gene
in question - that for the capsular polysaccharide of pneumococcal bacteria - led the way to
studying heredity not only by genetics but also by chemical means and was the dawn of the
age of molecular biology. McCarty was the youngest and longest surviving member of the
team responsible for this feat - which also included Oswald T. Avery and Colin MacLeod - at
his death on January 2, 2005 of congestive heart failure.

Mac was born in 1911 in South Bend IN, the second of four sons of a branch manager for the
Studebaker Corp. whilst it was still a firm for horse-drawn carriages. In his teens, he set for
himself the goal of becoming a physician-scientist; and, he pursued a successful strategy of
preparing himself for admission to, and early success in, The Johns Hopkins University
Medical School. Priescently, as an undergraduate at Stanford University, he began his studies
in the nascent field of biochemistry, working with James Murray Luck on protein turnover in
the liver. In 1937 he began his clinical training in pediatrics at the Harriet Lane Service at
Johns Hopkins. There Mac developed a special interest in infectious diseases - in particular,
with the antibacterial sulfonamide drugs treatments that were just entering medicine - which
he subsequently pursued by moving to New York University to work with William Tillett. A
National Research Council Fellowship in the medical sciences and an opening in Oswald T.
Avery’s laboratory spurred his move to Rockefeller University in 1941.

At that time, research in the Avery laboratory was focused on the pneumococcal
transformation, the heritable alteration of a pneumococcal strain from a nonvirulent rough
form to a virulent smooth encapsulated form. McCarty’s arrival at The Rockefeller Institute
in September 1941 marked 13 years since the discovery of the Griffith phenomenon, as
pneumococcal transformation was also known. Prior to this discover, the 1920s had been
marked by a medley of disparate observations on pneumococcus that seemed to involve
"exchange of receptors" amongst diverse bacteria grown together in liquid media or exposed
to various kinds of extracts and supernatants. With rare exception, the early workers in this
area were utterly confused about the distinction of genotype and phenotype. No single
experiment was carried forward to confirmation by other observers so that the entire field of
“para agglutination" was in some disrepute.

However in 1928, Fred Griffith, a leader in public health research in Britain, demonstrated
that the conversion of one strain to the other could happen in vivo, in a mouse. Shortly after
the publication of his results they were confirmed in several quarters, including Avery’s lab.
The analysis relied on serotyping: it was known that phenotypic differentiation of
pneumoccocal groups could be diagnosed by their reactions with specific antisera, already
recognized to reflect chemically distinct capsular polysaccharides. Griffith had neither the
resources nor inclination to purify and identify the responsible agent in pneumococcal
extracts that induced the changes of serotype. But, the phenomenon of transformation was at
least vaguely understood to comprise an alteration of what we would now call genetic factors.

Though interrupted, sometimes years at a time, from 1928 onwards these studies were the
centerpiece of the Avery lab agenda. Around 1940, they were activated by Colin MacLeod’s
efforts to purify the chemical agent in question - be it protein, nuclei acid or some other class
of molecule - and demonstrate that it was necessary and sufficient to cause the Griffith
phenomenon. Studies on pneumococcal transformation were grossly burdened by a wide
variety of variables that needed to be controlled to allow quantitative estimation of
transforming activity in extracts undergoing various stages of purification. MacLeod, in work
over a number of years, had resolved several thorny technical issues to render the
experimental system somewhat more reliable as an assay for biological activity. By the time
McCarty arrived at The Rockefeller the Avery team had just about decided that the active
reagant was not a protein. But what then? Could it be a soluble saccharide? RNA? Least
likely, but perhaps DNA? The progress of this research over the next three years is
beautifully described in Mac’s memoir "The Transforming Principle" written in the early
1980s (1).

As purification progressed, they determined that the extracts’ biological activity was not
dependent on RNA or protein by exposure to crystalline RNAse and to proteinase
preparations. Crystalline DNAse was not available until 1948, but biological activity was
rapidly reduced by tissue extracts rich in DNAse. Mac’s arrival was also marked by another
milestone, namely the development of a diphenylamine reagent assay to positively correlate
DNA with biological activity. It gradually became evident that the active material in purified
extracts had astonishingly high potency in micrograms of DNA that could consummate the
pneumococcal transformation in vitro.

McCarty, MacLeod and Avery wrestled with the standard of proof required to claim that they
had accomplished pneumococcal transformation with highly purified DNA from extracts.
After much self-enquiry, they published in the Journal of Experimental Medicine, issue of
Febr.1, 1944 that the active material was indeed DNA, (2) bereft of protein or any other
known polymer (3,4).

The vicissitudes of the acceptance of the concept that "genes are DNA" deserve the scholarly
adumbration they have received (5,6). Suffice it to say the claim was subject to a formidable
but predictable round of organized skepticism. Some would say even worse - that it was
simply ignored - but that is manifestly untrue, at least in the case of the New York research
institutions. The scientific community does not accept major scientific claims with ease, and
in this case, there were challenges associated with research on the pneumococcus, which made
it especially difficult to attract other investigators to pursue this research. To begin with, so
few people had the necessary expertise with this pathogen from a biological perspective - it
was dangerous to work with and at the same time, it was finicky to grow. In order to assay
its virulence, one needed to use mice as a selective filter. Most urgently lacking as
corroboration was the examination of other phenotypic markers besides the capsular
polysaccharide to see how far the findings on the gene for one pneumococcal antigen would
also apply to other metabolic markers of the organism.

However, by 1953, influenced by the enormous impact of Watson/Crick’s bihelical structure
of DNA, the preponderance of opinion had fully accepted the 1944 paper. In fact, formal
proof that DNA encoded genetic material might be said to have been approximated only much
later by the laboratory synthesis of oligonucleotides, and demonstration of their biological
activity as, for example, genes for t-RNA’s or for small DNA viruses. Long before that,
though, most commentators had accepted the untrammeled heuristic value of the proposition
that, indeed, genes were made of DNA.

Meanwhile, a physician-scientist through and through, Mac turned his attention to diseases
promoted by streptococci. So it happened that upon the retirement of Homer Swift in 1946,
Mac was asked to head the laboratory established in 1922 to work on Streptococci and
Rheumatic Fever. This was the scientific home of Rebecca Lancefield, who developed the still
powerful serological classification of streptococci. From innumerable clinical observations,
combined with the Lancefield classification, it was clear that acute rheumatic fever, a severe
sterile inflammatory condition affecting particularly the joints and the heart, was a
complication of group A streptococcal pharyngitis, following it by several weeks. The causal
chain of events still eludes us. Mac attacked this problem both by studying the biology of
group A streptococci and patients with acute rheumatic fever admitted to the Rockefeller
Hospital.

Together with his students and collaborators over the next twenty years, his work moved the
understanding of the organism from a Gram-positive streptococcus with a particular
serological characteristic to one of the best characterized bacterial species. Work on bacterial
cell wall anatomy and chemistry were just beginning. His work led to the isolation of the
streptococcal cell wal] as a structural entity suitable for anatomic inspection by
electronmiscroscopy. Chemical dissection led to characterization of the group A-specific
polysaccharide, the peptidoglycan, and identification of its serological specificity in the
terminal hexosamine. In order to prove this specificity, he first had to identify and purify a
specific enzyme that specifically cleaved hexosamine (a hexosaminidase) from a soil
organism. Treating the polysaccharide with this enzyme abrogated its serological reactivity.
Mac further demonstrated the precise configuration of the hexosamine linkage, by
synthesizing both a- and b-N-acetyl-glucosamine ovalbumin and demonstrating that only the
second reacted with group A antisera. A similar analytical strategy indicated that the
polysaccharide of group C streptococci differed by having a terminal b-N-acety]
galactosamine as the serological determinant.
In parallel, Mac studied patients with rheumatic fever admitted to the Rockefeller Hospital as
well as valuable specimen collections from WWII military outbreaks of the disease. He and
his collaborators found that the antibody responses to several streptococcal antigens were
significantly higher in the group of individuals that progressed to develop acute rheumatic
fever as compared to uncomplicated infection. However, the response to unrelated antigens,
for instance diphtheria toxoid, was not enhanced. He found that group A streptococci
secreted unusually high amounts of DNase and established a test for the detection of
antibodies to this antigen. This led to the discovery that streptococci were able to produce
multiple isozymes with this activity. He purified human C-reactive protein (CRP) by
crystallization, produced a highly specific antiserum and using this much simpler and more
sensitive test found that CRP levels responded more rapidly and reliably than other
inflammatory markers and could serve as the most accurate indicator of rheumatic
inflammatory activity. Measuring CRP levels to detect inflammation is now routine in medical
practice.

In his later years, Mac increasingly served as a statesman of the biomedical sciences. He
served for 14 years as the Physician-in-Chief of the Rockefeller University Hospital, and as a
trusted adviser and Vice President of the University. Outside the university his leadership
was sought by the New York City Health Research Council, the Helen Hay Whitney
Foundation, as a charter member of the Institute of Medicine and by numerous university
visiting boards. For more than 40 years as editor he placed his stamp of excellence and
integrity on the Journal of Experimental Medicine.

Mac’s scientific interests and energy had a counterpoint in his rich personal life. Together
with his vivacious wife Marjorie, Mac had a wide circle of very close friends both in this
country and abroad who cherished his personal warmth, his low key, spare and pragmatic
world view, his wit and his wide-ranging intellect. He loved English literature, the theater,
and concerts of symphonic music. He loved to wander the streets and the museums of the
great cities of the world particularly Paris, New York and London and frequently visited
overseas following his retirement. Moreover, he remained close to his family; the four
brothers, living in different parts of the country, never failed to meet for annual reunions.
Mac lived a full life and he continues to inspire all who knew him.

REFERENCES:

(1) M. McCarty. The Transforming Principle: Discovering thatGenes Are Made of DNA.
1985, W.W.Norton, New York. (252 pages).

(2) O.T. Avery, C. M. MacLeod and M. McCarty. Studies on the chemical nature of the
substance inducing transformation of pneumococcal types. Induction of transformation by a

desoxyribonucleic acid fraction isolated from pneumococcus type III. J. Exp. Med. 79: 137-
158, 1944.

(3) McCarty M, Avery OT Studies on the chemical nature of the substance inducing
transformation of pneumococcal types. 2. Effect of desoxyribonuclease on the biological
activity of the transforming substance. Journal of experimental medicine 83:89-96. 1946
(4) McCarty M, Avery OT. Studies on the chemical nature of the substance inducing
transformation of pneumococcal types. 3. An improved method for the isolation of the
transforming substance and its application to Pneumococcus Types II, TI, and VI. Journal of
experimental medicine 83:97-104. 1946

(5) Amsterdamska, O. | From pneumonia to DNA: the research career of Oswald T. Avery.
Hist Stud Phys Biol Sci. 1993;24(pt 1):1-40

(6) Olby, Robert. The Path to the Double Helix. London: Macmillan 1974

Image: Maclyn McCarty (June 9, 1911 - January 2, 2005).