Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Berg, Paul

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stand how normal cells seemed spontaneously to become can-

cerous. He hypothesized that cells turned cancerous because

of some unknown interaction between genes and cellular bio-

chemistry.

In order to study these issues, Berg decided to combine

the DNA of SV40, which was known to cause cancer in some

animals, into the common intestinal bacterium Escherichia

coli. He thought it might be possible to smuggle the SV40

DNA into the bacterium by inserting it into the DNA of a type

of virus, called a bacteriophage, that naturally infects E. coli.

A DNA molecule is composed of subunits called

nucleotides, each containing a sugar, a phosphate group, and

one of four nitrogenous bases. Structurally, DNA resembles a

twisted ladder, or helix. Two long chains of alternating sugar

and phosphate groups twist about each other, forming the sides

of the ladder. A base attaches to each sugar, and hydrogen

bonding between the bases—the rungs of the ladder—con-

nects the two strands. The order or sequence of the bases

determines the genetic code; and because bases match up in a

complementary way, the sequence on one strand determines

the sequence on the other.

Berg began his experiment by cutting the SV40 DNA

into pieces using so-called restriction enzymes, which had

been discovered several years before by other researchers.

These enzymeslet him choose the exact sites to cut each

strand of the double helix. Then, using another type of enzyme

called terminal transferase, he added one base at a time to one

side of the double-stranded molecule. Thus, he formed a chain

that extended out from the double-stranded portion. Berg per-

formed the same biochemical operation on the phage DNA,

except he changed the sequence of bases in the reconstructed

phage DNA so it would be complementary to—and therefore

readily bind to—the reconstructed SV40 section of DNA

extending from the double-stranded portion. Such comple-

mentary extended portions of DNA that bind to each other to

make recombinant DNA molecules are called “sticky ends.”

This new and powerful technique offered the means to

put genes into rapidly multiplying cells, such as bacteria,

which would then use the genes to make the corresponding

protein. In effect, scientists would be able to make enormous

amounts of particular genes they wanted to study, or use sim-

ple organisms like bacteria to grow large amounts of valuable

substances like human growth hormone, antibiotics, and

insulin. Researchers also recognized that genetic engineering,

as the technique was quickly dubbed, could be used to alter

soil bacteria to give them the ability to “fix” nitrogen from the

air, thus reducing the need for artificial fertilizers.

Berg had planned to inject the monkey virus SV40-bac-

teriophage DNA hybrid molecule into E. coli.But he realized

the potential danger of inserting a mammalian tumor gene into

a bacterium that exists universally in the environment. Should

the bacterium acquire and spread to other E. colidangerous,

pathogenic characteristics that threatened humans or other

species, the results might be catastrophic. In his own case, he

feared that adding the tumor-causing SV40 DNA into such a

common bacterium would be equivalent to planting a ticking

cancer time bomb in humans who might subsequently become

infected by altered bacteria that escaped from the lab. Rather

than continue his ground-breaking experiment, Berg voluntar-

ily halted his work at this point, concerned that the tools of

genetic engineering might be leading researchers to perform

extremely dangerous experiments.

In addition to this unusual voluntary deferral of his own

research, Berg led a group of ten of his colleagues from around

the country in composing and signing a letter explaining their

collective concerns. Published in the July 26, 1974, issue of

the journal Science,the letter became known as the “Berg let-

ter.” It listed a series of recommendations supported by the

Committee on Recombinant DNA Molecules Assembly of

Life Sciences (of which Berg was chairman) of the National

Academy of Sciences.

The Berg letter warned, “There is serious concern that

some of these artificial recombinant DNA molecules could

prove biologically hazardous.” It cited as an example the fact

that E. colican exchange genetic material with other types of

bacteria, some of which cause disease in humans. “Thus, new

DNA elements introduced into E. colimight possibly become

widely disseminated among human, bacterial, plant, or animal

populations with unpredictable effects.” The letter also noted

certain recombinant DNA experiments that should not be con-

ducted, such as recombining genes for antibiotic resistanceor

bacterial toxins into bacterial strains that did not at present

carry them; linking all or segments of DNA from cancer-caus-

ing or other animal virusesinto plasmidsor other viral DNAs

that could spread the DNA to other bacteria, animals or

humans, “and thus possibly increase the incidence of cancer or

other disease.”

The letter also called for an international meeting of sci-

entists from around the world “to further discuss appropriate

ways to deal with the potential biohazards of recombinant

DNA molecules.” That meeting was held in Pacific Grove,

California, on February 27, 1975, at Asilomarand brought

together a hundred scientists from sixteen countries. For four

days, Berg and his fellow scientists struggled to find a way to

safely balance the potential hazards and inestimable benefits of

the emerging field of genetic engineering. They agreed to col-

laborate on developing safeguards to prevent genetically engi-

neered organisms designed only for laboratory study from

being able to survive in humans. And they drew up professional

standards to govern research in the new technology, which,

though backed only by the force of moral persuasion, repre-

sented the convictions of many of the leading scientists in the

field. These standards served as a blueprint for subsequent fed-

eral regulations, which were first published by the National

Institutes of Health in June 1976. Today, many of the original

regulations have been relaxed or eliminated, except in the cases

of recombinant organisms that include extensive DNA regions

from very pathogenic organisms. Berg continues to study

genetic recombinants in mammalian cells and gene therapy. He

is also doing research in molecular biologyof HIV–1.

The Nobel Award announcement by the Royal Swedish

Academy of Sciences cited Berg “for his fundamental studies

of the biochemistry of nucleic acids with particular regard to

recombinant DNA.” Berg’s legacy also includes his principled

actions in the name of responsible scientific inquiry.

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