I'm Paul Berg.
I'm a retired professor at Stanford Medical Center.
And I'm going to reminisce a little bit
about how the recombinant DNA idea emerged.
I had been carrying out experiments
with a colleague at Stanford, Charles Yanofsky,
and as part of the experiments we were doing
we were using phage-mediated transduction,
the delivery of genes from one strain into another
using a virus as the carrier
of that new genetic information,
a very important technology
that had been widely used,
and very important in the development of our current notions
of the molecular biology of microbes.
But, at the same time,
I had decided to move from my work in bacteria
to try to learn something about mammalian cells.
It was largely to try and test
whether the ideas that had prevailed
about gene function in microbes
was also applicable to mammalian cells.
And I took a year off and went to work
at the Salk Institute with Renato Dulbecco,
and to work on the tumor viruses.
And I chose the tumor viruses
because they were known to have very small genomes,
and they were known to transform normal cells
into cancer cells.
And that seemed like a nice place to start
because, again, using viruses
that have limited genetic information,
being able to follow the expression of their genes,
seemed more tempting than trying to study
the whole mammalian cell genome.
During the time I was down there,
it was reported that
when SV40, or polyoma, infects mammalian cells,
virus are produced,
but amongst the progeny there are virions
which contain only cellular DNA, and not viral DNA.
And that was very similar
to the bacterial transduction system P1,
which infects E. coli
and comes out as P1 particles,
but containing E. coli segments of DNA,
and it seemed to me interesting to think about the idea of
trying to develop a transduction system
for mammalian cells to facilitate
the genetic modifications of cells,
much as we had done with microbes.
But very quickly it became clear
that a single virus particle
could not contain very much DNA,
in fact the limit is about five kilobases.
And the prospect of being able to find a gene
that was present in the human genome
of three billion basepairs,
and find it in a virus particle
that contained five [kilobases],
seemed pretty small.
So... but I liked the idea that we could actually
introduce new genes into mammalian cells,
and we could do it without a virus particle.
We could actually take the SV40 DNA,
which was known to integrate into the cell's DNA that it infects,
and link up to it some foreign DNA,
anything we wanted to put into the mammalian cell.
And so the first idea was:
could we get two different DNAs,
join them together covalently,
and then use them as a way...
as a transducing agent?
And we had in the lab a plasmid,
lambdadvgal.
It was a small DNA molecule
about ten kb,
which had lambda genes
and appropriate genes that it could replicate in E. coli,
and linked to it were the three bacterial genes
that encode the gal operon...
that encode the three genes
necessary for metabolizing galactose.
And the idea was to take the two DNAs
and join them together.
And we had to develop a way
to join DNA molecules together,
and we employed what was already known as
cohesive ends in the bacteriophage lambda.
The bacteriophage has these ends
which are cohesive,
that is, the two ends can be joined
to each other to form a circle,
or they can be joined to different molecules
that have the same kind of ends.
And so the idea...
could we synthesize synthetic ends
onto the two molecules we wanted to join?
And that was already known how to do it.
There was an enzyme that would polymerize A or T
onto the ends of one molecule,
and A or T on the other,
so you have two molecules,
one which A ends and T ends,
and if you mix them they come together.
And that was the scheme we set out to do,
and that turned out to be pretty straightforward,
I think it was actually done within less than a year,
and so we were able to make the first recombinant DNA,
although it wasn't called that at the time.
The first recombinant DNA molecule
was part SV40 and part lambdadvgal.
And the idea was to be able to
propagate these molecules in E. coli,
and maybe make mutations in the SV40 sequence,
but also to transfect them into mammalian cells
and to test whether we could get expression
of the exogenous genes.
So, that was the first idea
and the first fulfillment of that idea,
and that of course led, ultimately,
to the evolution of the whole recombinant DNA technology,
except that it became much easier
to join DNA molecules together
through cohesive ends created by restriction enzymes.