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20
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September 15, 1952

Dear Jim:

Esther and I were delighted to hear from you. Just what are you
up to these days?

The reprint you asked for, and some you didn't, are in the mail.

Your letter raises the question of the F+ agent = a temperate virus.
This seems purely a matter of definition, of which one can make an
arbitrary decision. Curiously, I wrote a review called Cell Genetics
and Hereditary Symbiosis (in press in Physiol. Revs.) which belabors
this point, before we stzated running seriously into the question
in our experiments. The main points that seem to be established
are 1) that F+ is not lambda and 2) that the F+ agent is not the actual
carrier of hereditary’ material, as Hayes originally suggested. This
is important because we do have systems of genetic exchange, viz.
transduction in Salmonella in which phage particles do act as gene
vehicles for the host bacteria. In the transduction system, however,
we have no difficulty securing the phage in cell-free filtrates, and
apparently only single factors (1.6. very small chromosome fragments)
are carried over on any occasion. In the K-12 crosses, the occasional
complete diploids show that the entire genotype of each parent may
participate, although I am still somewhat embarrassed by the rather
regular elimination of two factors, Mal, S, out of the many that have
been tested.

Unless great care is taken to avoid premature generalizations, there
is going to be an unholy mess. Larry Morse here has picked up what seems
to be a bon afide transduction in E. coli K-12, via lambda. This is quite
different from the "sexual" system because 1) the agent is filtrable,i.e.
lambda; 2) the transduced changes are usually unstable, and 3) so far pnly
one marker gene has been able to work, which is the main reason it was
not picked up before this. Obviously we are quite pleased to have this
system, for we can apply the more powerful techniques of genetic analysis
provided by "sexual" recombination for this special probken,

I am going to be very mich interested in whether genetic transductions
turn out to be a fairly general attribute of viruses, in situations where
the latter can infect without destroying the new host. It will certainly
raise some thorny questions in the delineation of the host genotype from
that of the virus. The story is not too unexpected except for the last
stage, how doessthe transduced fragment reenter the genotype of the
new host? For a possible answer to this, I am looking to the examples
of apparent crossing-over in pseudo-alleles which in some instances
(A locus in corn; Q in Neurospora; Lac, in K-12) involves a genetic
exchange between chromosomes by either exceptional double—crossovers
or some new mechanism altogether.

Sincerely,

Joshua Liderberg