THE UNIVERSITY OF CHICAGO

CHICAGO 37-ILLINOIS

INSTITUTE OF RADIOBIOLOGY AND BIOPHYSICS

1165 East 57th St,
Chicago 37, Tllinois

June 16, 1948
Dr. J. Lederberg
Department of Genetics

University of Wisconsin
Madison, Wisconsin

Dear Lederberg: peed. tree FOU

This is a sequel/to my letter of the 4th of June. I have been
uneasy about the behaviour of various of your strains towards phage 3, and
have therefore applied a more elaborate way of testing resistance to this
phage. I have not tried Yio, which Novick previously characterized as sen-
sitive, but I tried the original K strain, the 58-161 strain, the threonine-
less strain which you sent us, as well as the strain§ requiring both lucine
and threonine which you sent us. I also tested the prototrophes obtained by
mating the 58-161 strain with W, and by making the F/6 x W/1 cross.

The K12 strain which we obtained from you was most resistant, and
the prototrophes mentioned above were least resistant, The other strains
mentioned were intermediate, Whether the various degrees of apparent sensi-
tivity are due to host range mutants of the viruses, or whether we have to
deal with differences with regard to absorption petitint, phage 3 itself,
I am not able to say as yet, On the basis of what I have seen so far, it is
possible that the degree of sensitivity of the various growth factor defic-
iency strains is determined by the genes which are responsible for these
growth factor deficiencies, And it is further possible that the K12 strain
itself has comparatively recently undergone a mutation to phage resistance
while being subcultured, To sum up, I am quite uncertain at. the moment how

the difference between the prototrophes and the Kyo strain with regard to
Dr. J. Lederberg -2- June 16, 1948

resistance to phage 3 has to be interpreted,

Sincerely yours,

fovLe~ Lee

Leo Szilard