January 16, 1952

Dr. Felix L. Haas
Laboratory of Genetics
University of Texas
Austin, Texas

Dear Dr. Haas:

This letter is in refcrence to the paper by you and your colleagues,
"Wntabions und actagenic agents in oacteria", published in the American
Naturalis$, 84: 261-274, 1950, @specially pp. 268-267. I have long been
interested in the apparent differance between strain K-12 and strain B
with respoct to the phenotypes of Tl-resistant mutanis. In the latter,
the "5/1" (i.e. sensitive to T5) is usually associated with a requirement
for tryptophane, whereas I have not been able to find this with K-12.

In your paper, you repert an asscciaticn between resistance and
auxotreph mutations. My older experleace was not in accord with this,
and cone of my students has more recently attempted to repeat the experi-
ments on a larger scale, using replicasplating for simple and direct
tests of auxotrophy among Tl-resistant mutants. “ith the exception of
two prolinaless colonies out of a very large number (ca 10/), again no
association wae found. Tha two prolineless mitants were readily separable
from the Tl-resistance in recombination tests and undoubtedly represented
random, spontaneous mutante.

Your paper, which mentions this aspect only in passing, did not give
experimental details that might permit e closer comparison of our discrepant
findings. Were the Saataé colonies from a single plating, and from what sort
of mutagenic treatwent, if any? In view of the seeming concordince of their
nutritional pequirenents, might you no$ have picked up a clone that was
coincidentally auxotrophic and resistant? If your analysis indicates to
you that there is indesd a signfficant asaocdation, have you carried out
or considered genetic teste? I would appreciate a clarification of the
details of this published experiment that might permit the possible dis-
crepancy in our findings to be either explained or to be rectified by fur-
ther experiments.

Yours very sincerely,

Joshua Lederberg
Associate Professor of Genetics