Novenber 11, 1950.

Or. Ge lL. FPraenxel,
Dept. of Entomology,
University of Tllinois,
Urbena, Illinois.

Dear Dr. Proenkel:

Ag you surmised, we have repeatedly isolated bacterial mrtants which
appear to require imidentified prowth factors in yeast extract. Many of
theee turn cut te requires unusual combinations or critical concentrations
of known factors. However, we have not attempted to analyze all of our
mutants, since we are not primarily interested in nutritional research,
ond some of them may well raquire unique factors.

It would be rather inconvenient to send you "yeast-extract"® mutants as
they turn up. However, we will be glad to test uncharacterizable mutants,
as they oceur, on the coneentrates you mentioned. I can hardly promice
any results, but if any interesting culturee should turn up, we will be
gind to send the to you forthwith.

May I, hovever, make another sucrestion? Current metheds male it feasible
to search for specific mutants, of the kind you are interested in, vrovided
there is com assurance that the proposed growth factor le a -etabolite for
the orgoniem otudied. If wild-type Ee. coli, or other bacterium, containa
By » it should be feasitle to search for By-dependent mutants by the use

of the penicillin method. Techniques are summarized in detail in Volume IIT
(just published) of Methedc in Yedical Research; Dr. Luria in your Bacteriology
Departcent has probably had some direct experience which might be of additional
help. ,

Since our generalized sores of growth factors 4s yeast oxtract ¥ neptone,
it ie rather wilikely that any of our cultures could be B,-dependent unless
your new factor ecow’s in these materials, -

Yours aincerely,

Joshua Lederberg,
Associate Prefessor of Genetics