June 2, 1948.

Prof. P. R. Burkholder,
Osbora Botanical waboratory,
Yale University,

New Haven, Connecticut.

Dear Burkholder,

« Shaking machine nuving finally arrived, I've first gotten around to
a few experinents with some of the 5. subtilis mutunte you were kind enough
to send me. Strains 16 and 164x can be plated very nicaay on minimal agar:
they show little enouch “background” or residual growth that prototroph
colonies can be distinguished very reudily. 164x seeag yulte atuble (to
reversion), and 16 shows only occassional reversions. There seen to be
many mope prototrophs in mixtures of the two, but this may De due entirely
to the increasod syntrophie backeround growth, shich is quite zpparent,
and which would yield a sarser population in which reversions could occur.

At this point (which is about to years ago in full cycle), some adidtional
genetic markerszzre needed. I om asking .bout for subtilis phages that aight
be used to select for resistant mutunts. cn the other haad, additional
nutritionil markers would serve just as well. 4y memory is rather discouraging
on this point, but may I ask whether you had accumulated any maltiple
mutants such as could be used in tests for recombination in ©. subtilis?

The physiological work on lactose-ferienting sutunts of cold that I
referred to during ay visit is going uhead rather slowly due to equipment
delays, which in part accounts for this digression. However, we hive gotten
weal into the produetion of mutants in Salmonella, and hope before tw long

to have conpleted some eritic:il tests for gonetic exchunge in that group.

With best -resards,

Yours sinesrely,

Joshua Lederberg.