November 20, 1951

Dear Gunny and Roger:

I was somewhat chagrined at your negative experience with the
silica tubes. Perhaps they would have grown out in 46 hours, but
your result conflicts with all my previous experience. I have pre-
pared another set of PF-11 and PF-20, together with nutrient agar
slants. Would you teat the silica tubes as well as the slants for
viability, and let me know the result? If they don't work for you,
I have d duplicate set here for confirmation.

I happened fot to have PF-12 on hand on a ajant, so sent you PF-20
instead. This is a diauxctroph, leuc- trypt- S', one step removed from
PF-12. The S* marker should be convenient to you in standardizing your
selection procedure, as it may be somewhat easise to follow when the
auxotroph:prototroph ratio is low. Nutrient agar with 100 ug/ml scores
at/s® very micely.

Roger: 38. griseus behaved very nicely for production of auxotrophs,
and in what was supposed to be a preliminary experiment to calibrate
the UV survival, and check on the possibility of replica~plating, two
auxotrophs (different?) were obtained. They are being tested now for
difference, heterokaryon formation, etc., and more are on the way. Tech-
nically, this problem is turning out mich easier than I had anticipated.

Nothing more from Uginsky. Her cultures, both S and R, are very hetero-
genefous on EMB agar: what this might have to do with the results, I don't
know.

Sincerely,

Joshua Lederberg