July 29, 1946.

Dear Dr, Roepke,

Dr, Tatum has just shown me a uanuscript of your Letter on your
pab-less mutant. This was of special interest to me because I'd just
finished isolating and testing two different pab-less strains. One,
Y9, is an X-ray mutant in the threonine-leucineless strain 679-680.
it grows very nicely on pab, or #¢¥f methionine + hydrolysed yeast
nucleic cc. Y44 is an ultraviolet mtant in a histidineless strain
(¥39, obtained from B/r different from both K=12 and L-15) which again
grows well on pab, but shows no trace of response to methionine +
yna. I should be very much interested to determine, by syntrophism,
whether your strain is blocked at a step different from these two.
Would you be amenable to sending us a culture?

when I visited your lab last May, I was very mich intrigued by
your mutant ‘switch! (from lysine to thiamin as I remember) Have you
done anything more with this? I am rather more of a geneticist than
a chemist and that sort of problem is of greater concern to me than
say,pab synthesis.

From Dr. Lampen&s letter to ELT about 'syntrophism' I gather that
you have gotten it to work. “lad to hear that. That may mean that the
uracil-citrulline m.ation involves an inhibition as well as or instead
of a simple biochemical block, does it not, from the previous results
you mentioned to me using that mutant?

I woukdappreciate very much reprints of your papers as they appear.

Sincerely yours,

Joshua Lederberg,