Apull
MewetT-8 1916.
Dear Francis,

I'm sending you '679-680-Y9' which is a triple mttant, requiring leucine,
threonine and methionine. I don't know about the specificity of the lastnamed
requirement in re choline or homocysteine, etc.It should be useful as a marker
however, I would suggest re-isolating it from a single colony before using
using it; I can't give you any information on the quantitative requirements.

It was obtained by the new mutant detection techniques from 679=680 irradiated
with 65,000 roentgens in the culture medium in a small Petri plate. I may

have another triple soon, but am in no hurry about that now that this one has been
identified,

In trping to find stable mutations for genetic studies, I made rough tests
of the mustard induced mitants, to find that they are grossly not different from
the X-Ray ones,some backmtating more than others, No further study is indicated,
but the conclusion must be drawn that backmtation must involve some rather
unusual chemical events; if the mstard produces mitation by introducing an ethyl
group into the gene, backmitation is presumably a reversal of this process,a
disruption of what are ordinarily very stable chemical bonds. The premises may not
be correct, however.

In re growth tube paper, I have no copy of it, unless perchance the one I
sent back was given to m; I have no recollection of when I borrowed it. If you
think you can ppagw one, I would appreciate it, but it is not vital. However please
return a reprint of Roepke's paper which I have loaned you, and forgot about.

We are trying UV for mitations now, and the results look encouraging. Sex,
however will be the main problem for a while; Ed has some very encouraging data.

The plate method for identifying mutants (aumanography) is working quite well,

but any skggestions on how to cut down diffusion through agar would be appreciated.
*2e

In the course of other things, I{ve noted that underlaying a pour plate
with another thin layer of agar does help counting, as those troublesome
flat things at the glass interface are pretty much eliminated. Colony shape
seems to be due to stresses in the agar, if the shhpe of gas bubbles is any

criterion; one of the girls is working on that simple, Mat zke-an project

Enclosed are the Stanford comments; alse some thoughts of mybown on the

implications of coincidental mutations. Please let us know what's cooking
in Schermerhorn,

I received word from the Navy Dept. the other day that I am a free man

as far as my plans are concerned; they are ao longer concerned that I remain
in medical school, but you know my sentiments about that already,

Sincerely yours,

Joshua Ledegberg.