Jane 27, 1953

Dear Dr. Belser:

I would not recommend streptothricin-resistance. Many of the mtants

show a variable cross-resistance to streptomycin, and there is not a cleas
single-step development of resistance. However, it might be usable in
Serratia—— in any case you could not immediately extrapolate the expectatiohs
from E, coli. You may be able to get some from Woodruff at Merck. Its
non-availability through commercial channels is ancther disadvantage too.
I wish I could recommend another resistance marker that would be as good as
streptomycin, but I do not khow of any. For an unselected marker, however,
"furadroxyl" (see Szybalski) might serve; sc might Co**, or gid. (Have you
seen Cavalli's paper in Bull. W.H.O. 6: 185-206 19527)

I have been hoping that I might hear from Dr. Treffers or yourself
about the mtability modifier from 58-278. I have not so far.

For your Serratia problem, why do you not use additionaklauxotroph
markers? Historically, these served very well for K-12 (v. Nature, 158:
558, Oct. 19, 1946).

Sincerely,

Joshua Ledérberg

& Q