Bvenber al, 1949

Dear Lu-

We can afford to wait any reasonable length of time for the

refund.

is}

_Conserning your question, I think you fused two commente:

1. That a threonine—histidineless mitant, obtained via Tz, in
two steps, yields "double~-reversion" prototrophs almost as
frequently ae single reversions. This was not inkK-12, and I
have no good idea of the mechanism, unless by chance a suppressor
of T~ has a like effect on H-~.

2. Pairs of antibacterial agents (e.g. Streptomycin and aside gan
be used to select doubly-resiatant recombinants from a mixture of
comlemantary parents...e.

3? This may be it: Streptomycin resistant coli also resists strepto-
thricin., (Wakeman agrees).

Sincerely,

Joshua Lederberg