CARNEGIE INSTITUTION OF WASHINGTON

DEPARTMENT OF GENETICS

COLD SPRING HARBOR, LONG ISLAND. N.Y.

July 17, 1946

Dear Lederberg:

I got your letter and the K-12 stock, which is
now being tested. As for lyophilizing phage, it is possible
to do it from broth. I am, however, convinced that the best
way of oreserving phage is to keep liquid suspensions in screw-
cap bottles or sealed vials in the refrigerator. Not only does
it keep the activity, but also the titer. This is true for all
our phages except T3, which is slowly inactivated.

As for the recombination problem, I agree that
the nutritional recombinations should be used for selection.
I have some comments on your suggestion for studying the sevara-
tion of characters from complex mutations - Nethod l.

A - x B-/n, m ——3 A+Bt+/n, A+B+/m

This method should work provided either we do not use the muta-
tion B/1, which involves tryptovhane requirement, or we find the
conditions needed for growth of B/1l in tryptovhane agar (prob-
ably 500 gamma of tryptovhane/ml.). One vossibdle anvlication
would be to try splitting the common resistance groups.

Method 2. If all recombinations may occur at
random, I doubt that one can rely on failure to isolate a certain
type. As a eheck, I think one should try to test for all vossible
classes of recombinations from

A-/n x B-+-/m

There should be four classes of prototrophes as
regards resistance, but I doubt they can all be found. aAn addi-
tional method is to mix A-/n, m x Be/n, p and obtain A+Bt/n.

The only real use of selecting for phage resistance
seems to be in the detection of reciprocal classes.

A+B-/n/m x A=B+/p/q

could give A+tBt susceptible; one should find in the same culture
an A-B-/n/m/p/@ (unless lethal) detectable by phage resistance.
There are serious difficulties involved in such test, since we
do not have four comoletely independent, bona fide single
resistant mutants.
When do you think you may have the multiple
biochemical mutants? If it were not for the advantage of
using B/r, I would suggest to make some biochemical mutants
from my complex resistant strains, thus saving the serious
trouble of hunting for very rare complex mutants again. If
you want, I can send you four sood complex mutants for this

purpose.

Best regards,
Crea
“ob sea tobel Weis Gaessr te val mt of)

ht, pow on _