December al, 1949.

Dr. L. Cavalli,

Dept. Genetics,
University of Gambridge,
44, Storey's Way,
Cambridge, England.

Dear Cavalli:

I have just received your strains 123 and 123 Lac-. The latter is
almost certainlg not my Lac,~ (of Genetics, 33:617 '48), since your
strain ferments glucose, whéreas none of the Lac,~ I have so far isolated

in K-L2 will do so. Unfortunately, I have not been able to remain in
town long enough to do anything with them, but hope to do some testcrosses
next month.

Some preliminary experiments with UV on diploids show no striking differ-
ences in sensitivity. However, since both the diploids and the haploids are
maltinucleate, and the latter perhaps more so, this may not be pertinent.
At about 10% survival, there is a striking conversion|?— selective killing
is not yet excliaded)] of diploids to haploids. As yet there is little
evidence for diploids stabilized by balanced lethals, which may mean that
the complete medium actually induces segbegation, rather than merely
permitting the growth of auxotrophic segregants. However some multiple
dominant combinations may be occurring, which are rare on a recombination
basis and these may be actually diploid. By diploid and haploid above I
meant only the vy aadepposed to + orv— colony type on FMB medium, and this
may need further consideration.

I have a létter from Lominski requesting coli strains, or rather per-
mission for same. Please feel free to send out these strains generally to
anyone who wants to work with them, although I would like to learn who is
interested. However, for the strains carrying Het , I would prefer a direct
request.This might well be to mutual advantage, as we hope to be developing
improved strains with the passage of time.

The factor 100x for Hfr/Nfr accords with my own "intuitive" conclusions. .
I hope that you will be able to obtain derived Hfr strains from the s™ x Az®
with which you can further test the oppositional character of Hfr. As far
as salts go, Davis and I have found dome discrepancies in frequency of
"plate recombination”, which he believes now to be due to differences in
minimal aedium composition, which he has been evolving for some time. However,
in an experiment I did, no effect of Ng or citratebcould be discernedg
although I am still comvinced that Mg is important.

On the reverse side is the formula for Davis! current minimal medium.
This forma is quite good, giving dense growth without aeration (owing to high
buffer capacity). However, recombinants coming up on this medium often tend to

be mecoid, for reasons I do not well understand. Conceivably, this might have
something @o do with lysogenesis.

Per Liter:
K HPO, 7
KHoPO, 2

Na,citra te .5H,0 05
igSO, «71,0 wl

Autoclave separately

Agar as required ;
Glucose $1 (sic).

The diploids are cytologically distinguishable from haploids, but so
far uninterpretably.

Sincerely,

3. Lederberg