PURDUE UNIVERSITY
BIOPHYSICAL LABORATORY
LAFAYETTE. INDIANA

Dee 1, 1955

Dear Joshua,

You probably have gone through the calculation we talked ahout by
now, but anyway - 1. grow cells at level of 100 microcuries/micro-
gram P 2, Coli contains about 1079 micrograms DNA-P 3. therefore
incorporate 10-7 microcuries/cell, or 4.1072 4 osec/cells and
presumably transfer from Hfr to F around 5.1074 dis sec/cell. This
level is too low to count by geiger tube techniquw because of
background which is difficult to reduce below 10 counts/min. Special
techniques can go to still lower backgrounds but not to the level
this experiment demands. The beauty of emulsions is that only "stars"
are counted, i.e. multiple tracks originating from a single point
(the cell), and these are readily distinguished from background. The
level of activity in this experiment is well within the emulsion
range.

Your proposed experiment interests me for two reasons. First. it may
provide a test of the Levinthal et al model for crossing over. If
erossing over does not require the incorporation of the parental _ .\ones
chromosome then the Hfr DNA may be lost“in those eee which form
recombinants. Of course a negative result (retention of DNA) would
not mean anything since the DNA may hang around as nonfunctional
material or be broken down and efficiently reutilized. Second, it
may tell something about the kind of material being transferwed, If
only nuclear DNA is transferred then stars should still form even

if the F cells are disrupted before plating on the emulsion. Small
molecular components (and RNA espescially if RNAase is added) should
not register. You probably have other reasons for thinking about
this experiment.

In your diploids carrying Lp® and Lp® (which are immune to virulent
lamda) do the infected cells still continue to segregate sensitives
after infection? I would guess not. Also Benzer would like to find
out if such diploids can grow his T4ryz . Would you be willing to
test it if he sent the strain? The experiment is quite simple.

I will be off to CSH in a week to finish things with DAve. I hope
we will be in a position to send you a complete story before long.

Please give my regards to Esther,

best wishes,

Alan