August 4, 1952

Dear Hayess

I'll be leaving fairly soon myself for a vacation — it's
usually too hot and damp here in the summer to get much work
done — and doubt whether there will be any further progress
this summer. To answer a couple of your questions:

Virtually all progeny of Hfr x F- are F-,/ both in their
colpatibility reactions, and in their’&%bility to transduce F+.
The F- pregeny, like the parent, can reveivedi+ from other
strains. I suspect that the Hfr factor, like Mal, is regularly
eliminated. This would make sense: the Hfr (imx i.e. F++) seems
to determine the direction of elimination, and may itself be the
controlling site. I am trying to set up experiments involving
lifr//F- diploids (1.e., exceptions that have not eliminated) that
might help to pxsouxhkiax test this idea,

Our experiments on F+ filterability are quite indecisive. Cur
pusitive results claust certainly cependec cn the presence of P+
celis in our"steriie" filtrates. SterLlity tests on Lysates are
very tricky: I am still suspicious as to the possibility of relatively
dornant filter-passing particles, which may regenerate bacteria only
very slowly. In some tests, it has required two or three days for
a "contaminated" filtrate to give turbid growth when added to fresh
broth. In all such experivents, one has to use murkod otrains ag the
source of the F+ agent (e.g/ an ST’ F+) and steam demonstrate the absence
of this marker in the Fetransduced culture. This is obvicusly a sine
qua non of transformation experiments ganerally.

I haven't been able to quite make up my mind which strains to d:velop
further. Some new people are coming in next mcnth, and I expect to get
that program going by then. We have some strains which are ercssable
with each other, and with F- K-12, but which show no signs of an infective
F+ agent. I haven't decided whether it would be more aavantageous to
invest a lot t&mx of effort on such a system, or to choose another F~

strain over which we have some control by importing F+ agents from
K-1 2 s

Sincerely,

Joshua Lederberg