April 4, 1952

Dear Cavalli:

I have interpreted your remarks in your last letter as best I could, and incor-
porated them into the attacked manuscript. This has been submitted to GENETICS. Un-
fortunately, they have received a deluge of manuscripts just in the past two weeRs,
and our contribution is not likely to appear in print before the issue of next
January. I do not duppose, however, that you are likely to find a aore expedibious
response from JCM. Hayes may wish to see this: unfortunately, I do not have another
copy that I can spare now. If it is inconvenient for you, I will arrange to have
a microfilm prepared. I have queried in red a few places for yoursspecial attention.
I hope the authorship arrangement here suits you (as anticipated in my last letter > .
It can be reversed without difficulty if you would prefer our earlier arrangelpent.

To discuaa our expertints:  I have not yet had occasion to reproduce your DNase
experiments, but will do so when rpd have time. Miss Lively has aattf been putting
F+ and F- cells together in close proximity under the microscope, and allowing them
to form mixed microcolonies of varying size before plating out. Remarkably, no trans-
duction has been noticed so far under these conditions. I have not yet done mrdinary
scale experiments on agar to see whether this is the source of the difficulty.
Like yours elf, I have thought that Hfr sight carry a ttboundll F+ agent, especially
because the attenuated F+ culture that no longer behaved as Hfr would transduce
F+. However, prototrophs (and segregant auxotropha) from Hfr x F+ cultures have behaved
all as ordinary F+, while prototropb from Hfr x F- (W-677) have not traquced F+.
I am first testing these prototrophs now by SRP X W-1177 and B&3&%&$ the same F+
to test the F.phenotype of such prototrophs.

In the course of the experimen&&s to test the transduction of F+ from Hfr to W-1177,
f was streaking out the mixed cultures on EMB lactose streptomycin. I noticed a
remarkable number of Lac+ Sr, and have followed this up. The results are somewhat
startling. The best experinents have been as follows: Add 1 ml each of overnight
cultures of Hfr and W-1177 to 10 ml fresh Penassay. Incubate 60-90 minutes. Dilute
to give about 100-200 colonies per plate, and spread on EMB Lac and EMB Lac sm.
Redombinants (Lac+ Sr) occur at a frequency of between 1 and 6 k!! A good proportion
of the Lac+Sr colonies appear as sectored coloniesm and may indeed represent segregating
zygotes, but many of them are already pure Lac+! This is important, for it rules out
the possibility of delayed recombination of a sm-inhibited Hfr sell with a developing
clone of W-1177.  The segregating colonies can be detected with no selection whatever
on EN3 lactose. They tend to be ;rr3E little smaller than the others, and have an unmistakable
variegatad appearance. With one doubtful exception, the variegation does not persist,
except by the expected rate of rediploidization. 1 am just be&inn&&g to test other
markers, but Vl alse segregates. Among Lac+Sr ttselections n, about half were r, half s for
V,!  The same occurs with other F- x Hfr, and probably to a lesser extent with F+ x Hfr.
I have begun some microscopic examinations, so far have seen only clump&g. I have every
confidence in the validity of these experiments, and will be surprised if the next
several months does not per&t a morphological demonstration. I am most pleased at
the opportunity created by our collaboration to continue to check our observations and
ideas with each other, as this phase of the work is bound to have more than the usual
sub jet tive element.
You must have some older data parallel to my own on the occurrence of Mal/Lac recom-

binants in Hfr crosses. Do they$ have anything useful for the present? Also, would you
review the evidence for the high fertility of Hfr x Hfr, and the loss of Hfr after
crossing?                              Sincerely,