May 10, 1954

Dr. P. D. Skaar
Biological Laboratories
Gold Spring Harbor, L.I., N.Y.

Bear Dave:

I hope everything has gone well with Linda and your second-born.

Thanks very much for the succinct summary of your immnogenetic stu les.
I will give 1t closer study at my first opportunity, but so far have been
able only to glamce at it. IT think my earlier belated conclusion still holds—
thet it would have been better to look for crossable strains among materials
that had been serologicaily schematized: it would have put the burden of inter
pretation of cross-resx tions and the like on the schamatists. Anyhow, Aleck
rnstein has found a few apparently fertile isolates in the 0-55, O~111, 026
series, and 1s in the midst of the slow job of manufacturing the auxotrophs
needed for a more complete analysis.

Should we resuscitate the question of publishing 2t least two aspects of
your work here? The material on the cattle serums is certainly interesting,
especially the ingenious application of the twin analyses. This ought to make
a quite satisfactory contribution to the Journal of Inmunology, but I suggest
that you confer directly with Wilmer on this, unless you would prefer that
I medizte. Second, of course, is the production of F- by transfer in motility
agar. As you know, I had done a fairly extensive series myself late last fall ;
fully confirming the application, and the method is now in routine use here.
It has only occasionally failed, usually by failure to elgicit motility. This
question comes up, howewr: I have a distinct recollection that you had recorded
that many of the F- isolates from F+ straina proved to be refractory to reconversion
(back to F+), This has not been our experience more recently, but before we
go into this rather tedious question, I want to be sure of the refrac tory
experiences you have hai. Can you dig these out of your notes? Were any of them
saved (as i~ stocks or otherwise). One possibke source of error might be the
occurrence of strains that have become almost incompatible » but are still F+
by the criterion of their conversion of other testers. Tom tella me that
the stored stock of W-2301 (your 3.26/4, W-1325 H, F~) is now F+, But as I mentdonec
I would like a more complete dissertation from you before tangling with the
details of this. Gg F-'s obtained from Hfr have, of course, proved refractory
(and these, by the way, tell against my suggestion that the conversion consists
merely in establishment of optimal conditions for the survival and expression
of spontaneous F~ variants in a pyrely passive sense), but I had some (not com
f{t6/ pletely convincing evidence that such F- can be tempprarily converted by

mixed culture with F+ (in menages a trois). Boris Rotman did one more
extended chenostat run with 58-161, but unlike the experiment with Novick, this
gave no conversions, so the matter is still wide open. Anyhow, I think that
when some of the minor details are clarified in our own minds, it would be appro-
priate to submit a brief descriptive account as a Note (ca. 500 words) in the
Joprnal of Bacteriology. I have no objection to your unpublishing the same
material in MGB, but am not enthusiastically in favor of it.

Right now, I have been continuing the single cell isolations of Hfr = F-
zygotes, and you maybbe interested in this reeent finding. In a setup involving
motile Hfr x non-motile F- (another line), pairs of conjoined cells are not
difficult to find. So far, of 16 such pairs satisfactorily analysed, 13 have
shown recombinants among the immediate progeny after disjunction (which takes
30 - 150 minutes). [The other three could also have represented zygotes, of course,
which segregated no detected recombinanta]. The recombinants so far have always
coma from the P. exconjugant. The recombination frequency among these cells taken
at random is only about 1% under similar conditions. This looks like quite com
petent evidence for the process of temporary conjugation, with migration of a
gamete nucieus from the lifr to the F~ cell. There has been a suspicious amount of
lethality anong the exconjugants iConceivably, but improbably, incompatibility
is merely a reflection of such lethality]. But there have been encugh cases where
both exconjugants have survived, though it will be difficult to exclude the possi-
bility that one of the parental calls was alrealy effectively divided. There is
really very little more I can say about this at the instant. Tha mode of conjune-
tion is not yet certain, but still consistent with the pictures of fixed ang stained
material I had last year.

Before I forget, let m rewind you of the tajor phint of this letier-- details
on refractory F-.

\ Yours ,
Hd,

at
Joshia Lederberg