Dear Francis,

How did the Colloquium go? Nowm that it has gone, perhaps I shall be hearing
exciting news from you about coli reverse-mutations. Still have no further triple
mutants, but may have soon. Also, what about doubles in the'Coli B!' strain
where susceptibilty to any of the B viruses can be used as the genetic marker?
For various reasons, I am planning to do mich of the mutation work in this
strein, but as yet have no muitations,

Are you familiar with Lincoln and Gowen's work on Phytomonas? They have a
paper in Genetics, 27: 441 1942 "utation of P, stevartii by X-Ray irradistion!
which is of considerable interest. they worked nainiy with morphological
characters, but they scen fairly clear cut. There, the incidence of multiple
mutations was much greater than predictable from that of the single mutdiongs,
although the same qualification may hold as for the multiple resistances
picked up by Demerec and “ano in their study of mutation to virus resistance?
that these may be pleiotropic effects. Such an argument copld not reasonably
hold for these nutritional reversions, Ed tells me that the incidence of
coincidental mitations in the Neurospora series was that expected on the bakés
of the singles. This stuff is of the very highest importance. If you would
run a single experiment on any material available with results indicating
sucii a phenomenon here, I'd be happier about putting in a fair amount of
effort in getting a series of different multiples; otherwise it would not be
worth the considerable investment. One can determine the number of prototrophs
in the same plate as 4 cetermination of one mutant by plating into minimal,

and later adding to the surfifce an excess of, say,threonine. This recuires
covering the tnitial plate witha layer of unseeded agar. As for identification
of any colonies that do come up, lactose fermentation and Gram stain allow for
some assurance(that one has coli, at any rate.)
The time of my talk has been set tentatively for June 12, Can you make it?
Have some fairly cleancut experirents that just - bout tie up 'syntrophism+!
There are critical levels of substrates- €eg.e in the system Biotin-Methioninless
+ Threonine-Prolineless, + excess Biotin and Threonine, there is a threshold between
ol end .3 ug/l0 ml of methionine that has to be added, The growth response is
all or none. Apparently, the .3 ug of methionine allow enough growth of its homologous
cells that they secrete enough proline, etc., This amount of growth is the least
when grown with
visible turbidity. On the other hand, analyses of wild type filtrates (even, wham
excess of various precursors as anthranilic ac., glutamic acid) is disappointing,
as only traces (hut definite traces) of growth factors are present in the medium,
However, a more detailed study, with samples taken at di. ferent times in growth has
to be undertaken. No evidence yet of recombination.
Heard from lieClintock which strains to use for cytology (Chilton # “merson )
and will try same in hopes of incorporating factors in standard isogenic stocks.
These ere alreauy at F-3 , so perhaps some time has been wasted,
No other news here; what's new with you? “egards to Betty end Lil,

Sincerely yours,