18 May 1946.

Dear Francis,

How did the Colloquium go?  Nat that it has gone, perhaps I shall be hearing

exciting news from you about coli reverse-mutations. &ill have no further triple

mutants, but may have soon. Also, what about doubles in the'Coli 8' strain

where susceptibilty to any of the B viruses can be used as the genetic marker?

For various reasons, I am planning to do much of the mutation work in this

strain, but as yet have no mutations.

Are you famil3ar with Lincoln and Cowen's vfork on Fh;ytomonas? They have a

paper in Genetics, 27: &l 1942 ~i:utation of P. ste:~arti3. by X-&.~y irradiation'
.&ich is of cgnsiderable interest. The; worked r:ain:'.y witil morphological

charczcters, but they sws fairly clear cut. There, the incidence of multiple

mutations was much greater than predictable from that of the single mut&ion~,

although the same qualification RELY hold as for the multiple resistances

picked up by Demeec and sane in their study of *mutation to virus resistance:

that these may be pleiotropic effects. Such an argument co$Ld not reasonably

hold for these nutritional reversions. ti tells me that the incidence of

coincidental mutations in the Neurospora series was that expected on the b&&s

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

suci~ 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 a c:eterm&nation of one mutant by plating into minimal,

and later adding to the surUce an excess of, say,threonine. This requires


cohering the mnitial plate u.$t.hra layer of unseeded agar. As for identification

of any colonies that do come up,  Lctose 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 experiments that just bout tie up 'syntrophism+'
There are critical levels of sub&z&es- e.g. in the system Motin-Kethioninless

+ Threonine-Prolzlneless, + excess Uiotin and Threonine, there is a threshold between
.1 and .3 ug/lO ml of methionine that has to be added. The growth response is

all or none. Apparently, the .3 ug of mzthionine alloiv 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 otbzr hand, analyses of wild t:pe filtrates (even PdsEA
                                            4

excess of various precursors as anthranilic a~., glutamic acid) is disappointing,
as only traces (but, di?finite traces) of growth factors are present in the me&urn.

However, a more detailed study, with szm~les taken zt dL':erent tires in growth has

to be undertaken. No evidence yet of rechmbinztion.

Heard from k:cClintock which strains to use for cytology (Chilton # Xmrson)

and Willtry same

'i'hese sre alrentiy

  do other news

in hopes of incorprating factors in stzndsrd isogenic stocks.

at P-3 , so perhaps some time has been wasted.

here; what's new with you?  3egardea to Betty end ML,

            Sincerely yours,
                   .