September 22, 1948.

Dr, Martin Kristensen,
Statens Seruninstitut,
Kobenhavn, Danmark,
Dear Dr, Hristensen,

- Thank you fcr your letter of the lst September, and the culture of
S. typhi II which arraived in good condition. I am glad that pou clari-
fied the status of typhi II for me. If you should ever encounter any
examples of "shift mutations" in the sense I used the term, I would be
very grateful to hear from you about it.

As to the simultaneous ucquisition of two or more fermentative capacities:
My work on F. coli K-12, so far unpublished has led me to the conclusion
that the genetic control of fermentative enzymes is highly complem, i.¢.,
that there is no simple 1:1 relationship here beween gene and enzyme.
We have for example a coli mutant which is negative for lactose, maltese
and gluconate, and as far as could be tested withsany organisnf, suffers
a mitation of a single gens. As you might expect, such mitants occasionally
Arevert", and when they so do, could be spoken of as acquéring (rather re~
ac juiring)several fernentutive capacities, in analogy to your cases. As I
am studying lactose ferment:tion most intensively, I should be interested
to hear if you ever encounter such complex "reversions" in which lactose is
implicated.’
Yours sincerely,

Joshua Lederberg
Assistant Professor of Genetics. -