July 31, 1943.
Dr. S. C. Rittenberg,
Department of Bacterlology,
University of So. Cal ifornig
Los Angeles 7, Calif.
Dear Dr. Rittenberg, -

You may be interested to hear the disappointang results of our first
moderate.y c.nclusive taste for recombination in a Salmonella strain.
Working with 5-21, a lysogenic, monophasic typhi=mriun received from
P.R. Edwarda, we first igoluted a pair of biochetcal mutants requiring
tryptophane and isoleucine/valine respectiveiy. Then an arabinose—negative
mutation was obtuined in the first, and a galactose-nezative mtation in
the second of the mitants. When plated together on glucose-synthetic agar,
the mixed cultures gave rdse to a very few prototrophs, but ag none of
those showed any recombination of the fermentative characterist&cs (1.¢.,
all were either ir-Galf or Arf@al-) these were undou:tedly reversions of
the biochemical rejuirenentsyg such as could be observed occasionally in
the single cultures. Of courss, we my merely not huve hit upon the right
conditions, but we are owing on to other straths. I hope that ur. Oppen-
heiner has had more eucourageuent.

The coli work, on the other hand, has progressed considerably with the
finding of a heterozygote (delayed reduction) whieh is continually splitting
off all the recombination classes.

Yours sincerely,

Joshua Lederberg
Assistant Professor of Genetics.