Mr. D. A. Hopwood
Botany School
University of Cambridge

Dear Mr. Hopwood:
Thank you for your letter of the 21st.

My first experiments on streptomycetes, to look for genetic
recombination, were staréed in November, 1951. Auxotrophic mtants
were obtained in S. griseus. When mixed, they showed a pronounced
syntrophism, but this was sufficiently abated in some combinations
that discrete prototrophic colonies could be detected at low yields.
However, the spores from the prototrophs segregated almost completely
into the parental types, and I could conclude only tentatively that
heterokaryotic mycelia had formed. At this point, Dr. Cavalli ani I
were just finding out about the compatibility system (F, Hfr, etc.) in

E, coli Sxxmmtand this work was temporarily suspended. These rudimentary findings
were quoted only obliquely and unsatisfactorily in the review you
mentionsd, and in "Perppectives and Horizons in Misrobiology" ‘Rutgers
University Press, S. Waksman, ed., 1955).

Dr. S. G. Bradley came here as a postdoctoral fellow in Sep-
tember 1954 to contime this program. Our efforts to demonstrate
genic recombination (e.g., as stable prototrophs or other non=parental
combinations of auxotrophic ang giver cpenfers) joparxkaen in S. griseus
and S. cyaneus have heen guite ere must be a real dif-
ference (homo- vs, hetero-thallism?) between these spedies and coeli-~-
color. However, we have been able to substantiate the occurrence of
heterokaryons by the recovery of parental genotypes from isolated
single hyphal fragments. Dr. Bradley is continuing his studies largely
in an effort to elucidate the difference between the various species.

We are in the throes of completing a manuscript on the hetero-
karyons, and I will be hapny to send Dr. Whitehead a copy when it is
ready (during the next few weeks, I hope). I am sure that Dr. Bradley
and I will be happy to keep in touch with you on the grounds of com
mon interest; we are planning to communicate with Dr. Sermonti in the
same vein,

Yours sincerely,

Joshua Lederberg
Professor of Genetics