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NATIONAL RESEARCH COUNCIL
CANADA

ATOMIC ENERGY PROJECT

CHALK RIVER. ONT.

November 28, 1950.

Dr. Joshua Lederberg,
Department of Genetics,

The University of Wisconsin,
College of Agriculture,
Madison 6, Wisconsin.

Dear Joshua:

I am extremely glad that your proposed volume of
reprints on microbial genetics is materializing, and will
be pleased to have the two papers of mine which you men-
tioned (Nature 164: 150, 1993; and Genetics 33: hh? 1948)
@ included in it.

Thanks for the news concerning recent developments.
Your results with S?/SS heterozygotes have a bearing on the
interpretation of the delayed expression of radiation induced
mutationse It would seem that there is a delay quite apart
from either "phenotypic delay” or “segregation delay" (since
as you have shown there is no phenotypic delay in the case of
SS mutants, and in the case of colour response mutants not all
are sectored colonies). From the photoreactivation data it
appears that the photoereversible mutagenic effect of uev.
does not produce gene changes until after there has been meta~
bolic activity, and the above considerations would indicate
that this is true for both the photo-yreversible and the photo-
stable mutagenic effects. I am beginning to wonder if induced
mutation is not associated with gene reproduction in the same
way as spontaneous mutation, if the effect of radiation is to
produce chemicals which can react with the genes only at the
critical time. We have evidence that a few sectored colony
mutants arise from what are presumably single irradiated gene
complements (I think I mentioned this to you), and this might
mean either that a small amount of the mutagenic chemicals

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@ NATIONAL RESEARCH COUNCIL

CANADA
ATOMIC ENERGY PROJECT
CHALK RIVER. ONT.
moe
To: Dre Joshua Lederberg November 28, 1950.

persist until the time of the second gene division, or that
in the presence of the mutagenic chemical the first gene
division can occasionally give rise to a mutant and a non-
mutant daughter.

Your crossing experiments with other strains are very

satisfying. It seemed unlikely that Kel2 should be the only
"perfect" bacterium.

© Sincerely,

/

HBN /am Howard B. Newcombe.