550 HF
February 21, 1956

Miss Lois Dickinson
Boots Pure Drug Co. Ltd.
Research Department

The Priory

Thurgarton, Nottingham
England

Dear Miss Dickinson:

Dr. Joshua Lederberg has referred to me your recent
letter requesting a phenylalanine auxotroph in E. coli.
I am enclosing K12 mutant 58-278, which we have used
extensively for phenylalanine assays in the course of
isolating prephenic acid (Science 118, 251 (1953) and
119, 774 (1954)). It has the disadvantage of an additional
requirement for biotin, but it is much more stable in its
pNenylalanine requirement than any other phenylalanine requiring
mutant that we have on hand,

eg , Sincerely yours,

Fwd

Bernard D. Davis

ec: Dr. Joshua Lederberg «-’

* Like its K1l2 parent, it is sensitive to inhibition by valine,
but secondary mutants that have lost this sensivivity are
readily available by screening in the presence of valine.