146 MUTATION AND PLANT BREEDING 



The experiments presented here were carried out with an 

 appreciation both for the simultaneous study of biochemistry and 

 biology, and for uniformity of the biological material. Various strains 

 of the bacterium Escherichia coli were employed, and in all experi- 

 ments the bacteria used were synchronized as to cell division and 

 development by a cold treatment method (17). 3 In most of the experi- 

 ments the syntheses of RNA, DNA, and protein were followed in the 

 cultures simultaneously with mutation induction and expression. 



Materials and Methods 



Description of bacterial strains and the mutation followed in each 



E. coli strain B. — Originally obtained from Oak Ridge National 

 Laboratory stock of Dr. A. Hollaender. Mutation — aberrant colonial 

 color response on Difco eosin-methylene blue (EMB) agar after 2 days 

 incubation at 37° C. 



E. coli strain WP2. — Originally isolated by Witkin (26) as a 

 tryptophan-requiring mutant of E. coli strain B/r. Mutation — rever- 

 sion of the tryptophan requirement to the non-requiring state. 



E. coli strain 15 T - Me -T.vr- — A triple auxotroph of E. coli strain B/r 

 requiring thymine, methionine, and tyrosine isolated in our labora- 

 tory from thymine-requiring E. coli strain 15 T - following ultraviolet 

 light exposure. Strain 15 T - was obtained from Dr. S. Zamenhoff of 

 Columbia University. Mutation — reversion of tyrosine requirement. 



Description of growth media 



M medium. — A salts-glucose basal growth medium to which 

 various supplements under test, or various metabolic inhibitors, were 

 added. The composition of the medium has previously been described 

 (7). In experiments using auxotrophs appropriate amounts of the 

 required growth factors were added. All preirradiation and post- 

 irradiation incubation was carried out in liquid M medium supple- 

 mented as indicated in the various experiments. 



Agar plating medium 



1. For color mutants. — Difco EMB agar was used as the final 

 plating medium. Survivors and mutations were scored on the same 

 plates. 



3 See References, page 169. 



