276 Raymond Latarjet 



Material and Methods 



1. Bacteria and bacteriophages. Escherichia coli, non- 

 lysogenic strain B, and the phages of the T series, lysogenic 

 strain Kl2, its temperate phage X, and the strain K12S 

 sensitive to X, were used according to the classical techniques 

 for growth and plaque formation. 



2. The transforming agent TP Sr, which confers resistance 

 in Pneumococcus to 2 mg. of streptomycin per ml. without 

 inducing bacteria of intermediate resistance, was chosen. 

 TP of several stocks were used, containing about • 6 mg. of 

 DNA per ml. The techniques for preparation and purification 

 of this nucleic acid, for producing the bacterial transforma- 

 ations, and for quantitative titration of the active agent have 

 already been described in detail (Ephrussi-Taylor and Latarjet, 

 1955). 



3. The X-ray source was my usual molybdenum target tube 

 operating at 37 kv and up to 42 mA. Its radiation, filtered 

 through 0-05 mm. aluminium, delivered up to 1 krad./sec. 

 to the preparation, with an average wave-length of 0-9 A. 

 The samples were irradiated in plexiglas cups containing 0-4 

 ml. spread in a layer which absorbed about 10 per cent of the 

 incident radiation. In some experiments, the cups were 

 placed in vacuum chambers with aluminium windows. 



4. Two organic peroxides have been used: 



(i) Commercial cumene hydroperoxide (Hercules Powder 

 Co.), a viscous liquid which contained 40 per cent of 

 active product (Formula I). Its aqueous solution 

 reached saturation for a concentration by weight of 

 about 10-4. 



CH,— C — CHj 

 O — OH 



Formula I 



