126 MICROSOMAL PARTICLES 



logarithmic-growth phase in this medium. The cells had a generation time 

 of about 50 minutes. 



Cobalt 60 irradiation: Twenty-milliliter samples of bacteria were sealed in 

 culture tubes and placed in a cobalt 60 source which delivered 380,000 r/hour. 

 The temperature in the source was approximately 30° C. After irradiation, 

 the tubes were placed unopened in a 37° C water bath, and allowed to rise to 

 that temperature. 



Cyclotron bombardment : Bacterial cells were spun down in a Sorvall Model 

 SS-1 centrifuge, and the pellet was resuspended at a concentration of 1 X 10 10 

 cells/ml in minimal medium with no glucose added. One-tenth milliliter of 

 this suspension was placed on fine-pore filters (Millipore Filter Corporation) 

 and kept moist with a coarse filter backing containing distilled water. Irradia- 

 tion was performed in air at 0° C [10]. After irradiation, the bacteria were 

 resuspended in 10 ml of minimal medium with no glucose and allowed to 

 come to 37° C. 



After irradiation and temperature equilibration, the bacterial suspension was 

 added to an equal volume of minimal medium containing glucose which was 

 aerated at 37° C. This incubation mixture contained 0.2 |jc of the radioisotope 

 to be studied. L-Methionine-S 35 , 5.5 mc/g, and L-cystine-S 35 , 14.5 mc/g, were 

 obtained from the Abbott Laboratories, Oak Ridge. L-Proline-C 14 , 8.9 mc/mM; 

 L-leucine-C 14 , 7.95 mc/mM; and D-glucose-C 14 , 2.06 Mc/mg, were supplied by 

 the Nuclear-Chicago Corporation. At various times during the incubation, 

 2-ml samples were taken for the "whole cell" and "TCA-insoluble" fractions 

 [2]. The filters on which these fractions were placed were then dried in air 

 and counted under a thin window (less than 150 ug/cm 2 ) Geiger-Miiller coun- 

 ter. Background was about 17 counts per minute. 



In order to provide the variety in distribution of ionization densities as men- 

 tioned in the introduction, cyclotron bombardments were carried out with vari- 

 ous thicknesses of aluminum absorber between the bacteria and the beam. 

 Since the beam has a definite range, the amount of inactivation it produced 

 varied with the absorbers, falling to zero when the range in absorber was ex- 

 ceeded, so that no deuterons hit the bacteria. From these curves the equiva- 

 lent absorption of each bacterial preparation could be measured and thus the 

 effective energy of the deuterons hitting them estimated. Since the density of 

 ionization varies with energy in a known way [5], the appropriate value for 

 each bombardment can be determined. 



RESULTS 



The effect of irradiation with cobalt 60 on the uptake of L-methionine is 

 shown in figure 2. Three sets of curves are presented. The first, on the left, 

 applies to the uptake of unirradiated E. coli. The counts per minute are plotted 

 against time for two samplings: the whole cell, as represented by the upper 

 line, and the fraction insoluble in cold trichloroacetic acid (TCA). The differ- 



