68 



MICROSOMAL PARTICLES 



(a) (b) (c) 



Fig. 4. Patterns, after 16 minutes' centrifuging at 187,000^, for extracts from cells grown 

 in a glucose mineral salts medium and then resuspended in solutions containing 1 g 

 (NH 4 ) 2 S0 4 per liter of 0.13 M phosphate buffer, pH 7, with no further addition (a); 

 addition of 0.01 M glucose (b); addition of 0.2 g MgS0 4 '7H 2 per liter (c). Cells were 

 incubated for 90 minutes in the same volume of solution as that from which they were 

 harvested. 



preparation was increased, the proportion of 40 S relative to the other compo- 

 nents was reduced. The patterns shown in figure 5 suggest a split of 40 S 

 macromolecules into 29 and 20 S on dilution, favored by high phosphate con- 

 centration. Thus, no 40 S boundary was visible in 0.2 M phosphate: the lead- 

 ing peaks in the extract which contained 7.5 mg protein/ml sedimented at 

 measured (uncorrected) speeds of 27 and 20 S respectively; and from their in- 



w 



(*) 



Fig. 5. Patterns, after 16 minutes' centrifuging at 187,000g-, for extracts prepared in 

 three strengths of phosphate buffer. Extract concentrations were: 10 mg protein/ml (a); 

 7.5 mg protein/ml (b). Sedimentation to the left; in 0.066 M and 0.04 M phosphate, 

 boundaries of 40, 29, 20, and 8 S are visible. The three photographs of either scries were 

 taken at the same schlieren angle; but an adjustment was made between series to provide 

 comparable areas for the 8 S peak. 



