14 



MICROSOMAL PARTICLES 



Crude extract 



Centrifugal! y fractionated RNP (40 S) component 



Without buffer-DI 



Using buffer-DI 

 observed initially 



Using buffer-DI 

 observed 4 days later 



Fig. 4. Sedimentation diagrams illustrating the influence of the stabilizing medium 

 (buffer-DI) on the nature of the RNP fraction obtainable from E. coli extracts by differen- 

 tial centrifugation. Diagrams were recorded about 5 minutes after full field strength was 

 reached, 250,00%. 



that it consisted entirely of protein and ribose nucleic acid in a proportion about 

 3 to 1 by weight. Since the material appears to be essentially monodisperse, it 

 is tentatively concluded that the 40 S particles contain protein and nucleic acid 

 in combination as a ribonucleoprotein (RNP) . 



Sedimentation, diffusion, and partial specific volume measurements were 

 carried out with the RNP fraction, the more detailed data being obtained with 

 water as the solvent. The results are summarized in table 1. Although, as is 

 seen in figure 4, very small amounts of more slowly sedimenting material were 

 present in these preparations, they are believed not to cause serious errors in 

 the use of these data to determine particle characteristics. 



TABLE 1. Sedimentation and Diffusion Constants of Centrifugally Fractionated 

 Ribonucleoprotein from Extracts of E. coli 



Concentra- 

 Prepa- tration 



ration Solvent mg/ml 



a 0.1 ionic strength 6.3 

 phosphate buf- 

 fer, pH 7.0 

 b Water 8.0 



6.0 

 2.0 

 



11) 



13, 



•*20, W 



sec 



45.0 



43.6 

 44.4 

 46.0 



46.9 * 



10 7 D 2(V 



cm- sec 



-l 



2.67 



V 20 



0.657 



M 



1,240,000 



f/fo 



.16 



* Value obtained by extrapolating \/s. 



