164 



MICROSOMAL PARTICLES 



E 

 en 



(a) 



(b) 



CO 



o 

 o 



CO 



12 



12 



16 



16 

 .14 

 .12 



10 



EDTA P H9.o 



12 



(c) 



16 



m 9/cc 



12 16 



(d) 



Fig. 7. Reduced viscosity of particles under dissociating and nondissociating conditions. 

 (a) In 0.025 fx. P0 4 , pH 6.5, with 1 X 10~ 4 M MgCU. (£) In 0.7 M KCI, pH 6.5, 0.02 p. 

 K-P0 4 , 0° C, for 3 hours, (c) In 0.03 M EDTA, pH 6.8, 0.02 /* K-P0 4 , 0° C, for 3 hours. 

 (</) In 0.05 M EDTA, pH 9.0, 0.01 M Tris buffer, 0° C, for 3 hours. 



dissociation products. At infinite dilution these components have sedimenta- 

 tion coefficients of 59 to 61 S, 40 to 43 S, and 25 to 27 S. 



2. All these units contain nucleic acid and probably have RNA/protein ratios 

 similar to that of the 80 S particles. 



3. The mixtures, and perhaps the individual units, of 60, 40, and 25 S com- 

 ponents have frictional coefficients similar to that of the 80 S particles. 



4. Under conditions in which only 60 and 40 S units are formed by dissocia- 

 tion of 80 S particles, the two components occur in an amount ratio of 2.2-2.3 

 to 1 respectively in both schlieren and absorption optics. Treatment with EDTA 

 over a wide range of concentration of both EDTA and particles results in the 

 formation of only 40 and 25 S units. These are present in an amount ratio 

 of 2:1, indicating that the two units in the system are in a stable state. 



On the basis of the above observations, the molecular weights of the 60, 40, 

 and 25 S components can be estimated to be 2.6 to 3.0 X 10 6 , 1.3 to 1.5 X 10 6 , and 

 6.5 to 7.5 X 10\ respectively. The ratios between the molecular weight of the 

 80 S particles and the molecular weights of the successively smaller units are 

 then in the series 6:4:2:1. Chao has reached a similar conclusion for the 60 and 

 40 S components of yeast particles [4|. Reports of Wagman, and of Tissieres 

 and Watson, in this meeting also tend to support this formulation. Both groups 

 find that the 40 S particle from Escherichia coli is spherical, with an RNA/pro- 

 tein ratio close to unity, and a molecular weight of 1.3 to 1.8 million. Since the 

 40 S particle is spherical, the difference between its sedimentation coefficient and 



