302 



U. B. HOHKHTS, H. .1. BRITTKN, AND B. J. MCCARTHY 



optical density) in ribosomes separated by setlinientation. Similar results 

 are shown in Fig. 6 using P*- and C'*-leucine. Thus the ribosomes of 

 E. coli, whether 308, 508, 708. or 1008. all have the same nucleic acid 



4000 - 



3000 - 



in 2000 



c: 



o 



CO 



ro 

 CL 



1000 



Fraction number 



Fig. 6. Sedimentation analysis of a total cell extract from E. coli cells in tris- 

 HCl 0.01 M, pH 7.4, MgCl; 0.01 M, randomly labeled with C'-leucine and P^^Or"' . 

 Centrifugation 90 minutes at 37,000 rpm, 4°C. Note same ratio of protein/nucleic 

 acid in ribosomes of different sedimentation coefficients. Protein indicated by radio- 

 activity of C"-leucine, nucleic acid indicated by P^l 



content of 63% or possibly higher. Notable exceptions to this statement 

 are the particles which accumulate during growth in chloramphenicol 

 and the precursors of the ribosomes (see Sections V-VII). 



C. PROTEIN COMPONENTS 



1. Structural Elements 



The protein and nucleic acid components of ribosomes are readily 

 dissociated either by treatment with 4M urea or by removal of mag- 

 nesium with chelating agents such as disodium cthylenediaminetetra- 

 acetate. Bolton showed the presence of several components separable by 

 chromatography (Bolton, 1958). More extensive studies by Waller and 

 Harris show at least 20 components which are resolved by electrophoresis. 



