310 B. B. BOBEBTS. R. J. BRITTEN. AND B. J. MCCARTHY 



Robert*, 1960 •. Figure 10 shows the purity of SOS ribosomes prepared 

 in this way. When small quantities of ribosomes are used in ver>' thin 

 layers excellent resolution can be achieved, as shown in Section V,B 

 below. 



C. DENSITY GR.\DIENT B.\NDINC. 



Since DXA. RXA. and ribosomes have different densities they can be 

 separated by prolonged centrifugation in appropriate solutions of high 

 density. The materials forai bands at the levels where their density 

 equals the density of the solution. Materials differing by only a few per 

 cent in density can be well separated since the density of the solution 

 varies only slightly from top to bottom of the centrifuge tube. Density 

 banding was used to detect the DXA-RXA hybrid complex (Hall and 

 Spiegelman, 1961; Spiegelman et al.. 1961) and to show an association 

 between DXA and ribosomes (Xisman, 1961). 



Density banding also provides a unique method of separating newly 

 formed ribosomes from old ones. The cells are first grown in media 

 containing hea\y isotopes (C", X^^. and H-» and then transferred to 

 media containing light isotopes. The ribosomes formed before and after 

 the transfer can then be separated by density' gradient banding. The 

 value of this technique is illustrated in the work of Brenner et al. in their 

 studies of phage-infected cells (Brenner et al.. 1961 ». 



D. ELECTROPHORESIS 



Electrophoresis has been used occasionally in the study of ribosomes. 

 Pardee et al. showed a difference in the electrophoretic pattern caused by 

 growth of the cells in chloramphenicol (Pardee et al., 1957). Electro- 

 phoresis was used to isolate the RX'A formed after phage infection 

 (X'omura et al, 1960 ». 



E. r«'0-PHASE SYSTEMS 



Successful separation of ribosomes from other cellular components by 

 partition in two-phase systems is described by Albertsson (1960i. This 

 method has not been exploited extensively in the study of ribosomes 

 but it gives promise of being extremely useful in the search for complexes 

 of ribosomes with DX'A or for other studies of cellular organization. 



F. CONCLL'SION 



There is still no general technique for distinguishing ribosomes of 

 slightly different composition. Hence there is no method for determining 

 whether there are two or a thousand different types of ribosomes. The 

 only technique applied to date is the specific precipitation of the ribo- 



