318 



R. B. ROBERTS, R. J. BRITTEN, AM) B. J. MCCARTHY 



:± 

 E 

 O 



CO 

 CM 



d 



- 600 



- 400 



200 



Bottom 



15 20 



Top 



Bottom 



- 800 



- 600 



- 400 



200 



3 

 O 



o 



20 



Top 



Fraction number 



Fig. 14. Sedimentation analysis of four total cell extracts taken from cells given 

 (a) 10-minute, (6) 20-minute, (c) 40-minute, and (d) 55-minute exposures to 

 C"-uracil. Centrifugation 90 minutes at 37,000 rpm. Cell washing and breakage and 

 centrifugation carried out in tris-HCl 0.01 M, pU 7.4, MgCL 0.01 M. 



but the increased centrifugation time in Fig. 15d produces higher 

 resolution. In this instance the front-running peak of radioactivity is 

 clearly resolved from 50S ribosomes and appears to have a sedimentation 

 coefficient of about 43S. 



The resolution of early labeled RNA from the bulk ribosomes sug- 



