KEMPNER AND POLLARD 



125 



10* 



I0 3 10 I0 3 



MOLECULAR WEIGHT 



10" 



10' 



Fig. 1. Comparison of accepted molecular weights and those determined by target 

 analysis. We wish to thank W. R. Guild of our laboratory for permission to use this 

 figure. 



same fraction the same analysis cannot hold; instead, the best fit to the data 

 is found for a long, thin, sensitive volume of radius roughly 11 A and length 

 roughly 2.2 microns. Thus the methionine incorporation can very well be identi- 

 fied with a process taking place in a microsomal particle, usually estimated as 

 having a radius of 100 A [8], while the proline incorporation appears to impli- 

 cate a whole chain, probably of nucleic acid, and may mean that the incorpora- 

 tion of proline is dependent on the integrity of a system that binds together 

 several microsomal particles. The experiments forming the basis for these con- 

 clusions can now be described. 



MATERIALS AND METHODS 



Cultures of Escherichia coli B, maintained in this laboratory for a year, were 

 grown with aeration at 37° C in an inorganic salt medium ("Minimal C 

 medium" [9]) containing 5 g of glucose per liter. Aeration was stopped when 

 the bacteria reached a concentration of approximately 5 X 10 s cells/ml as read 

 in a Bausch and Lomb spectrophotometer. This is about the middle of the 



