134 MICROSOMAL PARTICLES 



reported in extracts of E. coli [13], our work can be interpreted as support for 

 the functional character of these particles. 



If the incorporation of exogenous amino acids into a TCA-insoluble form 

 corresponds to protein synthesis, then the conclusion that at least two different 

 amino acids are incorporated by two different structures within the cell logically 

 leads to a model in which protein synthesis takes place in at least two steps. 

 If the two units found by radiation were simultaneously required and both 

 essential for protein synthesis, then methionine and proline would be expected 

 to give the same radiation targets. The fact that targets of quite different char- 

 acter are found indicates that there must be some difference between the two. 

 It is clearly interesting to continue the studies to see whether groupings of 

 types of target exist. 



Actually, that an ionization in a cellular unit which is probably composed 

 of a dozen or so subunits should destroy its function is surprising. It seems 

 certain that the effect of radiation on one part can precipitate a disruptive effect 

 on the whole. This suggestion has already been made by Billen and Volkin 

 [14]. Our data tend to support their conclusions. 



ACKNOWLEDGMENTS 



We wish to thank Messers P. Hanawalt, P. Schambra, and J. Lowry for 

 assistance in running the cyclotron. We are also grateful to Drs. R. Roberts, 

 D. Cowie, R. Britten, and E. Bolton for advice in rapid filtration procedures 

 as well as for many stimulating discussions. 



SUMMARY 



Ionizing radiations of different character, y rays, deuterons, and a particles, 

 were used to determine radiation targets for the incorporation of methionine, 

 proline, and glucose into the cold-TCA-insoluble fraction of Escherichia coli. 

 Spherical targets were found for methionine (260 A diameter) and glucose 

 (160 A diameter) incorporation. The target for proline incorporation is a long, 

 thin rod, 22 A in diameter and 2.2 u long. That the units associated with 

 methionine and glucose correspond closely in size to microsomal particles found 

 in cell debris is therefore evidence for the functional importance of these 

 structures. 



The size and shape of the proline-associated incorporation target appear to 

 be those of a nucleic acid unit, although it is not possible to distinguish be- 

 tween RNA and DNA by this method. 



The fact that the target determined for glucose incorporation is smaller than 

 that found for the incorporation of the exogenous amino acid methionine indi- 

 cates that glucose is not directly connected with the binding of incorporated 

 amino acids into bacterial protein. 



