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Structure of Microsomal Nucleoprotein 

 Particles from Pea Seedlings 1 



PAUL O. P. TS'O 



Division of Biology, California Institute of Technology 



Microsomal nucleoprotein particles from pea seedlings have been isolated 

 and characterized in our laboratories [1|. These particles have an RNA/pro- 

 tein ratio of 4/6 and a molecular weight of 4 to 4.5 X 10 r> , and they appear in 

 the electron microscope as oblate spheroids (fig. 1). The microsomal nucleopro- 

 tein particles of pea seedlings thus seem to be similar to those of yeast [2], liver 

 [3], and perhaps also to those of bacteria as reported in this meeting. 



The problem of the structure of the particles can be approached from at 

 least three lines of inquiry. (1) How are the smaller units of nucleoprotein put 

 together in the 80 S particles? (2) How are the RNA and protein put together 

 in the nucleoprotein? (3) If protein synthesis takes place in the particle, what 

 is the structural relation of the newly synthesized protein to the nucleoprotein 

 that constitutes the particle? The present paper is principally concerned with 

 the first of these questions. 



Two salient features of the approach and of the interpretation should first be 

 noted. Experimentally, every precaution has been taken to assure that the 

 dissociation agents employed do not exert hydrolytic action on covalent bonds. 

 In interpreting the results of dissociation studies, it is assumed that all particles 

 in the preparation have similar gross structure. The simple ultracentrifugal 

 patterns of the dissociated particles, and the reversibility of certain dissociating 

 processes, seem to support this assumption. It is entirely conceivable, however, 

 that though in general features the subunits of all the particles may be very 

 similar, the cohesive strength with which they stick together inside the particle 



1 This work was supported in part by grants Nos. Rg-3977 and Rg-5143 from the 

 National Institutes of Health, United States Public Health Service. 



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