64 S. S. COHEN 



majority at 80 to 150 m//,. Chantrenne (1947) considered that Claude had 

 fractionated the microsomes in the course of differential centrifugation, 

 thereby limiting the observable range of particle sizes. This worker first 

 described the heterogeneity of size, composition, and enzymatic activity of 

 the microsomal fraction. However, electrolytes rather than sucrose had been 

 employed in this work, and the results of the fractionation were therefore 

 more suspect than most. More recent studies with the sucrose technique 

 appeared to have confirmed the existence of marked chemical and enzymatic 

 heterogeneity within this fraction (Novikoff, 1957; Hogeboom and Schneider, 

 1955). However, the recognition of the existence of a class of labile small 

 mitochondria or lysosomes, which readily liberate their enzymes and thereby 

 contribute to the apparent heterogeneity obtained in the usual fractionation, 

 makes it difficult to be certain of the significance of numerous earher reports. 



The work of Jeener (1952a,b) on the particles of the colorless flagellate, 

 Polytomella coeca, has provided strong evidence of compositional and 

 functional heterogeneity. Petermann and Hamilton (1957) have recently 

 described the ultracentrifugal heterogeneity of the particulate fraction of rat 

 liver after removal of nuclei and mitochondria and have isolated a major 

 ribonucleoprotein constituent, S = 77.5. 



Nevertheless, some studies on bacterial organization have revealed a high 

 order of uniformity among particles whose size would place them within the 

 isolated microsome fraction of animal cells. For instance, electron microscopy 

 of lysing E. coli infected with bacteriophage revealed great numbers of 

 apparently uniform particles of about 150 m/x ia diameter (Luria et al., 1943). 

 Uniform spherical particles of this size (S20 = 40, MW = 10^), have been 

 isolated and characterized by Schachman et al. (1952) who found them to 

 contain, most of the RNA of all bacteria examined (e.g., Pseudomonas 

 fiuorescens, E. coli). In yeast, a similar, somewhat larger structure (S20 = 80) 

 can be found (Chao and Schachman, 1956). The relatively large quantity of 

 such particles in a yeast extract and their apparent homogeneity with respect 

 to sedimentation may be discerned in Fig. 8. The particles are present in the 

 organisms in each stage of development of a synchronized culture of E. coli 

 (Cohen and Earner, unpublished data). However, the S = 40 particles 

 disappear under conditions in which E. coli is starved (Dagley and Sykes, 

 1957). 



Ultracentrifugal analysis of extracts of some plant tissues, e.g., pea epi- 

 cotyl, similarly has revealed a high concentration (ca. 25 % of the acid- 

 precipitable protein) of a major class of ribonucleoprotein particles of 

 S20 = 74 and apparent molecular weight of 4 to 6 X 10'' (Tso et al., 1956). 

 Extracts also contain about 10 % as much of another particle of llOS. On 

 isolation these particles are found to have about 35 % RNA and far less 

 lipid than the microsomal fraction of animal tissues. This chemical evidence 



