kidney cells which have a high physiological activity synthesize very little 

 protein and have a relatively low RNA content. The RNA content of 

 cells varies under different physiological conditions. This variation ap- 

 pears to be reflected in varying capacities for protein synthesis. For 

 example, during development of the reticulocyte or immature mam- 

 malian erythrocyte there is a simultaneous reduction in RNA content 

 and incorporation of labeled amino acids into protein. Likewise, in 

 growing bacterial cultures, especially during the logarithmic phase, the 

 RNA content is proportional to the growth rate and RNA synthesis 

 parallels that of protein. 



Brachet and his group were the first to suggest that specific cytoplas- 

 mic structures may function in protein synthesis. It is now known that 

 the RNA content of the microsomes, like that of the whole cytoplasm, 

 is closely correlated with the cells' capacity for protein synthesis. Fur- 

 thermore, the initial site of incorporation of isotopically labeled amino 

 acids into cytoplasmic protein is generally assumed to be the microsomal 

 fraction, or more specifically, the ribonucleoprotein granule component 

 of this fraction (Figure 3-27). The first step in amino acid incorporation 

 into the ribonucleoprotein granules is considered to be the enzyme- 

 catalyzed activation of amino acids. This reaction leading to formation 

 of an activated amino acid is considered to be as follows: 



AA + ATP + E ^ [AA - AMP] -E + PP 



AMINO ENZVMZ ACTIVATED 



ACID AMINO ACID 



The reaction, which is reversible, involves the natural amino acids 

 (L-forms) and has a specific requirement for ATP. The second step is 

 considered to be the addition of specific nucleotides to end groupings of 

 RNA in the soluble portion of the cytoplasm. The third step involves 

 the binding of the activated amino acid to the RNA of the soluble 



Figure 3-26. (Contin.) 



sists of fragments of the rough-surfaced membrane elements of the endo- 

 plasmic reticulum. Many of the fragments still retain the characteristic, 

 flattened appearance of the cisternae in intact tissue as illustrated in Fig. (a) 

 above. A normally sectioned fragment of the endoplasmic reticulum is shown 

 at n, fragments cut at increasing degrees of obliquity are marked ob, and 

 ob2. Particles attached to membranes (p); ring-shaped profiles (rg); rough- 

 surfaced membranes (mb); matrix or content of membrane fragments or 

 vesicles (c); smooth-surfaced profiles (ss). (From Palade, G. E. and Sieke- 

 vitz, P. 1956. "Liver Microsomes. An Integrated Morphological and Bio- 

 chemical Study," /. Biophys. Biochem. CytoL, 2, Fig. 4, Plate 29, and Fig. 

 10, Plate 32.) 



STRUCTURE AND FUNCTION OF CYTOPLASMIC ORGANELLES / 57 



