424 



< IIAI'TER 33 



WHOLE PARTICLE 







00 oooo 



Size 



2(30s) 



2(50s) 



2170s) « KlOOs) 



Molecular Weight 

 x 10* 



0.85 0.15 1.80 0.15 



2.8 0.2 5.9 1.0 



RNA CONTENT 



Size 



Molecular Weight 

 x 10' 



16s 



0.55 



0.10 



23s 



1.15 



0.20 



FIGURE 33-1. 



Characteristics 

 of E. coli ribosomes. 



50% RNA by weight.) The smaller units 

 aggregate to form the larger ones when 

 Ml: or other divalent cations are added. 

 Mammalian ribosomes behave similarly, al- 

 though the basic particles, 40s and 60s, are 

 somewhat larger than those in E. coli. The 

 mammalian 80s particle (homologous to the 

 E. coli 70s particle ) results from the com- 

 bination of one 40s and one 60s ribosome. 



About 80% of the RNA in a cell is con- 

 tained in ribosomes. (Small amounts of 

 RNA are also reported in mitochondria.) 

 Ribosomal RNA is single-stranded and has 

 a relatively high molecular weight: 0.55 ± 

 0.10 X 10 6 for the 16s RNA component of 

 the 30s particle and 1.15 ± 0.20 X 10° for 

 the 23s RNA in the 50s particle; the number 

 of nucleotides in 16s and 23s RNA is about 

 1000 and 2000. The 23s RNA, however, 

 is not to be considered a dimer of 1 6s RNA 

 because of evidence ' that their genetic de- 

 rivations are different. Moreover, the RNA 

 of ribosomes complexes best with denatured 

 homologous DNA, suggesting that the ribo- 

 somal RNA of different organisms differs in 

 base sequence, if not base content. 



The synthesis of the RNA of certain mam- 

 malian viruses is similar to 18s and 30s 

 ribosomal RNA of mammalian cells in that 



1 See S. A. Yankovsky and S. Spiegelman (1963), 

 and S. Spiegelman (1964). 



both viral RNA replication and ribosomal 

 RNA synthesis are inhibited by the drug 

 puromycin. On the other hand. RNA 

 viruses such as TMV are ribonucleoproteins 

 whose protein portion is composed of a num- 

 ber of identical subunits (Figure 28-2), 

 whereas the 30s ribosome contains ten, prob- 

 ably all different, polypeptide chains with a 

 molecular weight of about 30,000. Clearly, 

 the protein structure of ribosomes is more 

 complicated than that of RNA viruses. 



After radioactive amino acids are injected 

 into the body, tissues which synthesize pro- 

 teins rapidly can be examined at intervals. - 

 When a large dose of labeled amino acid is 

 injected, the ribosomes are labeled almost 

 immediately. When a minute dose of la- 

 beled amino acid is injected it is expected 

 to be used up rapidly in protein synthesis; 

 the label in the ribosome increases quickly 

 at first but then decreases. Finally, the 

 labeled amino acid which moves out of the 

 ribosomes is actually incorporated into pro- 

 tein, for example, hemoglobin. These ex- 

 periments give us clear evidence that ribo- 

 somes are associated with protein synthesis. 

 Since the amino acid sequence in hemoglobin 

 is found to be a primary effect of the func- 



- The following is based on the work of P. C. 

 Zamecnik and co-workers, and of M. Rabinovits 

 and M. E. Olson. 



