196- J- D. WATSON 



faster to the 30s and 50s forms when the Mg' level is lowered. It is often 

 convenient 16 when using mammalian ribosomes to add a chelating agent to 

 rapidly break down the Sos ribosomes (homologous to the 70s ribosomes of 

 bacteria) to their 40s and 60s sub-units. Bacterial ribosomes are thus not 

 significantly smaller than mammalian ribosomes. It is merely easier to ob- 

 serve the smaller sub-units in bacterial systems. 



Ribosomal RNA 



Already in 1958 there were several reports 17 that ribosomal RNA from 

 higher organisms sedimented as two distinct components (18s and 28s). We 

 thought that the smaller molecules most likely arose from the smaller sub- 

 unit while the faster sedimenting RNA came from the larger of the ribo- 

 somal sub-units. Experiments of Mr. Kurland 18 quickly confirmed this 

 hunch. The E. cell 30s ribosome was found to contain one RNA chain (16s) 

 with a molecular weight of 5.5 X io 5 . Correspondingly a larger RNA mol- 

 ecule (23s) of mol. wt. 1.1 X io 6 was found in most 50s ribosomes (Fig. 3). 



Ribosome proteins 



Analysis of the protein component revealed a much more complicated pic- 

 ture. In contrast to the small RNA viruses, where the protein coat is con- 

 structed from the regular arrangement of a large number of identical protein 

 molecules, each ribosome most likely contains a large number of different 

 polypeptide chains. At first, our results suggested a simple answer when Drs. 

 Waller and J. I. Harris analysed E. coli ribosomes for their amino terminal 

 groups. Only alanine, methionine, with smaller amounts of serine, were 

 present in significant amounts. This hinted that only several classes of protein 

 molecules were used for ribosomal construction. Further experiments of Dr. 

 Waller 19 , however, suggested the contrary. When ribosomal protein frac- 

 tions were analysed in starch-gel electrophoresis, more than 20 distinct bands 

 were seen. Almost all these proteins migrated towards the anode at pH 7 

 confirming the net basic charge of ribosomal protein 20 . A variety of control 

 experiments suggested that these bands represent distinct polypeptide chains, 

 not merely aggregated states of several fundamental sub-units. Moreover, 

 the band pattern from 30s ribosomes was radically different from that of 

 50s proteins. 



As yet we have no solid proof that each 70s ribosome contains all the 



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