Polypeptide Synthesis and RNA 



431 



attachment requiring the presence of gua- 

 nosine triphosphate), much of the final, 

 three-dimensional configuration of the poly- 

 peptide may be attained before its synthesis 

 is completed. 



Since only one specific ribosomal site ex- 

 ists for peptide bond formation, the mes- 

 senger RNA cannot remain in a constant 

 position on a ribosome. Consequently, the 

 messenger RNA template must move over 

 or through the ribosomal surface. The di- 

 ameter of a 70s ribosome is only about 

 230A. Messenger RNAs containing more 

 than 1500 nucleotides have been detected; 

 if the average internucleotide distance is 

 3.4A, this RNA would extend lengthwise 

 more than 5000A. These facts suggest 

 that several ribosomes can utilize the same 

 messenger RNA simultaneously, the poly- 

 peptide that each is making being at differ- 

 ent stages of growth (Figure 33-3). Sup- 

 porting this hypothesis is evidence that in 

 some systems protein synthesis occurs 

 among aggregates of five to eight 70s (or 

 80s) ribosomes. Such protein-synthesizing 

 ribosomal aggregates — polyribosomes or 

 polysomes (ergosomes) — can be seen in 

 electron micrographs. When long messen- 

 ger RNAs are involved, polysomes can con- 

 tain dozens of ribosomes. Ribosomes that 

 have completed a polypeptide synthesis are 

 free to start the process again; each ribo- 

 some is available for protein synthesis sev- 

 eral times. Since ribosomes are found in 

 the nucleus, it is not surprising that some 

 protein synthesis occurs there.- Data ob- 

 tained from in vitro studies suggest an in- 

 termediate stage in protein synthesis in 

 which DNA is joined to functionally com- 

 petent ribosomes by means of mRNA. 

 Such an intermediate stage in vivo would 

 have important implications for the stabili- 

 zation of mRNA and polarity of mRNA 



20 As shown by A. E. Mirsky, V. G. Allfrey, and 

 others. 



attachment to the ribosome, as well as the 

 regulation of protein synthesis.- 1 



The synthesis of all normal bacterial pro- 

 tein ceases within several minutes after the 

 addition of the base analog, 5-fluoro uracil, 

 to the medium in which the bacteria are be- 

 ing cultured. Since this analog is quickly 

 incorporated into the RNA being synthe- 

 sized, the analog is expected to be incor- 

 porated into new messenger RNA; to lead 

 subsequently to the acceptance of incorrect 

 adapter RNAs; and, therefore, to lead the 

 manufacture of defective proteins. This 

 expectation suggests that, in bacteria, fresh 

 messenger RNA is made continuously and 

 that old messenger RNA does not persist 

 very long. As noted earlier, the synthesis 

 of new messenger RNA from DNA is 

 blocked by the addition of the antibiotic 

 actinomycin D. Studies of bacteria treated 

 with actinomycin D show that messenger 

 RNA has a half-life of about two minutes 

 and is available as a template only ten to 

 twenty times. In other organisms, certain 

 messenger RNAs — the one for hemoglobin, 

 for example — persist for a longer period. 

 The mechanism by which messenger RNA 

 is degraded is still unknown. Perhaps ribo- 

 nuclease is involved in this degradation, for 

 much, if not all, of the bacterial cell's RNase 

 is found, in latent form, attached to the 

 30s particle.-- Moreover, where messenger 

 RNA does not appear to be degraded, as 

 in reticulocytes whose mRNA shows little 

 turnover, there is also no evidence for a 

 latent structural RNase on the ribosome. 

 On the other hand, some evidence - 3 indi- 

 cates that polynucleotide phosphorylase de- 

 stroys messenger RNA in microorganisms. 



21 See R. Byrne, J. G. Levin. H. A. Bladen, and 



M. W. Nirenberg (1964). 



-- See M. Tal and D. Elson ( 1961 ), and also I. D. 



Raacke and J. Fiala ( 1964). 



--Obtained by M. Sekiguchi and S. S. Cohen 



(1963): see also W. C. Hymer and E. L. Kuff 



(1964). 



