RNA IN THE SYNTHESIS OF PROTEINS 



S-RNA 



Growing 

 polypeptide chain 



Messenger RNA 



Fig. 7. Messenger RNA attachment to several ribosomes. (This illustration is schemat- 

 ic since the site of messenger attachment to ribosomes is not known.) 



the 150-200S region 52 . Fixation of a single poly U molecule (mol. wt. = 

 100,000) to a 70s ribosome (mol. wt. = 3 X io 6 ) should not significantly 

 increase ribosomal sedimentation. Nor is it likely that a very large number 

 of poly U molecules have combined with individual ribosomes. In these 

 experiments, the molar ratio of fixed poly U to 70s ribosomes was less than }. 

 Instead, the only plausible explanation involves formation of ribosomal 

 aggregates attached to single poly U molecules. The 300 nucleotides in a 

 poly U molecule of mol. wt. ~ io 5 will have a contour length of about 1000 A 

 if the average internucleotide distance is 3.4 A. Simultaneous attachment is 

 thus possible to groups of 4-8 ribosomes (diameter ~ 200 A) depending 

 upon the way the messenger passes over (through) the ribosomal surface. 

 This estimate agrees well with the average aggregate size suggested by the 

 sedimentation rate of the « active » complexes. Sedimentation of extracts after 

 incorporation reveals most polyphenylalanine attached to the rapidly sed- 

 imenting « active » ribosomes. 



Single messenger molecules thus most likely move simultaneously over 

 the surfaces of several ribosomes, functioning on each as protein templates 

 (Fig. 7). A progression of increasingly long polypeptide chains should be 

 attached to successive ribosomes depending upon the fraction of the mes- 

 senger tape to which they were exposed. When all the messenger has moved 

 across the site of synthesis, some mechanism, perhaps itself triggered by a 



s-129 



