[962 J. D. W A I SON 



mdan forces to a cavit) in the 50s ribosome. Extensive dialysis against 

 5 to -5 M Mg (which leaves intact 30s and 50s ribosomes) strips the 

 n.is,\nt chains off the 50s ribosomes 50 * 51 . The released polypeptides sediment 

 about 4s and it" the latent rihonuclcasc is not activated, most likely still have 

 terminally bound sKNA. When the Mg 1 ! level is again brought to io~ 2 M 

 main released chains again stick to ribosomes. 



Movement of the Messenger Template over the Ribosomal Surface 



At any given time, each functioning ribosome thus contains only one nas- 

 cent chain. As elongation proceeds, the NH 3 -tcrminal end moves away from 

 the point of peptide bond formation and conceivably may assume much of 

 its final three-dimensional configuration before the terminal amino acids are 

 added to the carboxyl end. The messenger RNA must be so attached that 

 only the correct amino-acyl-sRNA molecules are inserted into position for 

 possible peptide bond formation. This demands formation of specific hy- 

 drogen bonds (base-pairs?) between the messenger template and several 

 (most likelv three) nucleotides along the sRNA molecule. Then, in the pres- 

 ence o( the necessary enzymes, the amino-acyl linkage to the then terminal 

 sRNA breaks and a peptide bond forms with the correctly placed incoming 

 amino-acyl-RNA (Fig. 5). This must create an energetically unfavorable 

 environment for the now free sRNA molecule, causing it to be ejected from 

 the sRNA binding site. The new terminal sRNA then moves into this site 

 completing a cycle of synthesis. It is not known whether the messenger tem- 

 plate remains attached to the newly inserted amino-acyl-sRNA. But if so, 

 the messenger necessarily moves the correct distance over the ribosomal sur- 

 face to place its next group of specific nucleotides in position to correctly 

 select the next amino acid. No matter, however, what the mechanism is, the 

 messenger tape necessarily moves over the ribosome. They cannot remain in 

 static orientation if there is only one specific ribosomal site for peptide bond 

 formation. 



Attachment of Single Messenger RNA Molecules to Several Ribosomes 



Addition of the synthetic messenger poly U to extracts containing pre- 

 dominantly 70s ribosomes creates new active ribosomes which sediment in 



s-128 



