394 



RICHARD SCHWEET AND JOHN BISHOP 



fact that tliree U's code I'ur a single amino acid, it is likely that the 

 poly-U messenger acts catalytically, even in the cell-free system. 



The data shown in Table V indicate that poly-U or TMV-RNA stim- 

 ulated ribosomes synthesize protein in similar amounts to the original 

 crude extracts. The somewhat lower values are undoubtedly due in part 

 to the manipulations involved in removing DNA and endogenous mes- 

 senger RNA from the artificial systems before assay. If we assume that 

 ribosomes stimulated by poly-U or other added messenger do not incorpo- 

 rate more amino acids i)cr ribosome than in the crude extract, then these 



TABLE V 



Incorpohatiox of Amino Acids in Cell-P^ree Systems 



" Values are given as m/umoles of amino acid per mg of ribosomal protein. 



data indicate that most of the ribosomes which were making many 

 proteins in the crude extract are now making only polyphenylalanine 

 or a few proteins involved in TMV synthesis. Thus, from the infonnation 

 transfer standpoint there seems to be little ribosome specificity. A point 

 which will be returned to is the fact that reticulocyte ribosomes, which 

 contain a stable messenger RNA, incorporate amounts of amino acids 

 that are similar to the stimulated bacterial cell-free system (Table V). 

 Ribosome specificity has, however, been noted by Novelli et al. 

 (1962). Also, as noted earlier, there is an enzymatic specificity, since 

 transfer enzymes from rabbit reticulocytes and E. coli were not inter- 

 changeable (Nathans et al., 1962). Since it ajipears that transfer and 

 messenger RNA are not species specific, it follows that the enzymatic 

 specificity serves another function, not involving the template (which 



