372 



RICHARD SCHWEET AND JOHN BISHOP 



with H'-lt'uciiu'. and rabbit roticiilocyto RNA with C"-lcucine. To 

 eliminate posi-ible effects of contaminants of the RNA preparations, the 

 two hibeled RNA's were mixed and incubated in j)arallel experiments 

 with either rabbit reticulocyte or guinea pig liver ribosomes. More 

 H^-leucine than C"-leucine was transferred to guinea pig liver ribo- 

 somes, and more C^^-leucine that H''-leucine to reticulocyte ribosomes. 

 Two explanations of these results are suggested. (/) There exist subtle 

 differences between rabbit reticulocyte and guinea pig liver leucine- 



TABLE IV 



RAniOACTIVITY OK PEPTIDES OF AlPHA ChAIX OK RaBKIT llE.\I()(il-OBIN" '' 



- From Bishop et al (1961). 



* Radioactivity is expressed as the percentage of the total radioactivity in all peptides. 

 denotes less than 0.1%. 



transfer RNA which affect the rate of the transfer reaction without 

 influencing the site to which the leucine is transferred. (2) Different 

 types of transfer RNA, specific for the same amino acid, are present and 

 have different transfer specificities, perhaps transfer to different types 

 of sites. If, for example, two kinds of leucyl-RNA's exist, but in different 

 relative proportions in guinea pig liver and rabbit reticulocytes, the 

 results may be explained if, in addition, different relative frequencies 

 of two corresponding types of template sites are found in the ribosomes 

 of these ti.^sues. 



Specificity in transfer enzymes has been reported. Transfer enzyme 

 fractions of rabbit reticulocytes and E. coll were not interchangeable 



