370 HICflAUl) SCinVKKT AM) .TOIIX HISIIOP 



fore, that wliik' tlici'c must he some initial association of amino ac_\'l- 

 HNA and ribosouR's, this is very transitory and the annuo acid is 

 transferred from the amino acyl-RNA to an alkali-stahh' foim in the 

 ribosonie veiy rapidly. The RNA is then fice to foi'in amino acyl-RNA 

 once again. 



TABLE II 

 RiBosoMAL C"-Leuci.\e IN Alkali-Labu^e Li.\ka(;e" 



" G. Favclukes and R. Schweet (unpublished data). 



'' DOC-ribosomes (2.8 mg), C'Meuoyl-RNA (0.8 mg), enzyme (3 mg) and GTV 

 (1 /miole) were incubated at 37°C for 3 minutes. 



b. The Role of GTP in the Transfer Reaction. An understanding of 

 the role of GTP is essential to the elucidation of the mechanism of the 

 transfer reaction; but to date little information is available. Nathans 

 et al. (1962) have shown, in the reticulocyte transfer system, that the 

 breakdown of GTP to GMP and pyrophosphate is several orders of 

 magnitude lower in extent than amino acid transfer. This would appear 

 to eliminate the breakdown of GTP to GMP as a significant factor in 

 the transfer process. Webster (1961), using a pea seedling system, has 

 reported a reversible breakdown of GTP accompanying amino acid 

 transfer. Incubation of ribosomes charged with amino acids and soluble 

 RNA resulted in the formation of amino acyl-RNA and GTP. On the 

 other hand, Nathans et al. (1962) have observed a rapid breakdown of 

 GTP to GDP upon mixing E. coli ribosomes and soluble enzyni(>s in 

 the absence of amino acyl-RNA. 



Several pieces of evidence which suggest a role for GTP have come 

 from studies of "release" in the reticulocyte ribosome system (Morris 

 and Schweet, 1961; Morris et al, 1962). In these studies it was shown 

 that ribosomes prelabeled with C^*-leucine could be incubated in a com- 

 plete system with C^ --amino acids and the radioactivity "chased" into 

 soluble hemoglobin. In this system, incorporation of C^--amino acids 

 finishes partially completed chains in the ribosome and the formation of 

 labeled, soluble protein is a measure of these reactions. It has now been 

 found that GTP will also i)roniote release of soluble C^''-protein from 



