392 MAHLON B. HOAGLAND 



whether some specific transfer enzyme may reside in this fraction is un- 

 known at present. 



Acs 187 has performed an experiment which sheds light on a hitherto 

 puzzling aspect of amino acid incorporation studies. As was mentioned 

 earlier, transfer RNA in the natural state contains amino acids; it is, 

 in essence, a pool of activated amino acids. These workers showed that 

 if this natural complement of amino acids is first stripped from the RNA 

 (by treatment in dilute alkali) and then this RNA is labeled with a single 

 amino acid (threonine-C 14 ), the resulting compound is less effective in 

 donating its amino acid to microsomal protein. This would appear to 

 mean that the other unlabeled amino acids naturally bound to transfer 

 RNA are required for the incorporation of the single labeled amino acid, 

 and most likely are incorporated into protein with it. This conclusion 

 would be supported by the finding described in Section II, 2, d, that 

 all of the transfer RNA ends accompany a single amino acid into the 

 ribosomes during transfer. Amino acid incorporation studies have long 

 embarrassed their practitioners by failing to show a requirement for other 

 free amino acids in the incorporation of a single radioactive one. The 

 explanation may be that there is a sufficient pool of transfer RNA-bound 

 amino acids to account for the small quantity of C 14 -amino acid which 

 enters protein in these in vitro experiments. 



An encouraging advance in the study of protein synthesis has been 

 reported by Webster, 111 as was mentioned in Section II, 1, c. He reports 

 that ribonucleoprotein particles from pea seedlings similar to those char- 

 acterized by Ts'o et a/. 46 will carry out the net synthesis of considerable 

 quantities of soluble protein when supplemented with Mn ++ , ATP, GTP, 

 a nucleoside triphosphate generating system, and a full complement of 

 amino acids. Omission of any one of these components prevents synthesis. 

 The particles possess indigenous amino acid activating activity. The sys- 

 tem is said further to require the addition of a "polynucleotide" fraction, 

 as yet not fully characterized. The unequivocal demonstration that trans- 

 fer RNA is an obligatory requirement of such a highly active protein 

 synthetic system will go far toward removing any lingering doubts about 

 its role. 



This section cannot be concluded without reference to a number of 

 investigations which are claimed not to support the role of transfer RNA 

 in protein synthesis. 



Beljanski and Ochoa 188 " 190 have isolated and purified from Alcaligenes 



187 G. Acs, unpublished data, (1959). 



188 M. Beljanski and S. Ochoa, Proc. Natl. Acad. Sri. 44, 494 (1958). 



189 M. Beljanski and S. Ochoa, Proc. Natl. Acad. Sri. 44, 1157 (1958). 



190 M. Beljanski, Compt. rend. acad. sci. 248, 1446 (1959). 



