390 MAHLON B. HOAGLAND 



amino acid to peptide linkage. The C 14 -amino acid, in its transit to protein, 

 did not pass through a free state since the addition of its C 12 -homolog 

 in excess had no effect on the transfer. 117 



A more careful examination of the in vitro system reveals that during 

 the course of incubation the microsomal RNA acquires a small amount 

 of attached amino acid which accumulates rapidly and remains at the 

 same level during the period in which the amino acid is accumulating in 

 protein. This confirms the in vivo observation mentioned above. That this 

 amino acid accumulation in microsomal RNA is significant is supported 

 by the finding that it is stimulated by GTP. 96 ' 184 



We are prompted next to inquire: does the transfer RNA molecule ac- 

 company the amino acid into the ribosome? This question could be an- 

 swered by labeling different parts of the transfer RNA molecule and 

 studying the appearance of label in the RNA subsequently sedimentable 

 as ribosomal RNA. Presumably, however, once the transfer RNA has 

 completed its mission of giving up its amino acid to protein in the ribo- 

 some it would return again to the soluble milieu. Thus, one might expect 

 to find a steady state of labeling of ribosomal RNA by transfer RNA in 

 parallel with the behavior of the amino acid. Preliminary studies are en- 

 couraging in that, they indicate that sRNA labeled by using orotic acid-C 14 

 as a precursor, does in fact become incorporated into microsomal RNA. 14 ' 184 

 This incorporation is increased twofold or more by GTP, an essential 

 criterion for assessing the significance of such a labeling. In more critical 

 time studies, as Fig. 6 shows, sRNA labeled in its terminal adenine moiety 

 rapidly reaches a steady state on the microsomal RNA, as did the amino 

 acid, and this is again GTP dependent. 96 On a molar basis, approximately 

 12 times as much terminal adenine enters the microsomal RNA as does 

 a single amino acid valine (Fig. 6). This ratio agrees well with the ratio 

 of total terminal adenine to valine in the original sRNA used and indi- 

 cates that all of the adenosine ends, with their attached unlabeled amino 

 acids, are accompanying the valine-C 14 into the microsomal RNA. 



What may prove to be the in vivo counterpart of this event has been 

 demonstrated by Lacks and Gros. 124 They have shown that the exposure 

 of whole E. coli cells to adenine-C 14 results in a rapid equilibration of 

 this base with the sRNA. The subsequent addition of nonradioactive 

 adenine results in a decrease in the specific radioactivity of sRNA but 

 a continued increase in radioactivity of particulate RNA. Of course, 

 in this case, it is hard to distinguish phenomena related to RNA synthesis 

 from those related to the terminal group of sRNA. 



It will be most interesting to examine the fate of the other parts of 

 the transfer molecule: in particular, the whole terminal pCpCpA grouping 



'* 4 M. B. Hoagland, Rec. trav. chim. 77, 623 (1958). 



