VIII. PROTEIN SYNTHESIS AND GENE ACTION 355 



time (see review by Hoagland, 1960). However, the main line of investi- 

 gation which hiter led to the conclusion that the amino acyl-RNA 

 compounds were the important soluble intermediate in protein synthesis 

 was initiated by these studies in Zamecnik's laboratory (Hoagland et al., 

 1957). From these results and those of others came the recognition of a 

 particular type of RNA with a specific function, that is, transfer RNA. 

 Studies of the transfer of amino acids from amino acyl-RNA to ribosomal 

 protein have been reported from a number of laboratories in the past few 

 years and are reviewed in detail below. 



The development of two new systems for studying amino acid incor- 

 poration led to some of the more recent advances. A cell-free system from 

 rabbit reticulocytes (Schweet et al., 19o8b) provided a system where 

 incorporation into a known, soluble protein (hemoglobin) was readily 

 studied. Finally, the bacterial ribosomal preparation (Lamborg and 

 Zamecnik, 1960) made available a system which permitted studies on 

 information transfer in protein synthesis. These studies involving 

 "messenger" RNA are reviewed in detail below. 



A number of other pathways for protein synthesis have been reported 

 and some of these are discussed below. However, in this review the trans- 

 fer RNA to ribosome pathway is emphasized, largely because this is the 

 only system where the intermediate stages have been defined and w^here 

 the role of nucleic acids has been studied. Thus at least a framework, if 

 not the details, of information transfer from gene to protein can be 

 discussed. 



II. Formation of Amino Acyl-RNA Compounds 



This subject has been reviewed recently, either as part of general 

 reviews on protein synthesis (Raacke, 1961; Berg, 1961; Campbell, 1961), 

 or in reviews devoted to this topic specifically (Stulberg and Novelli, 

 1962; Hoagland, 1960). For this reason, only a brief summary of the 

 earlier findings are presented here plus more detailed discussion of recent 

 papers, particularly those relevant to specificity aspects of this reaction. - 



A. AMINO ACID-ACTIVATING ENZYMES 



These enzymes catalyze the following reactions: 



1. Amino acid + ATP + enzyme ^ (amino acyl-Amp-E) + PP 



2. (Amino acyl-AMP-E) + T-RNA ^ amino acyl-RNA + enzyme + AMP 



3. SUM - amino acid + ATP + T-RNA ^ amino acyl-RNA + AMP + PP 



" The abbreviations are : PP, inorganic pyropliospliate ; T-RNA, transfer RNA ; 

 A, adenylic acid; C, cytidylic acid; U, uridylic acid; G, guanylic acid; DOC, 

 deoxycholate; C-terminus, free carboxyl terminus; N-terminus, free amino terminus; 

 TMV, tobacco mosaic virus. 



