VIII. PROTEIN SYNTHESIS AND GENE ACTION 357 



The discovery of the pCpCpA end group was made by a number 



of laboratories. Enzymes which remove and replace these three nucleo- 

 tides in a very specific way have been studied (see Hoagland, 1960; 

 Raacke, 1961). There is a rapid turnover of the end gioup of T-RNA, 

 but the significance of this is not known. The finding of guanosine di- 

 phosphate in alkaline hydrolyzates seems to have established this as the 

 major terminus at the other end of the molecule (Singer and Cantoni, 

 1960; Zillig et al, 1960; Herbert and Canellakis, 1960). In addition, 

 the base composition of T-RNA is unique in its content of "odd" 

 bases. These odd bases, particularly pseudouridine, have been studied 

 by several groups (see Hoagland, 1960 for earlier references). A very 

 complete study of the composition of the soluble RNA of Escherichia 

 coll has been reported by Dunn et al. (1960). They report the 

 base composition to be (moles per 100 moles): A = 20.3, G = 32.1, 

 C = 28.9, U = 15.0, pseudo-U = 2.1, thymine = 1.1, and 2-methyladen- 

 ine = 0.3. Smaller amounts of other odd bases were also present. Ribo- 

 somal RNA contains very little pseudouridine and also differs in having 

 a lower C and higher U and A content. The soluble RNA from other 

 species also contains high proportions of odd bases, but some differ- 

 ences exist. More recently, Cantoni (1962) has reported similar results 

 with rabbit liver soluble RNA, except that thymine nucleotides were not 

 found, but methylamino C was present. Thymine may be an artifact 

 derived from methyl C during hydrolysis (Dunn et al., 1960). The 

 ability of RNA fractions to form amino acyl-RNA compounds was 

 directly proportional to their pseudouridine content (Osawa, 1960). 

 Further discussion of the relation of base composition to function will 

 be given below in connection with the specificity of T-RNA. 



The nature of the amino acid to RNA linkage has been studied. This 

 linkage is an ester between the carboxyl group of the amino acid and the 

 ribose hydroxyl of the terminal adenosine. It is not known whether the 

 2'- or 3'-hydroxyl of the ribose is the esterified group. This finding was 

 first reported by Zachau et al. (1958) and was based on the isolation of 

 C^'^-leucyl-adenosine after RNase digestion of C"-leucyl-RNA. This 

 result was confinned and extended by Preiss et al. (1959) and Hecht 

 et al. (1959). The essentiality of the ACC tenninus for amino acid 

 attachment was shown by enzymatic studies and by periodate oxidation 

 of the free cis-hydroxyls of the ribose of the terminal adenosine. If an 

 amino acid was attached to the RNA, the ribose was protected from 

 periodate action. This findings led to methods of separating amino acid- 

 specific RNA fractions (see later). 



