CHEMICAL PATHWAYS 103 



1958), in bacteria (Berg, 1958) and in yeast (Osawa, 1960; Otaka and 

 Osawa, 1960; Monier et al, 1960). Although no systematic survey has been 

 reported, it seems probable that RNAs able to bind activated amino acids 

 will be found everywhere. It is important to realize that activated amino 

 acids cannot be transferred to all kinds of RNA. Ribosomal nucleic acids, 

 which make up around 85 per cent of total cellular RNA, do not accept 

 activated amino acids. It is only from the soluble fraction that adequate 

 acceptor RNAs have been isolated. 



The very first studies on soluble RNA (Hoagland et al, 1957, 1958) 

 already indicated that crude soluble RNA from rat liver can bind several 

 individual amino acids, and that there is no competition between these. 

 When the RNA had been saturated first with a given amino acid, it could 

 still bind all the other amino acids. This suggested that crude soluble RNA 

 contained independent specific acceptor sites for each kind of amino acid. 

 Similar observations were made for soluble RNA from E. coli (Berg and 

 Ofengand, 1958; Nisman, 1959) and from reticulocytes (Schweet et al., 

 1958). A similar system was found in plant cells (Webster, 1959). An im- 

 portant question was to find out whether there was one molecular species 

 of RNA able to bind the individual amino acids at specific sites distributed 

 along its molecule, or whether there were several molecular species of RNA, 

 each specific for one single type of amino acids. Fractionation of nucleic 

 acids is not an easy matter. However, Goldthwait (1958) using an Ecteola 

 column. Smith et al. (1959) with a cation starch exchanger, Holley et al. 

 (1959) by means of counter-current distribution, Lipmann, et al. (1959) 

 with column electrophoresis, all achieved some degree of resolution of 

 soluble RNA. When the fractionation was applied to a crude soluble RNA, 

 which had been loaded with various labelled amino acids, a definite al- 

 though only partial separation of fractions binding difl:'erent amino acids 

 was obtained. Thus Holley et al. (1959) quite clearly separated a RNA 

 fraction labelled with i^C alanine from another which was loaded with I'^C 

 leucine. The acceptor RNA for leucine was also partly separated from the 

 acceptor for threonine (Lipmann et ah, 1959) and from that for tyrosine 

 (Smith et al., 1959). Brown (1960) has succeeded in separating almost com- 

 pletely the acceptor RNAs for histidine and tyrosine from all the others, by 

 coupling chemically to polydiazostyrene the histidine and tyrosine bound 

 to RNA. The other amino acids do not react with the resin under the condi- 

 tions used; consequently, only histidine and tyrosine with their attached 

 specific RNA are retained with the polymer. Mild alkali treatment splits 

 the amino acid-RNA bond and releases the specific acceptor RNAs. 



Zamecnik et al. (1960) have recently developed a chemical method which 

 is, in principle, applicable to the isolation of all the individual acceptor 

 RNAs. 



The specific acceptors of the individual amino acids thus belong to 



H 



