104 THE BIOSYNTHESIS OF PROTEINS 



different molecular species of RNA, which can be separated by several 

 methods. 



The average molecular weight of soluble RNA has been estimated by- 

 different methods to be in the range of 10,000-50,000 (Hoagland, 1958; 

 Zamecnilc et al., 1958; Goldthwait, 1958; Hoagland et al, 1958). The 

 more recent figures being 16,000 (Brown, 1960), 25,000 for E. coli (Tissieres 

 et al., 1959) and 27,000 for yeast (Otaka and Osawa, 1960). These figures 

 can only be rough approximations since the molecular weight determina- 

 tions were made on preparations known to be mixtures of many different 

 molecular species of RNA. They indicate that soluble RNAs are molecules 

 containing of the order of 50-80 nucleotide residues. An end group deter- 

 mination gave an estimate of about 100 residues (Singer and Cantoni, 

 1960; Allen ^^ a/., 1960). 



It was shown, moreover, that the RNA acceptors of proline, leucine and 

 valine all have that same molecular size (Klee and Cantoni, 1960). When 

 crude soluble RNA is loaded with leucine, the maximum amount it can 

 bind corresponds to one leucine residue for about two thousand nucleotides 

 (Goldthwait, 1958; Hecht et al., 1959). Since there are twenty different 

 amino acids, and since the acceptor RNAs are all of the same size, assuming 

 equivalent proportions of each, one can estimate that there is one acceptor 

 site per one hundred or so nucleotides. Other estimates gave one amino 

 acid residue per 70-90 nucleotides (Allen et ah, 1960). This being roughly 

 the size of one soluble RNA molecule, it would seem that soluble RNAs 

 most probably carry one single acceptor site per molecule. 



Soluble nucleic acids differ from ribosomal nucleic acids on several 

 accounts: they are soluble in molar NaCl, whereas the bulk of cellular 

 RNA is precipitated (Smith, 1960), they contain considerable amounts of 

 5-ribosyl uracil nucleotide, which differs from normal uridylic acid by the 

 position in which uracil is bound to the Ci of ribose (Dunn, 1959, 1960; 

 Scannell, et al, 1959; Osawa and Otaka, 1959). Soluble RNA is also 

 adsorbed by charcoal (Brown, 1960) and by Ecteola (Otaka and Osawa, 1960) 

 much more readily than the bulk of RNA. Several precursors of RNA are 

 incorporated much more rapidly into soluble RNA than into microsomal 

 RNAs, but the results of recent investigations indicate that in homogenates 

 adenylic or cytidylic nucleotides can be incorporated at the end of the 

 chains of soluble RNA in a process which does not correspond to the 

 synthesis of new RNA molecules (Heidelberger et al., 1956; Paterson and 

 Lepage, 1957; Edmonds and Abrams, 1957; Canellakis, 1957; Canellakis 

 and Herbert, 1960; Herbert and Canellakis, 1960). It would seem that 

 soluble RNAs are able to bind in succession two cytidylic nucleotides, 

 followed by an adenylic residue which thus comes to occupy a terminal 

 position in the polynucleotide chain. This last adenylic residue is bound 

 through its 5' phosphate group to the 3' position of the last cytidylic 



