ROLE OF NUCLEIC ACIDS 65 



shows unequal distribution of 32p in the RNA of the collected fractions 

 (Roberts et al, 1958). 



Differences in nucleotide composition have been observed between 

 various RNA fractions isolated from a single tissue. For instance, the com- 

 position of RNA is not the same for all the fractions isolated from rat liver 

 homogenate (De Lamirande et al.., 1955 ; Elson et al., 1955). Vincent (1952) 

 showed that RNA of isolated nucleoli of starfish oocytes contains more 

 guanine and less uracil than the average cytoplasmic RNA (see also Harris, 

 1959; Vincent and Baltus, 1960). 



In yeast and in pancreas, soluble RNA contains a 5-ribosyluridine 

 nucleotide which is not found in particle RNA (Davis and Allen, 1957; 

 Kemp and Allen, 1958; Osawa and Otaka, 1959; Yu and Allen, 1959; 

 Cohn, 1959; Scannell et al, 1959; Otaka et al, 1959). Methylated purine 

 are also more abundant in this RNA (Bergquist and Matthews, 1959). 



Differential salt extraction, enzyme or acid hydrolysis (Jeener, 1948; 

 Sacks et al, 1955; Sacks and Samarth, 1956; Martin and Morton, 1956; 

 Osawa et al, 1958) also reveal the presence of RNAs which may differ in 

 the way they are associated with other cell constituents (Martin and 

 Morton, 1956; Brachet, 1959; Bell, 1959; Shigeura and Chargaff, 1960). 



All these results leave no doubt that a living cell contains a population of 

 many different molecular species of RNA which have independent 

 physiological activities, just as a cell contains many protein species. Good 

 methods of fractionation are urgently needed and there is little doubt 

 that progress in the study of the function of RNA will be crippled until 

 suitable methods are found. Attempts at fractionating ribosenucleic acids 

 by physicochemical methods have been made repeatedly (Chantrenne, 

 1945; Bacher and Allen, 1950; Ghuysen, 1950; Delcambe, 1950; Del- 

 cambe and Desreux, 1950; Desreux and Ghuysen, 1951; Ghuysen and 

 Desreux, 1952; Mallette and Lamanna, 1953; Roberts et al, 1958). The 

 fractions thus obtained from yeast RNA differ in molecular weight but not 

 much in composition (Ghuysen and Desreux, 1952; Miura and Suzuki, 

 1956; Miura et al, 1958; Smith, 1960). By countercurrent distribution, 

 RNA of different compositions can be partly separated (Kirby, 1960). 

 Electrophoresis also permits some fractionation (Harris and Davies, 1960). 



The development of methods for isolation of proteins has been facilitated 

 by the fact that many proteins have individual features which make it easy to 

 recognize and to determine them : typical solubility, characteristic colours, 

 specific enzymic activity. Nothing quite comparable is known for RNA. Two 

 recent developments indicate, however, that one might witness important 

 progress in this field in the coming years : the so-called soluble RNAs con- 

 tain abnormal nucleotides and bind amino acids by ester bond, whereas 

 the bulk of RNA does not. It is to be expected that several RNAs will be 

 characterized by their capacity to bind individual amino acids in this 



