64 THE BIOSYNTHESIS OF PROTEINS 



homogenate was fractionated by centrifugation into nuclear, mitochon- 

 drial and microsomal fractions, and a supernatant. All these fractions con- 

 tain RNA, and the specific radioactivity of their RNA phosphorus showed 

 marked differences: the nuclear RNA had always a higher radioactivity 

 than the other fractions. Difi^erences in rate of incorporation were also 

 observed between the cytoplasmic fractions (Marshak and Calvet, 1949; 

 Jeener and Szafarz, 1950; Barnum andHuseby, 1950; Jeener, 1952; Tyner 

 et ah, 1953; Moldave and Heidelberger, 1954; Sacks and Samarth, 1956). 

 The use of other precursors of RNA, like glycine, formate (Smellie and 

 Davidson, 1956) and orotic acid (Hurlbert and Potter, 1952) led to similar 

 observations. Changes in physiological conditions differently affect the 

 metabolism of these various fractions (Jeener and Szafarz, 1950; Reid and 

 Stevens, 1956; Moldave, 1954; De Lamirande et al., 1958). 



Even the more elaborate among the presently used fractionation pro- 

 cedures are still very crude, and each time it has been possible to further 

 separate one of the cellular fractions into several subfractions, e.g. by com- 

 bining centrifugation with extraction in salt, in phenol or in detergents, it 

 was found that the RNAs contained in the subfractions again differ in their 

 physiological or metabolic activity (Vincent, 1952, 1957, 1958; Bhargava 

 etai, 1958; Schneider and Potter, 1958; Logan, 1957; OsRwaetaL, 1958; 

 Sihataniet al, 1959, 1960; Osawa, 1959; Goldthwait, 1959; Georgky et al., 

 1960; Antoni et al, 1960; Scholtissek, 1960). 



Among the various RNAs so far detected, special mention must be 

 made of the so-called 'soluble RNA'. In their studies on incorporation of 

 amino acid into proteins of liver homogenates, Hoagland et al. (1957, 1958) 

 observed that labelled amino acids are bound to a special RNA fraction 

 which is not sedimented with the particle bound RNA (see Chapter IV). 



This 'soluble RNA' is not a single substance, but a mixture of several 

 molecular species (Bloemendal and Bosch, 1959). It could be separated by 

 chromatography on cationic starch exchanger or by counter-current distri- 

 bution into fractions selectively binding certain amino acids (Smith et al., 

 1959;Uo\\eyetal., 1959). 



It is quite possible that soluble RNA preparations contain, beside 

 amino acid binding RNAs, other RNAs which play a different role or have 

 a different type of metabolism (Canellakis and Herbert, 1960). 



In bacteria also, RNA fractions differing in their metabolism have been 

 detected and separated (Countryman and Volkin, 1959). Immediately 

 after infection by a bacteriophage, a limited amount of a special high turn- 

 over RNA is formed (Volkin and Astrachan, 1956; Astrachan and Volkin, 

 1958; Watanabe and Kiho, 1958). In ultraviolet irradiated (Suzuki and Ono, 

 1959) or in chloramphenicol treated bacteria (Neidhart and Gros, 1957), 

 labile RNA fractions have been detected. Column fractionation of extracts 

 of normal E. coli after short time incorporation of radioactive phosphate 



