37. NUCLEIC ACID AND PROTEIN SYNTHESIS 375 



TABLE V 



Relative Proportions' 1 of Additional Components in RNA from Rat Liver 

 Microsomes and Soluble Fraction* 



Component Microsome Soluble 



Pseudouridine 



5-Methylcytosine 



6-Methylaminopurine 



6 -Dimethyl aminopurine 



1-Methylguanine 



2-Methylamino-6-hydroxy purine 



2- Dime thylamino-6-hydroxy purine 



11 Values are mole/100 mole uridine. 



6 Data are from D. B. Dunn, Biochim. et Biophys. Acta 34, 286 (1959). 



vary considerably small differences which may ultimately prove to be sig- 

 nificant would not show up here. It will be noted that there is generally a 

 remarkable similarity in the base composition of sRNA from microorgan- 

 isms and animals. This is in contrast to the clear differences in base com- 

 position of ribosomal RNA from these two sources. The possible significance 

 of this finding will be discussed later. 



The discovery by Cohn, 137, 138 and Davis and Allen 139, 140 of "pseudouri- 

 dine" (5-ribosyluracil) has led to an exploration of its location in definable 

 cellular RNA fractions. The occurrence of this compound in yeast, particu- 

 larly in a relatively salt-soluble fraction perhaps equivalent to sRNA, 139 

 suggested that it might be a constituent peculiar to sRNA. This was found 

 to be the case by Dunn, 132 whose data appear in Table V. It will be noted 

 that this nucleoside is considerably more abundant in sRNA than in micro- 

 somes. It was also observed, as the table shows, that there was a relative 

 excess in sRNA of methylated derivatives of the bases. 132 



(3) Heterogeneity. Early ultracentrifugal and elect rophoretic analyses of 

 sRNA revealed that the preparations were nonhomogeneous. Furthermore, 

 it became apparent that sRNA must consist of a number of distinct species 

 since amino acids were attached to sRNA additively (thus to specific sites) 

 and terminally. For these reasons much work has been directed toward at- 

 tempts to fractionate sRNA, both to separate the transfer RNA's from one 

 another and to separate them from contaminating unrelated "junk" RNA. 



135 D. A. Goldthwaite, Biochim. et Biophys. Acta 30, 643 (1958). 



136 It. S. Schweet, unpublished data, (1959). 



137 W. E. Cohn and E. Volkin, Nature 167, 483 (1951). 



138 W. E. Cohn, Biochim. et Biophys. Acta 32, 569 (1959). 



139 F. F. Davis and F. W. Allen, /. Biol. Chem. 227, 907 (1957). 



»° C. Yu and F. W. Allen, Biochim. et Biophys. Ada 32, 393 (1959). 



