408 R. M. S. SMELLIE 



thymine may be derived from the /3-carbon of serine, the a-carbon of 

 glycine, or from formate. 



The subject has recently been extensively reviewed by Brown et al.,''^ 

 Christman,^^ and Franke.^* Most of the studies on pyrimidine metabolism 

 have been made with the aid of labeled orotic acid (uracil-4-carboxylic 

 acid). This pyrimidine has been shown to promote the growth of certain 

 pyrimidine-deficient strains of Neurospora^^ and to act as a precursor of 

 nucleic acid pyrimidines in rats,^®'*^'^" in rat liver slices, ^^ and in yeast.^^ 



The incorporation of labeled orotic acid into rat liver and viscera nucleic 

 acids has been studied by several workers^^'^"'^' and shown to be about five 

 times greater in a given time in PNA than DNA. 



IV. Ribonucleic Acids of the 

 Subcellular Fractions 



1. Contrast between the Metabolism of Nuclear 

 AND Cytoplasmic PNA's 



It has been generally accepted that the PNA of the cell is distributed 

 amongst the various subcellular fractions (Chapter 21). While the main 

 bulk of the PNA is found in the cytoplasmic fractions (mitochondria, 

 microsomes, and cell sap), a small but important part is present in the 

 cell nucleus (Chapter 18). Since these morphological fractions probably 

 play different parts in the economy of the cell, it is clearly important to 

 examine the relationships between the PNA's from these different sources. 



In 1948, Bergstrand et al.^^ observed that, after administration of gly- 

 cine-N^* to rats with normal or regenerating livers, the isotope content of 

 the PNA from the liver cell nuclei was greatly in excess of that in the PNA 

 of the cytoplasm. Using radioactive phosphate, Marshak and Calvet^^ fol- 

 lowed up earlier studies on the nucleoproteins of the nucleus^* '^^ and 

 showed that the PNA of the nucleus assimilated P'^ much more rapidly 

 than did the PNA of the cytoplasm. Moreover, while the activity of the 

 nuclear PNA rose rapidly after administration of the isotope and fell 

 sharply soon afterwards, the activity of the cytoplasmic PNA increased 

 much more slowly, reaching a maximum much later than the nuclear PNA. 

 Similar studies have been carried out by Jeener and his co-workers," ■'' ■''^ 

 Barnum et a/.,^^'^^ Mclndoe and Davidson,'^ Davidson et al.,^^ Payne et 

 al.,^° Anderson and Aqvist,^' Smellie et al.,^ and by Tyner et al.,^^ who are 



^* A. Marshak and F. Calvet, J. Cellular Comp. Physiol. 34, 451 (1949). 



" A. Marshak, /. Gen. Physiol. 25, 275 (1941). 



9« A. Marshak, J. Cellular Comp. Physiol. 32, 381 (1948). 



" C. P. Barnum and R. A. Huseby, Arch. Biochem. 29, 7 (1950). 



38 C. P. Barnum, R. A. Huseby, and H. Vermund, Cancer Research 13, 880 (1953). 



" J. N. Davidson, W. M. Mclndoe, and R. M. S. Smellie, Biochem. J. 49, xxxvi (1951). 



