BIOLOGICAL ROLE OF PENTOSE NUCLEIC ACIDS 515 



ever, reported that PNA remains constant both in amount and in chemical 

 composition during development of sea urchin eggs, even if abnormalities 

 are produced by lithium treatment. 



Papers by Anderson and Aquist,^^^ BennelTt,^^^ Tyner et al.,'^° and Ham- 

 marsten et alr"^^ on the incorporation of various precursors in the PNA and 

 in the proteins of homogenates confirm, on the whole, the results obtained 

 by SmeUie et al.^^'' which have been discussed at some length in the preceding 

 article. Regarding the role of the nucleus in PNA synthesis or turnover, 

 recent work with autoradiographic techniques (A. Ficq,-^- J. Taylor-''^ 

 shows clearly that the nucleolus plays a leading part: incorporation of P^- 

 or radioactive glycine is exceptionally rapid in the nucleolar PNA, a con- 

 clusion also drawn by Stich and Hammerling,-^^ who studied the radio- 

 activity of the nucleolus dissected out of giant Acetabvlaria algae treated 

 with P^-. In the starfish oocytes studied by A. Ficq,-^- incorporation of 

 glycine in the nucleolar proteins is also usually high and fast. While these 

 autoradiographic results lend support to Caspersson's theory, it has been 

 found by Brachet and Szafarz-^* that PNA turnover, followed with labeled 

 orotic acid, is not seriously effected by the removal of the nucleus in Ace- 

 tabularia. Such a finding is somewhat difficult to reconcile with the sugges- 

 tion that cytoplasmic PNA originates within the nucleus (Caspersson,^* 

 Jeener and Szafarz'-^). This hypothesis, indeed, is steadily losing ground: 

 Barnum, Huseby, and Vermund,^^^ on the basis of new experimental data 

 and extensive calculations, come to the definite conclusion that their re- 

 sults are not consistent with the assumption that nuclear PNA is the pre- 

 cursor of any fraction of cytoplasmic PNA. The same opinion is shared by 

 Moldave and Heidelberger,^^^ who found that the PNA's of the nuclei, 

 mitochondria, microsomes, and supernatant are all different from each 

 other in both the composition of bases and the nucleotides sequence. 



Coming now to the question of the part played by the nucleus in protein 

 synthesis, we have seen earlier that biochemical experiments on homogenates 

 have so far failed to show a larger incorporation into the isolated nuclei 

 than into the mixed cytoplasmic proteins. In contrast to these findings, 



"8 E. P. Anderson and S. E. G. Aqvist, /. Biol. Chem. 202, 513 (1953). 



"9 E. L. Bennett, Biochim. et Biophys. Acta 11, 487 (1953). 



"0 E. P. Tyner, C. Heidelberger, and G. A. LePage, Cancer Research 13, 186 (1953). 



2" E. Hammarsten, B. Thorell, S. Aquist, N. Eliasson, and L. Akerman, Exptl. Cell 



Research 5, 404 (1953). 

 2« A. Ficq, Experientia 9, 377 (1953). 

 2« J. H. Taylor, Science 118, 555 (1953). 



2" H. Stich and J. Hammerling, Z. Naturforsch. 8b, 329 (1953). 

 "6 J. Brachet and D. Szafarz, Biochim. et Biophys. Acta 12, 588 (1953). 

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

 2" K. Moldave and C. Heidelberger, /. Am. Chem. Soc. 76, 679 (1954). 



