498 J. BRACKET 



place around the nuclear membrane (Caspersson,^^''"^ Hyden*"**^); similar 

 observations, made with basic dyes, have been reported in several labora- 

 tories on various types of cell (see, for instance, Barr and Bertram, ^^^ 

 Lagerstedt,^^'' Brachet'^')- There is little doubt that the presence of large 

 amounts of PNA, presumably in the form of microsomes surrounding the 

 nuclear membrane, is a general phenomenon in all protein-synthesizing cells. 



Biochemical investigations with labeled isotopes as precursors of PNA 

 lend some support to Caspersson's interpretation of these cytochemical 

 findings: Marshak'^^ was the first to report that the specific radioactivity 

 of the nuclear PNA, studied with P^^^ is much greater than that of the 

 cytoplasmic PNA. He considered that the nuclei contain a special type of 

 nucleic acid which is neither PNA nor DNA, but has some properties 

 common to each and which is a precursor to cytoplasmic PNA. Somewhat 

 later, Marshak and Calvet^^* expressed the opinion, on the basis of experi- 

 ments in which the specific activity of P^^ in PNA was measured in isolated 

 nuclei and microsomes, that nuclear PNA behaves as a precursor to cyto- 

 plasmic PNA (see also Chapter 26). 



The higher specific activity of nuclear PNA labeled with P'^ as compared 

 with cytoplasmic PNA has been confirmed by Jeener and Szafarz,^^* by 

 Barnum and Huseby,'^^ and by Smellie et alP^ All agree that their experi- 

 mental findings are compatible with the possibility that nuclear PNA is 

 the precursor of cytoplasmic PNA: but, while Jeener and Szafarz^^^ take 

 this explanation as the most probable one, Barnum and Huseby^^* empha- 

 size that this conclusion is made much less likely by the fact that, in their 

 experiments, the relative specific activity of the supernatant PNA remains 

 a constant percentage of the relative specific activity of the nuclear PNA. 

 It should be noted, however, that Barnum and Huseby's^^^ findings could 

 be easily explained if some of the PNA present in the supernatant fraction 

 were derived from the nuclei, a possibility which Smellie et al}"^^ do not 

 rule out. In any case, Barnum and Huseby'^^ suggest an alternative ex- 

 planation: some unidentified intermediate, which could be of either nuclear 

 or cytoplasmic origin, is the immediate precursor of both nuclear PNA 

 and part of the cytoplasmic PNA. 



Another interesting ploint is made by Smellie et al. -.^"^^ while their results 

 do not preclude the possibility that nuclear PNA is the ultimate precursor 

 of cytoplasmic PNA, the activity-time curves are not consistent with a 

 process of simple diffusion from the nucleus into the cytoplasm. They 

 further point out that, if simple diffusion occurred, cytoplasmic PNA and 



1" M. Barr and E. Bertram, Nature 163, 676 (1949). 



130 S. Lagerstedt, Acta Anat. SuppL, 9, 1 (1949). 



1" J. Brachet, Compt. rend. soc. btol. 143, 1300 (1949). 



i'2 A. Marshak, J. Cellular Comp. Physiol. 32, 381 (1948). 



